13. sediment microfabric and physical properties record of late

Wise, S. W., Jr., Schlich, R., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 120

13. SEDIMENT MICROFABRIC AND PHYSICAL PROPERTIES RECORD OF LATE NEOGENEPOLAR FRONT MIGRATION, SITE 7511

Frank R. Rack2 and Amanda Palmer-Julson2

ABSTRACT

Through scanning electron microscope analysis of sediment microfabric, we have evaluated variations inhigh-resolution shipboard physical properties (index properties and shear strength), sediment components (smearslide determinations), and shore-based calcium carbonate and biogenic silica data from Site 751 (Kerguelen Plateau).The stratigraphic section at this site records a change in biogenic ooze composition from predominantly calcareous(nannofossil) to siliceous (diatom) ooze from —23 Ma to the present, reflecting expansion of Antarctic water massesduring the late Neogene. The profound change in physical properties and sediment character at 40.1 mbsf (—5-6 Ma)evidently records the northward movement of the Polar Front and a change in absolute accumulation rates ofsediment at this site.

Trends in geotechnical properties with depth at Site 751 allowed us to subdivide the sedimentary column into anumber of geotechnical units that reflect changes in depositional and postdepositional processes with time.Geotechnical properties are sensitive to changing sedimentary inputs of primarily siliceous and calcareousmicrofossils. This allows us to study the physical nature of biostratigraphically-identified hiatuses and variations inenvironmental conditions linked to the migration of the Polar Front across this region. The analysis of geotechnicalproperties permits a more detailed division of the sedimentary column than is possible from shipboard lithologicdescriptions alone.

Our study of the sedimentary microfabric indicates that randomly oriented, elongate pennate diatom valvescompose the sediments with highest porosity and water content values, and the lowest density values (wet bulk, drybulk, and grain density). Conversely, sediments composed of nannofossils and disassociated nannofossil crystallitesand little or no siliceous remains have the lowest porosity and water content values, and the highest density values.Samples of mixed siliceous/calcareous composition have intermediate physical property values, but these varyaccording to the nature of the sedimentary matrix and the state of preservation of individual skeletal elements.

INTRODUCTION

Most previous investigations of the Antarctic Polar Fronthave involved bio stratigraphic, isotopic, taxonomic, or quan-titative faunal analyses. In this study, we establish a geotech-nical stratigraphy for Site 751 and evaluate variations inphysical properties with depth as a record of paleoenviron-mental changes.

Geotechnical stratigraphy has been defined as "the studyof a vertical succession of sediment characteristics such asindex properties (wet- and dry-bulk density, grain density,porosity, and water content), state and behavior of consolida-tion, strength parameters, and other inherent physical andmechanical properties that identify a distinct aspect of a givensedimentary unit" (Taylor, 1984). Taylor (1984) establishedcriteria for geotechnical analyses that were based on identify-ing zones having similar downhole trends in properties. Thesezones had distinct boundaries separating them from oneanother, thus delineating geotechnical units and/or subunits.The boundaries for geotechnical units were identified by trendreversals, inflections of a particular trend, or by discontinui-ties in geotechnical properties.

We define geotechnical subunits for Site 751 using thecriteria described above (see Table 1). The use of geotechnicalsubunits provides a coherent framework that can be used tointerpret subtle changes in the sedimentary record at this site,

1 Wise, S. W., Jr., Schlich, R., et al., 1992. Proc. ODP, Sci. Results, 120:College Station, TX (Ocean Drilling Program).

2 Ocean Drilling Program, Texas A&M University, College Station, TX77845-9547, U.S.A.

which are observed as variations in physical properties. Wealso calculate accumulation rates using dry-bulk density dataand linear sedimentation rates derived from shipboard biostra-tigraphy and magnetostratigraphy. (Age assignments used inthe text of this paper are from Harwood and Maruyama, thisvolume). Accumulation rate data (Appendix A) provide abasis by which we can evaluate changing fluxes of individualsedimentary components, which reflect variations in surface-water paleoproductivity, microfossil preservation, and theaccumulation of inorganic materials such as ice-rafted debris(IRD). In addition, we address changes in the sedimentarymicrofabric with regard to the sedimentary and environmentalconditions represented at this site.

Oceanographic SettingThe Antarctic Circumpolar Current (ACC) system can be

described as a zonal sequence of strong currents (expressed asmultiple narrow jets) and contrasting water masses, separatedby density fronts, which appear to be circumpolar in extent(Emery, 1977; Hofmann, 1985; Nowlin and Klinck, 1986) (Fig.1). Changes in ACC mean transport rates are related tolatitudinal displacements of the frontal systems (Whitworth,1983), which serve as boundaries between various watermasses, and extend with little attenuation to the sea floor. Thegeographic position of the respective fronts are known to varyfrom one basin to another, according to Gordon et al. (1978)and Gordon and Molinelli (1982); these and other reviewpapers note that migrations of the Antarctic Polar Front (orsimply "Polar Front" in this paper) should be considered inthe context of the entire ACC system.

The Oceanographic fronts of importance to this paper arethe Antarctic Divergence (AD), the Polar Front (PF), and the

179

F. R. RACK, A. PALMER-JULSON

Table 1. Geotechnical stratigraphic units, Site 751.

Subunit(g/cm3)

Depth(mbsf)

Water content(% wet wt)

PorosityWet-bulkdensity(g/cm3)

Dry-bulkdensity

Graindensity(g/cm3)

Carbonatecontent (%)

G-IaG-IbG-IcG-IIaG-IIbG-IIcG-IIdG-IIeG-IIfG-IIgG-IIhG-IIiG-IIjG-IIkG-IIIG-IIm

00.00-9.5014.4014.90-39.9040.35-52.0054.00-60.4060.85-76.0076.40-90.9091.35-94.3595.00-102.35

103.00-109.00109.90-120.00120.50-125.00125.40-128.00128.40-139.90140.35-149.85150.50-164.50

81.84-48.8820.1881.57-56.8148.02-40.2043.96-35.7656.95-46.2048.12-38.1739.63-37.1754.44-41.4253.81-39.6056.81-33.5938.03-34.5851.51-40.8045.16-36.3740.25-36.2248.75-37.45

91.50-70.9835.1190.57-71.9170.83-63.4068.19-59.9174.75-68.0770.46-61.3663.48-59.5574.73-64.4673.15-63.0274.24-57.7462.00-58.0972.61-63.2667.17-60.5763.21-59.5770.77-60.28

.49-1.12

.811.30-1.171.65-1.481.76-1.55.54-1.37

1.68-1.51.72-1.64.63-1.39.67-1.39.84-1.37.78-1.65.62-1.48.76-1.54.76-1.67.74-1.51

0.76-0.201.450.51-0.220.97-0.771.13-0.870.82-0.591.04-0.801.07-0.990.95-0.641.01-0.641.21-0.591.16-1.050.96-0.721.12-0.851.11-1.001.08-0.77

2.58-1.892.172.72-1.612.71-2.372.82-2.422.65-2.152.74-2.382.82-2.552.67-2.222.63-2.352.89-2.202.81-2.512.79-2.522.78-2.352.95-2.502.80-2.46

71.46-00.00

24.39-00.0084.34-59.9290.84-77.8178.77-42.9484.83-67.5391.72-85.1084.82-42.9071.04-25.9094.48-24.2594.48-81.8572.43-47.5388.99-62.0392.61-87.5993.18-66.75

40°S

50c

• ODP Leg 119 Sites

β ODP Leg 120 Sites

Australian-Antarctic

Basin750

70'

vDavis Station

ANTARCTICA

80° 90"

Figure 1. Location map showing the Kerguelen Plateau and theposition of Site 751.

Subtropical Convergence (STC). The AD, nearest the Antarc-tic continent, is characterized by upwelling and the formationof cold Antarctic Surface Water, which flows north toward thePF (Gordon et al., 1978). The PF, or Antarctic Convergence,is marked by a complex stratification between antarctic andsubantarctic waters, and the down welling of surface water toform Antarctic Intermediate Water. The PF exists where thewater temperature minimum dips sharply below the 200 mlevel, and a salinity minimum (34.0%< 3̂4.1%o is present.Subantarctic Surface Water flows northward, away from thePF, and sinks near the STC (Gordon et al., 1978). Variationsin Subantarctic Surface Water (bounded by the STC and PF)are linked to simultaneous changes in the position and inten-

sity of the westerly winds and sea surface temperatures(Fletcher et al., 1982).

In this study, we have evaluated the history of Polar Frontmigrations at Site 751 by using geotechnical stratigraphy toreveal latitudinal shifts of pelagic sedimentary facies. Such faciesshifts have been linked to Neogene movements of the PF andSTC by Hayes et al. (1976), Wise et al. (1982), Ciesielski andWeaver (1983), Williams et al. (1985a, 1985b), Ciesielski andGrinstead (1986), and Morley (1989) among others. These andother studies have shown that the northern boundary of thesiliceous facies is marked by a change to calcareous depositioncoincident with the location of the Subtropical Convergence.The southern boundary of the siliceous facies is generallyacknowledged to be coincident with the Antarctic Divergence(~60°S in the Atlantic and Indian oceans), and is marked by achange to terrigenous sediment associated with the mean posi-tion of Antarctic sea ice (Cooke and Hays, 1982). Most terrige-nous material deposited north of the AD and south of the PF,such as at Site 751, has therefore been transported primarily bywind or ice (see Breza, this volume for discussion of NeogeneIRD). Furthermore, the bulk of the biogenic opal that is incor-porated into the geologic record accumulates in a latitudinal beltaround Antarctica between 45° and 65°S; thus making shifts inthis facies highly significant for global fluxes of silica over time(Lisitzin, 1972,1985;DeMaster, 1979,1981; Barron and Baldauf,1989).

Site 751

Site 751 (Fig. 1) is located at 57°43.56'S, 79°48.89'E, in1633.3 m of water, 900 km south of the present-day position ofthe Polar Front (at 50°S, the latitude of Kerguelen Island).Schlich, Wise, et al. (1989) described two sedimentary unitsfrom this site (see Fig. 2):

Unit I (0-40.1 mbsf): upper Pleistocene (>0.2 Ma) to lowerPliocene (to upper Miocene?) diatom ooze with minoramounts of ice-rafted debris, foraminifers, volcanic ash, andPorcellanite.

Unit II (40.1-166.2 mbsf): upper Miocene to lower Miocenediatom nannofossil ooze.

As discussed by Schlich, Wise, et al. (1989), the sedimen-tary section at this site records the late Neogene glacialhistory of Antarctica (and consequent expansion of Antarcticwater masses) reflected as a facies change from nannofossilooze to diatom ooze. Unit II represents the —17 m.y.

180

160 —

0.0 0.4 0 3 1.2 1.6 2.0 0 20 40 60 80 100 1.4 1.8 2.2 2.6 3.0 0

Dry- and wet-bulk density (g/cm 3 ) Water content and porosity (%) Grain density (g/cm3)

20 40 60 80 100 0 20 40 60 80 100 120 140

Carbonate content (%) Su (kPa)

Figure 2. Profiles of index properties, carbonate content, and shear strength vs. depth for Site 751. Core recovery and relative age of section is shown to the left of the figure. Geotechnicalsubunits are also indicated.

n50O

32n>zσ

n>r50O

WGO

50M

8σ


(~23-~6 Ma) during which 126.1 m of predominantly calcar-eous ooze accumulated under the influence of earlier watermasses. A disconformity (—2.5 m.y., from 8.4-5.9 Ma) existsbetween the units, marking the northward migration of the PFacross the Site 751 location. Above the disconformity, 40.1 mof biosiliceous ooze accumulated during the ensuing 5.9 m.y.(5.9 Ma to present) under the influence of various Antarcticwater masses.

This change in sedimentation regime is clearly demon-strated by the percentages of diatoms and nannofossils seen insmear slides of the sediment, geochemical (wt% CaCO3 andSiO2), and physical property data (porosity, bulk density, andgrain density) (Figs. 2 and 3). However, subtle variations alsoappear in these data, suggesting that a record of climatichistory may be present throughout the sequence. Our geotech-nical stratigraphic analysis was particularly valuable for re-vealing these less distinct variations.

We examined nine representative samples with the scan-ning electron microscope (SEM) (see Table 2) and evaluatedour observations with reference to physical properties andsedimentological data from the sediments. Our intention wasto investigate how and why variations in the physical proper-ties data corresponded to changes in sediment composition; inparticular, we were interested in fluctuations in the two-component (silica vs. carbonate) nature of Site 751 sediments,which the Leg 120 shipboard party interpreted as representingclimatic fluctuations (i.e., Polar Front migrations) (seeSchlich, Wise, et al., 1989).

A cursory review of the data (Figs. 2 and 3) indicatesstriking differences in physical properties correlated withsediment composition: siliceous sediments have higher poros-ity and water content values, and lower wet-bulk, dry-bulk,and grain density values than do calcareous sediments. Simi-lar observations have been made by previous investigators(e.g., Schreiber, 1968; Hamilton, 1976; Shephard and Bryant,1980; Bryant et al., 1981; Wilkens and Handyside, 1985;Wilkens et al., 1987; Pittenger et al., 1989; among others). Inaddition, the apparent consolidation state (as illustrated by aplot of the Su/overburden stress ratio) and bulk accumulationrate of sediments are shown to be inversely related; withhigher bulk sediment accumulation observed as apparentunderconsolidation, and lower bulk accumulation resulting inoverconsolidated or normally consolidated sediment se-quences (Fig. 3). We were interested in how the orientationand interrelation of discrete particles (skeletons or skeletalelements) influences the physical properties of the sediment,which led us to investigate the microfabric of the oozes.

Previous studies of the sedimentary fabric and its effects ongeotechnical properties have typically focused on nonbiogenicsediments, especially clays (e.g., see review in Bennett et al.,1977). However, there have been few fabric studies of bio-genic sediments, although biosiliceous sediments, in particu-lar, demonstrate intriguing physical properties that are clearlyrelated to sediment fabric characteristics. For example, theporosity and density of biosiliceous sediment may remainunchanged with depth because the porous, rigid, siliceousskeletons resist the compaction that typically occurs in othersediment types in response to overburden load (Hamilton,1976; Shephard and Bryant, 1980; Bryant et al., 1981; Wilkensand Handyside, 1985; Wilkens et al., 1987; Pittenger et al.,1989).

In the following sections, we (1) discuss our methods ofsample selection and analysis, (2) present a geotechnical stratig-raphy for Site 751, (3) present a summary of the physicalproperties data, microfabric, and calculated accumulation rates,(4) present an overall summary and discussion of our results as

a record of the movement of the Polar Front in response to thelate Neogene climate history of the Southern Ocean.

METHODSHigh-resolution measurements of physical properties (index

properties and shear strength), calcium carbonate, and smear-slide data were obtained from shipboard analyses detailed in the"Explanatory Notes" chapter (Schlich, Wise, et al., 1989). Weperformed additional carbonate analyses of the index propertysamples on a coulometer similar to the one aboard the JOIDESResolution. These data increase the resolution of the shipboardcarbonate profile and provide a check on the shipboard valuesmeasured in rough seas during the cruise. Biogenic silica (opal)contents were determined by D. DeMaster, University of NorthCarolina using the sodium carbonate leaching method describedby DeMaster (1979, 1981).

We selected nine of the physical properties samples forSEM analysis. These samples were chosen to represent sed-imentary intervals that we found to have distinctive physicalproperties, with one or two samples taken from each interval.Note that no samples were taken from Cores 120-751 A-1H to-3H because these had extremely high water contents and itproved impossible to obtain undisturbed material. The sam-ples we chose were trimmed and oriented so that we couldidentify the direction of bedding. We also studied smear slidesof these samples to make direct correlations of results.

Samples for SEM analysis were oven dried and coated withAu-Pd. They were mounted on aluminum stubs perpendicular tobedding (the orientation of the stratigraphic "up" direction waslost, but this information was not critical for the purposes of ourinvestigation). Samples were examined using the JEOL instru-ment at the Texas A&M Electron Microscopy Center at 10-15kV; an energy-dispersive X-ray spectrographic analyzer wasused to study carbon-coated splits of selected samples.

RESULTS

IntroductionThe goal of a geotechnical stratigraphic analysis is to integrate

measured physical and mechanical properties of the sedimentsfrom a particular site with geochemical data and calculated bulkaccumulation rates, to provide a better understanding of thedepositional history of a given sedimentary sequence. Thisapproach has been used to describe the sedimentary sectionsfrom numerous DSDP and ODP drill sites.

Recently, attention has been focused on sedimentary se-quences that contain biogenic silica in high-latitude regionssuch as the V0ring Plateau (Leg 104; Eldholm, Thiede,Taylor, et al., 1987, 1989), the Weddell Sea (Leg 113; Barker,Kennett, et al., 1988, 1990), across the South Atlantic (Leg114; Ciesielski, Kristoffersen, et al., 1988), and along thelength of the Kerguelen Plateau (Legs 119 and 120; Barron,Larsen, et al., 1989, and Schlich, Wise, et al., 1989, respec-tively). These sequences demonstrate the anomalous nature ofbiosiliceous sediment properties (see Schreiber, 1968; Hamil-ton, 1976; Bryant et al., 1981; Wilkens et al., 1987) comparedwith those of clays or carbonates, where, typically, porositydecreases and density increases with increasing depth andoverburden pressure.

We evaluated the physical properties data from Site 751and identified geotechnical subunits as follows: three in litho-logic Unit I and 13 in lithologic Unit II (Table 1 and Figs. 2 and3). These subunits reveal contrasts in sediment compositionbecause siliceous sediments have lower wet-bulk, dry-bulk,and grain density values and higher porosity and water con-tent values than calcareous sediments. In the following sec-

182

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100 80 60 40 20 0

Nannofossils (%)

20 40 60 80 100

Diatoms (%)

0.01 0.1 1.0

Su / overburden stress

5 10 15 20 25 30

Bulk accumulation of sediment (g/m2/k.y.)

Figure 3. Smear slide data of diatom and nannofossil content vs. depth compared with shear strength/overburden stress ratio, and bulk accumulation of sediment vs. depth. Core recovery,relative age, geotechnical units and lithologies are also shown. Arrows indicate intervals where decreases in bulk-accumulation of sediment correspond with increased consolidation in Unit

II.

>rT3

O*üffl?σH


Table 2. Data from samples investigated with the SEM, Site 751.

Core, section,interval (cm)

120-751A-

4H-4, 67-694H-6, 30-325H-5, 70-726H-1, 70-726H-3, 70-728H-5, 30-328H-5, 70-7212H-4, 115-11713H-6, 30-32

Depth(mbsf)

28.931.539.943.446.468.068.4

105.4117.0

Age

early Plioceneearly Pliocenee. Plio./l. Mio.late Miocenelate Miocenelate Miocenelate Miocenemiddle Miocenemiddle Miocene

D

957593353753401515

Biogenic

coniµuiiems

R

22623213+

N

000

636044507975

(Vc)

SF

1100011++

F

+000000+5

CaCO3

(%)

0.30.20.1

67.171.242.342.365.791.1

SiO2Δ

(%)

62.164.545.413.513.324.0

10.21.5

Porosity

(%)

83.8682.4971.9166.4665.7671.4973.2463.3660.39

Wet-bulkdensity(g/cm?)

:•

.24

.231.37.57.59.38.40.65.84

Dry-bulkdensity(g/cm?)

0.380.380.550.890.890.640.630.981.20

Graindensity(g/cm1)

2.342.101.952.582.462.152.252.592.89

Notes: Major component abundances as estimated from smear slides. D = diatoms, R = radiolarians, N = nannofossils, SF = silicoflagellates, F =foraminifers (or foraminifer fragments).

tion, we discuss the important results of our geotechnicalstratigraphy.

Index Properties

The shipboard physical-properties program completed atSite 751 was designed to provide high-resolution (50-cmsample spacing) profiles of index properties to investigate thedetailed behavior of biogenic oozes deposited during theNeogene in the Raggatt Basin. In addition, this site providedthe opportunity to investigate changes in index propertiesover most of the range of natural carbonate contents from 0%to 100%. This fortuitous situation allowed the relationshipsbetween percentage of calcium carbonate and index proper-ties to be determined for the ~165-m-thick sequence at Site751. Plots of individual index properties vs. carbonate contentshow strong correlations between these measurements (Figs.4-6). Over the range of carbonate contents measured, thefollowing nonlinear regression equations were determined forindividual index properties at Site 751 on the basis of calciumcarbonate percentage:

WBD = 2.592 × l(r5 (JC2) + 0.003 (x) + 1.216, r2 = 0.901 (1)

DBD = 3.046 × I0"5 (x1) + 0.005 (x) + 0.345, r2 = 0.919 (2)

GD = 2.617 × I0"5 (x2) + 0.003 {x) + 2.160, r2 = 0.720 (3)

POR = -3.186 × I04 (x2)- 0.220 (x) + 84.45, r2 = 0.899 (4)

WC = 1.354 × I0-4 (x2)- 0.387 (JC) + 71.85, r1 = 0.911 (5)

where x = calcium carbonate percentage,WBD wet-bulk density (gravimetric),DBD = dry-bulk density (gravimetric),GD = grain density,POR porosity, andWC = water content.

According to the relationships derived in our study, depth-related changes in index properties are controlled primarily bydifferences in lithology; these, in turn, are the result ofchanging environmental conditions on the Kerguelen Plateau.For example, the effects of compaction vary with depth as aresult of changing microfossil assemblages and absolute accu-

mulation rates: siliceous tests typically resist overburdenforces more than do calcareous tests.

The poor correlation of carbonate content and grain den-sity (relative to the other index properties) may result from thepresence of ice-rafted and aeolian components, particularly inLithologic Unit I (see Breza, this volume). However, it alsomay be a function of the calculation method (see Boyce,1976), which is imprecise for high-porosity sediment. Therelationships shown for index properties and carbonate con-tent also apply to the majority of the observed siliceous vs.calcareous microfossil assemblage and sedimentological vari-ations with depth. The close association between sedimento-logical and physical properties data at Site 751 allowed us todevelop a detailed geotechnical stratigraphy that reflects thedynamic process of Neogene pelagic sedimentation in thecentral Raggatt Basin.

State of Consolidation

The sediments at Site 751 have low shear strengths, al-though values are somewhat higher than would be expectedgiven such noncohesive, high-water content oozes at shallowburial depths. Higher shear strengths could also be an artifactcaused by the drainage of fluid in the cores during testing (thuscausing increased vane shearing resistance). Lee (1982) con-sidered the effect of biogenic silica on the shear strength andbulk density of deep-sea calcareous sediments. He noted thatthe grain size and shape of pelagic sediments are controlled byproductivity and dissolution factors; furthermore, the pres-ence of biogenic silica has such a strong effect on the bulkdensity that the effects from variations in other sedimentconstituents are completely masked.

At Site 751, zones of increased shear strength correlatewith increased water content (Fig. 7), and decreased bulkdensity and carbonate contents (Fig. 8). These trends areanomalous since shear strength normally increases with de-creased water content and increased density in marine sedi-ments (Bryant et al., 1981). Changes in the bulk accumulationrate of sediments are often inversely related to shear strengthvariations. The decreased accumulation of calcareous micro-fossils (possibly indicating dissolution of tests), and increasedaccumulation of siliceous microfossils, in zones of increasedshear strength may point to a diagenetic cause for this trend.These zones of increased shear strength may also indicate therelative positions of sedimentary hiatuses.

Compaction effects are variable downhole because of thechanging composition of microfossils and the typical resis-tance of siliceous tests to overburden forces that act uponthem. A plot of void ratio to overburden stress shows thesubstantial decrease in the range of consolidation for the

184

SEDIMENT MICROFABRIC AND PHYSICAL PROPERTIES RECORD

1.9

1.7

O

3

5α>

T3

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1.5

1.3 0

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,7o

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u3

§ 0.7•o

o0.5

0.3

0.1

0 j 00 0

20 40 60 80

Carbonate content (%)100 20 40 60 80 100

Carbonate content (%)

Figure 4. Scatter plots showing the relationship between laboratory wet- and dry-bulk density measurements and carbonate content of samplesfrom Site 751.

calcareous ooze below 40.1 mbsf, as indicated by the largereduction in void ratio above -25 kPa (Fig. 9).

A field consolidation curve of shear strength/effectiveoverburden stress was calculated according to the relationshippresented by Skempton (1970) for the sediment at Site 751 toevaluate their apparent state of consolidation (Fig. 10). Valuesof the ratio Su/overburden stress between 0.2 and 0.5 areinterpreted to represent normally consolidated sediment; val-ues <0.2 indicate underconsolidation and values >0.5 areconsidered to indicate overconsolidation.

The upper portion of this curve shows the sediment to behighly overconsolidated except in narrow zones that correlate toincreases in water content and decreased carbonate content.This state of apparent overconsolidation is common in surficialmarine sediments, but generally does not extend very far into thesubsurface. The high values of the ratio of shear strength/overburden stress observed in the upper 40 m at Site 751 may bethe result of interlocking skeletal elements of elongate siliceousmicrofossils (i.e., diatoms). The framework developed by thesiliceous skeletons, combined with the low bulk density of thesehighly porous deposits, reduces effective overburden pressuresand thus increases the strength/stress ratio.

The shift to underconsolidated sediment at —52 mbsf islinked to decreased porosity, increased carbonate content

(i.e., presence of nannofossils), and increased bulk sedimentaccumulation in Subunit G-IIb with respect to Subunit G-IIa.The fine-grained calcareous matrix causes tight grain packing,which restricts fluid flow in this subunit. Reversals in therelative underconsolidation of lithologic Unit II are observedfor Subunits G-IIc, G-IIf, G-IIh, and G-IIj, indicating morenormal consolidation behavior correlated with increased bio-genic silica content and decreased bulk accumulation rates.The higher permeability of siliceous ooze relative to carbonateooze is likely to be the cause of this apparent increase inconsolidation state. The shear-strength measurement intervalbecame more widely spaced below 52 mbsf, however, we canidentify two general trends: (1) increased underconsolidationwith depth, and (2) narrow zones of increased relative consol-idation within highly biosiliceous intervals (having >—40%diatoms).

Water Content, Porosity, and Permeability

Biogenically precipitated silica (SiO2 n H2O) is not homog-enous in nature. Density variations in opaline tests are related towater content, specific surface area, and age (Hurd and Theyer,1977; Hurd et al., 1979). Wefer et al. (1982) estimate the boundwater in diatomaceous opal from sediment traps in the WeddellSea to be between 6 and 9 wt%. DeMaster (1979) suggests that

185


3.0

1.620 40 60

Calcium carbonate (%)

80 100

Figure 5. Scatter plot showing the relationship between laboratorygrain density measurements and carbonate content of samples fromSite 751.

for Antarctic siliceous sediments containing —55 wt% biogenicsilica, 10%-15% of the remaining weight is attributable to boundwater. This water must be taken into consideration when ana-lyzing the physical properties of siliceous sediments.

Previous workers have evaluated the effects of inter- andintraparticle water on the physical properties of calcareoussediments. For example, Bachman (1984) found intraparticlewater to be associated with foraminifers, but it appears tohave no effect on geotechnical properties unless the particlesare fractured during burial-induced loading (Demars, 1982).

Valent et al. (1982) proposed that the predominant com-pression mechanism in calcareous oozes is high, nonrecover-able deformation of the microfossil skeletons without physicalbreakage, as grain crushing was not observed in the naturalspecimens tested. However, Demars (1982) observed thatforaminifer particles are susceptible to fracture when sub-jected to only moderate compressional stress, thereby releas-ing intraparticle water. Nannofossils are more likely to exhibitchipping rather than fracture at particle contacts, since largerparticles transmit dynamic stresses and shield the fine-grainedmatrix (e.g., particles >5 mm in diameter tend to fracturewhen absorbing contact forces) (Demars, 1982; Bennett et al.,1989). Nannofossils do, however, show changes in pore sizeand shape in response to mechanical compaction caused byoverburden stress (e.g., dewatering and rearrangement of

grains), thus creating denser packing arrangements (Bryant etal., 1981; Kim et al., 1985).

Bennett et al. (1989) discuss how packing arrangement cancontrol permeability in calcareous oozes as the matrix ofnannofossil crystallites creates small pores and restricteddewatering pathways. They measured permeability coeffi-cients of I0"5 to I0"6 cm/s (porosity = 54%-67%) in calcare-ous sediments (composed of —10% foraminifers in a matrix ofnannofossils) from Exuma Sound. Bryant and Rack (1990)present permeability coefficients for samples from Maud Rise(Sites 689 and 690, Leg 113) that are from I0"4 to I0"5 cm/s,for shallow-buried samples of diatom ooze, and from I0"5 toI0"7 cm/s, for underlying nannofossil oozes.

The high permeability of diatom ooze at low overburdenpressures allows rapid drainage of fluid and, hence, resultsin the high degree of apparent overconsolidation observed infield consolidation curves. The data acquired from consoli-dation tests, using samples from Maud Rise, show theimportance of intraskeletal porosity in shallow-buried dia-tom oozes. Diatomaceous sediments subjected to verticalstresses (up to 3200 kPa) exhibit a higher level of voidreduction during testing than the calcareous samples, yet donot consolidate to as low a final void ratio (Bryant and Rack,1990). The results obtained during consolidation testing ofsiliceous and calcareous oozes from Kerguelen Plateau areconsistent with the findings from Maud Rise (Rack et al.,unpubl. data).

Pittenger et al. (1989) demonstrate that for a given changein permeability, a greater degree of consolidation (decrease invoid ratio) is observed for sediments with high biogenic silicacontents than for other sediment types. The high percentageof intraskeletal and bound water in opaline tests may also helpexplain the resistance to compaction generally observed indiatomaceous deposits. If dissolution and/or breakage of testsreleases intraskeletal and/or bound water (which can replacethe water expelled by consolidation), then the effective verti-cal stress felt by the sediment will be reduced. This concept isimportant in explaining the underconsolidated state of sedi-ments from the V0ring Plateau, although other factors (e.g.,overlying low permeability clay-rich sediment) are also impor-tant (Pittenger et al., 1989). Our interest in how the orientationof discrete particles (skeletons or skeletal elements) influencethe physical properties of the sediment led us to investigatethe microfabric of these cores.

Microfabric AnalysisWe examined nine samples utilizing the SEM, noting for

each such features as (1) nature, condition, and orientation ofthe component particles; (2) relationship of matrix to largergrains; (3) degree of fragmentation, disarticulation, or disas-sociation of biogenic elements; (4) distribution of intra- andinterparticle pore spaces; and (5) any other significant featureof the sedimentary fabric (detailed fabric descriptions for eachSEM sample are presented in Appendix B; physical propertydata and component abundances are listed in Table 2). Wethoroughly scanned each sample, noting fabric characteris-tics, and photographed representative features (e.g., diatomvalves showing preferred orientation or articulation of valvesand girdle bands); these are illustrated in Plates 1-5.

Biosiliceous sediments demonstrate intriguing physicalproperties clearly related to sedimentary microfabric. SEManalyses showed that some samples are predominantly com-posed of elongate, essentially intact pennate diatoms, whereasother samples are composed largely of centric diatom valves,and pennate diatoms are mostly represented by fragments.The matrix material in Unit I is primarily composed offragmented diatoms; large skeletal fragments in these samples

186

95


90

2020 40 60 80

Carbonate content (%)

20 40 60 80

Carbonate content (%)100

Figure 6. Scatter plots showing the relationship between laboratory measurements of porosity and water content and carbonate content forsamples from Site 751.

are scattered in the matrix, and are not generally in contact(even where clustered, as with the radiolarians). The almostcomplete absence of nannofossils and nannofossil fragmentsin this zone indicates the influence of carbonate-corrosiveAntarctic waters.

We make the following summary observations of themicrofabric with regard to the physical properties and sedi-mentological data from the samples (see Appendix B):

1. Porosity is positively correlated with diatom content andwith SiO2 content; all the density measurements (wet-bulk,dry-bulk and grain density) are negatively correlated withporosity; elongate pennate diatoms create the most open, leastdense fabric (e.g., Plates 1 and 2), as observed above 40.1mbsf; below this level, centric diatoms create a less open,denser sediment (e.g., Plate 4), as reflected by lower porosityvalues and higher density values relative to the upper se-quence.

2. Porosity is reduced in highly diatomaceous sediments inwhich the pore spaces of the individual tests are filled withmatrix material or overgrown with authigenic minerals, suchas those immediately above the major hiatus at 40.1 mbsf(Plate 3, Figs. 1-3). The apparent inorganic silica (Opal-A'?)composition of this material appears to be responsible for the

lack of difference in density values between samples of similarcomposition, with or without overgrowths.

3. Throughout the section, radiolarians are commonly filledwith finer grained material than present in the surroundingmatrix (e.g., Plate 1, Fig. 2) and may have shielded the finematerial from winnowing. The loss of this potential pore spacemay reduce the overall porosity of the sediment (and weakenthe correlation between silica content and porosity whereradiolarians are a large component of the sediment).

4. Arthur et al. (1980) noted that bioturbation in diatoma-ceous sediments of the Japan Trench created a disorderedfabric that caused the sediments to resist compaction andremain underconsolidated to —300 mbsf. In our samples, wetypically found an overall disordered fabric, probably theresult of bioturbation, but even in clearly bioturbated sedi-ments (as noted in core descriptions) we saw some (relict?)evidence of preferred orientation (e.g., the alignment of longaxes of diatom valves; Plate 1, Figs. 1-4). Bohrmann andStein (1989) interpreted orientation of diatom valves to be theresult of deposition directly from highly productive surfacewaters; they apparently did not consider bioturbation as apossible factor for creating a disordered fabric.

5. Porosity is negatively correlated with nannofossil con-tent and CaCO3 content in these sediments. Nannofossils pack

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160 -

18020 40 60 80 100 120

Strength (kPa) vs. water content (%)

140

Figure 7. Plot of vane shear strength (filled oval) vs. water content(open oval) for Site 751 showing correlation between increased waterand increased strength at specific depths downhole.

very closely and disassociate into crystallites, which close offeven more pore space (e.g., Plates 4 and 5); nannofossilscontribute most of the CaCO3 in these sediments.

Accumulation Rates and HiatusesRegions of high productivity, associated with upwelling,

produce large fluxes of phytoplankton (diatoms and cocco-lithophorids) and zooplanicton (foraminifers and radiolari-ans). Productivity variations, linked to climatic and Ocean-ographic factors, and dissolution or erosional effects arelargely responsible for causing changes in microfossil assem-blages.

Carbonate productivity fluctuations are difficult to identifyin the geologic record. Dean and Gardner (1986) note thatlarge fluctuations in carbonate content (>30%-40%) are noteasily explained by productivity alone; but rather, must in-clude some degree of dissolution or erosion because of themagnitude of the observed change. The goal of identifyingproductivity fluctuations is further complicated by the chang-ing importance of individual processes, such as dissolution,erosion, dilution by other sedimentary components, andchanging degree of preservation, in creating the observedsequence.

Dissolution events are most evident in regions exhibitinghigh diversity gradients, such as the Polar Front and Subtrop-ical Convergence. Dissolution can be evaluated using suchparameters as increased numbers of benthic foraminifers,

160

18020 40 60 80 100 120

Strength (kPa) vs. carbonate (%)140

Figure 8. Plot of vane shear strength (filled oval) vs. carbonate content(open oval) for Site 751 showing correlation between decreasedcarbonate content and increased strength at specific depths downhole.

increased fragmentation of tests, and changes in radiolarianabundance, contrasted with fluctuations in calcium carbonatepercentage (Williams et al., 1985a, 1985b).

Accumulation Rates

Absolute accumulation rates (flux rates) can be determinedfor any component of the sediment by multiplying the dry-bulk density and linear sedimentation rate and the weightpercent of the component in the sediment. Clemens et al.(1987) presented equations for estimating dry-bulk densityfrom carbonate content that give similar results to the equa-tions presented in this paper (their data came from five Eltaninpiston cores in the southeast Indian Ocean north of KerguelenIsland). They cautioned that productivity differences betweenwater masses may bias interpretations of paleowater massmovement from relative microfossil abundance data, andconsequently they recommended using absolute accumulationrates derived from dry-bulk density values to eliminate rate-dependent effects.

We calculated absolute accumulation rates by multiplyingthe percentage of each sedimentary constituent by the dry-bulk density (shipboard measurement) and the linear sedimen-tation rate derived from shipboard age assignments (note thatour results are contingent on future revision of biostrati-graphic determinations). We determined calcium carbonateaccumulation rates using shore-based laboratory data of car-

188


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1 10 100

Overburden stress (kPa)

1000

Figure 9. Plot of void ratio vs. overburden stress for Site 751. Note thesharp decrease in void ratio below 75 kPa indicating the boundarybetween Units I and II at this site.

bonate content (Fig. 11 A), and noncarbonate accumulationrates by assuming a two-component system (i.e., 100%-wt%CaCO3 = wt% SiO2) (Fig. HB). Although this assumptionincludes ice-rafted and aeolian components of the total flux,generally <10% of Site 751 sediments, it does give a goodfirst-order approximation of changing siliceous accumulation.This approximate siliceous accumulation is compared withrates determined using data from chemical analyses of wt%SiO2 provided by D. DeMaster (North Carolina State Univer-sity). The chemical data shows lower wt% SiO2 than would beexpected; however, if we include the wt% water associatedwith biogenic opal (β%-\5%, or a mean value of 10%), we canaccount for most of the difference between the calculated andanalyzed values. Ice-rafted debris (IRD) accumulation rateswere determined by Breza (this volume) on a grain/cm2/k.y.basis. We use IRD flux data from Breza's study (usingsamples located adjacent to our physical properties samples),to compare the calculated carbonate and noncarbonate accu-mulation rates derived from our study with IRD peaks (Figs.12A and 12B). The highest IRD flux was observed between-25 and 40 mbsf and at 7-10 mbsf. The interval from 15-20mbsf shows minimal IRD accumulation, yet shows 5-7 g/m2/k.y. accumulation of noncarbonate material; thus providing arough estimate of the average biosiliceous flux during thePliocene at Site 751. This estimate agrees well with thesiliceous accumulation rate of 10.0 g/m2/k.y. reported for thecentral Circumpolar Ocean by Lisitzin (1985), which wasdetermined for the time interval from 0 to 0.7 m.y. (BrunhesChron). The rate of noncarbonate accumulation calculated for

0

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80

100

120

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I i

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0.01 0.1 1 10

Su/Po' ratioFigure 10. Plot showing the ratio of undrained shear strength/over-burden stress vs. depth for Site 751. Values of this ratio above 0.5indicate apparent overconsolidation; values below 0.2 indicate appar-ent underconsolidation; and values between 0.2 and 0.5 indicatenormally consolidated sediment.

sediment in the upper few meters of Site 751 is equal to ~l .5g/m2/k.y. (during the Pleistocene).

The accumulation of biogenic silica is fairly constant below40.1 mbsf, averaging approximately 4.0 g/m2/k.y. throughoutlithologic Unit II. Peaks in siliceous accumulation, associatedwith low carbonate accumulation rates, occur at three distinctintervals in Unit II (Fig. HB). These intervals are: (1) -60-76mbsf (Subunit G-IIc); (2) 95-115 mbsf (Subunits G-IIf, G-IIg,and G-IIh); and (3) 125-135 mbsf (Subunits G-IIj and G-IIk).The interval between 95 and 115 mbsf shows extreme vari-ability in carbonate accumulation rates, indicating large vari-ations in productivity and/or erosion. Carbonate accumulationdrops dramatically at —109 mbsf and slightly above 115 mbsf.The decrease at -109 mbsf is marked by a 5% porosityincrease and sharply reduced density values. The decrease at-115 mbsf is clearly shown by increased shear strengthvalues, indicating enhanced silica preservation combined witherosion and/or dissolution from corrosive water masses flow-ing over the site.

Hiatuses

Four hiatuses in the sedimentary record were recognized onthe basis of shipboard biostratigraphy: (1) 1.9-2.2 Ma, 6.3 mbsf;(2) 4.7-5.3 Ma, -40.1 mbsf; (3) 6.2-9.5 Ma, 54.5 mbsf; and (4)12.1-14.8 Ma, 109.5 mbsf. The updated diatom biostratigraphyof Harwood and Maruyama (this volume) has been incorporatedinto this study to provide improved age control, and additional

189


160 -

180

B

0

20

40

60

£ 80

.πo.Q 100

120

140

160

180

0 Carbonate

| Noncarbonate

10 15 20 25 30 10 15 20 25

Bulk accumulation of sediment (g/m /k.y.) Accumulation rate (g/m /k.y.)

Figure 11. A. Plot illustrating the bulk accumulation of sediment vs. depth for Site 751. The increase in accumulation rate from —10 to —20 near55 mbsf is associated with a hiatus at ~8.8 Ma. B. Plot illustrating the absolute accumulation of carbonate and noncarbonate (assumed to be opal)sediment vs. depth for Site 751. Increases in noncarbonate accumulation often are associated with large decreases in carbonate accumulationrates. Carbonate accumulation above 40.1 mbsf is severely restricted.

hiatuses have been recognized (see Table 3). We have evaluatedthe geotechnical stratigraphy at Site 751 to determine the effectof each hiatus on physical properties changes (e.g., was thehiatus erosional or nondepositional). Of the hiatuses identifiedduring the cruise, the lower two appear to be erosional, withincreased bulk density and decreased porosity directly leadingup to the unconformity, with reversed trends above. Theseobservations may also reflect increased productivity or preser-vation of siliceous microfossils during episodes of increasedcarbonate dissolution. The upper two events are linked to sharplithologic discontinuities, which makes them difficult to identifyconclusively as nondepositional or erosional. However, thediscontinuity at 40.1 mbsf, associated with a 2.5-m.y. hiatus(8.4-5.9 Ma) is most likely an erosional event, marking thepassage of the Polar Front across the Site 751 location andintensification of the ACC system.

Highlights of Geotechnical Stratigraphic Analysis

The following are the most significant findings of ourgeotechnical stratigraphic analysis:

1. Subunit G-IIi (Figs. 13 and 14) is identified by a narrowzone of fairly constant, high bulk-density, low porosity, and

high carbonate values, associated with low shear strengths.Subunit G-IIj (—125-128 mbsf) is associated with a gradual10% porosity decrease, an increase in bulk density, and lowaverage carbonate content. The base of Subunit G-IIj (—128mbsf) is marked by a sharp decline in carbonate content,reversals in density trends, and a porosity peak. Harwood andMaruyama (this volume) suggest a hiatus near 128-131 mbsf(see Table 3). This event may be represented by smallreversals in index properties and a large change in carbonatecontent at —131 mbsf. Below Subunit G-IIj, there are reducedamplitude changes (5%-7%) in porosity and wet- and dry-bulkdensity for the remainder of the section.

2. Subunit G-IIh (Table 2 and Figs. 13 and 14) is partic-ularly interesting since it exhibits large changes in carbonatecontents and index properties over a — 10-m interval of Hole751 A. The significant changes in porosity, bulk density, andgrain density within narrow zones are superimposed on anoverall, gradual decrease in carbonate content from the baseof this unit to the middle of Subunit G-IIf. The highamplitude fluctuations of carbonate content allow us tocompare the effects of nondeposition, dilution of the carbon-ate flux by silica, or erosion during the early to middleMiocene. Each sharp carbonate decrease is marked by a

190


α>Q

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α.<D

Q

0 Carbonate

• Noncarbonate

200 400 600 800 1000 1200 1400

IRD accumulation rate (grains/cm /k.y.)

5 10 15 20

Absolute accumulation rates (g/m2/k.y.)

Figure 12. A. Plot illustrating the accumulation of ice-rafted debris (IRD) vs. depth (modified from Breza, this volume) for Site 751. Majorincreases in IRD flux occur between 40-27 mbsf and 6-10 mbsf. B. Plot illustrating the absolute accumulation of carbonate and noncarbonatesediment vs. depth for Site 751. Noncarbonate accumulation rates include biogenic silica and IRD accumulation.

sharp increase in porosity, generally reflecting increasedaccumulation of biogenic silica within that interval.

3. The top of Subunit G-IIh (-109.9 mbsf; Figs. 13 and 14)is marked by a sharp decrease in carbonate; porosity shows a10% increase, and bulk density decreases sharply at theboundary (before gradually increasing within Subunit G-IIg).Grain density values decrease from 2.76 to 2.35 g/cm3 acrossthis boundary. A hiatus from —13.3 to 14.1 Ma and an increasein shear strength are also associated with these changes.

4. Subunits G-IIg, G-IIf, and G-IIe are separated from eachother by sharp trend reversals and inflections in index prop-erties and carbonate content over the interval from —109 to90.0 mbsf (Figs. 13 and 14). These shifts are also seen inchanges of sediment bulk accumulation rates.

5. Subunit G-Hd and G-IIb exhibit similar physical proper-ties and carbonate contents (Fig. 15). The boundary betweenSubunits G-IIb and G-IIa is marked by a drastic change in bulksediment accumulation rate, a 20% decrease in carbonatecontent and inflections in index property trends. SubunitG-IIb is underconsolidated relative to Subunit G-IIa; probablyas a result of the increased accumulation of low permeabilitynannofossil ooze and the existence of a possible erosionalhiatus (at —8.8 Ma) across this boundary.

Table 3. Ages of hiatuses at Site 751.

Age (Ma)

0.8-1.61.8-3.0

4.1-4.3?4.4-5.6

5.9-8.4

-8.8?-10?12.1-12.813.3-14.1-14.2-14.515.1-16.5

Geologic age

PleistocenePleistocene/

late Plioceneearly Plioceneearly Pliocene/

late Miocenelate Miocene

late Miocenelate Miocenemiddle Miocenemiddle Miocenemiddle Miocenemiddle Miocenemiddle/early

Miocene

Depth(mbsf)

1.1-1.36.3

25.935.4

40.1

54.5-75-104.5109.5-115-120129-131

Evidence

D, M, L s s, Lc, P, CD, M, L s s, Lc, P, C, R

M, L s s, Lc, P, CD, M, L s s, Lc, P, C

D, M, L s s, Lc, P, C,R?L s s, Lc, P, C, FM, L s s, Lc, P, CD, M, L s s, Lc, P, CM, L s s, Lc, P, C, RL s s . Lc, P, CL s s, Lc, P, CD, M, L s s, Lc, P, C

Notes: Evidence for identification of hiatuses at Site 751 is as follows:D = diatom biostratigraphy (from Harwood and Maruyama, thisvolume); M = magnetostratigraphy; L s s = lithology (as determinedfrom smear slide analysis); Lc = lithology (as determined by visualcore description); P = physical properties data; C = calciumcarbonate content; F = planktonic foraminifer biostratigraphy; andR = radiolarian biostratigraphy.

191


100 -

110 -

nε

Q.

D

120 -

130 -

1400.0 0.5 1.0 1.5 2.0 2.5 3.0

Wet- and dry-bulk and grain density (g/cm3)

Figure 13. Plot of wet-bulk (filled oval), dry-bulk (open oval), andgrain density (open rectangle) vs. depth for Site 751; for the depthinterval between 90 and 140 mbsf. Geotechnical subunits are alsoshown.

Wei and Wise (this volume) present relative abundancedata for Reticulofenestra perplexa, which represents the coldwater end-member of Miocene calcareous nannofossil assem-blages. Positive shifts in the relative abundance of this speciesindicates decreasing water temperatures, whereas decreasingrelative abundance generally suggests warmer temperatures(see Wei and Wise, this volume). The relative abundancevariations of R. perplexa help to illustrate the differencesbetween Subunits G-IIf and G-IIa by graphically illustratingchanges in calcareous microfossil assemblages from —40 to100 mbsf (Fig. 15). Subunits G-IIe and G-IId, and SubunitG-IIb represent periods of high R. perplexa abundance; andtherefore decreased water temperatures. The decline in rela-tive abundance of R. perplexa in Subunit G-IIc is accompa-nied by generally increased noncarbonate (e.g., siliceous)accumulation from —80 to —60 mbsf (Fig. 16), and decreasedcarbonate accumulation. Colder water temperatures could beresponsible for decreased abundances of R. perplexa in partsof this interval because of the environmental stress placed onthe species by frigid Antarctic surface waters (Wise, personalcommunication). Subunit G-IIa is marked by decreased abun-dances of R. perplexa and decreased accumulation ratesbefore the change to lithologic Unit I.

We interpret the variations in physical properties observedat Site 751 to be the result of: (1) fluctuations in surface-waterproductivity that variably favored production of siliceous orcalcareous microplankton, and (2) variations in the nature ofthe water masses at the site that selectively favored preserva-

100 -

110 -

nE

Q.ΦQ

120 -

130 -

140

20 30 40 50 60 70 80 90 100

Porosity (%) vs. carbonate content (%)

Figure 14. Plot of porosity (open oval) and carbonate content (filledoval) vs. depth for Site 751; for the depth interval between 90 and 140mbsf. Geotechnical subunits are also shown.

tion of the siliceous or calcareous microfossils. These factorsare directly related to Oceanographic conditions controlled bythe position of the Polar Front and the intensity of theAntarctic Circumpolar Current system. The Kerguelen Pla-teau would probably have been most influenced by Antarcticintermediate water masses than by bottom water because ofits elevation above the deep basins of the southern IndianOcean. The site was certainly above the regional CCD duringthe Neogene; however, dissolution caused by corrosive watermasses appears to have been an important influence ondeposition.

CONCLUSIONS

Site 751 records the northward migration and strengtheningof the Antarctic Polar Front during the late Neogene. At ~ 110mbsf (13.3-14.1 Ma) there is an erosional hiatus that couldrepresent northward and southward pulses of the Polar Frontor increased erosion by the Antarctic Circumpolar Currentsystem. An erosional hiatus at 54.4 mbsf (—8.8 Ma) mayrepresent another expansion of the Polar Front. Although thisevent is not observed in the revised diatom biostratigraphy, itis accompanied by large changes in accumulation rates andinflections in all measured physical properties. The abruptchange from calcareous to siliceous sediments at 40.1 mbsf(the lithologic Unit I/II boundary) is marked by a hiatus of—2.5 m.y. (8.4-5.9 Ma; late Miocene, near Miocene/Pliocene

192


40 I 1—π 1 1 1 1

100 -

1100 0.2 0.4 0.6 0.8 1.0

Carbonate (%/100) vs. R. perplexa abundance (%)

Figure 15. Plot of decimal percent carbonate content (open oval) andpercent relative abundance of calcareous nannofossil species Reticu-lofenestra perplexa (filled oval) vs. depth for Site 751; for the depthinterval between 40 and 110 mbsf (Relative abundance data for R.perplexa taken from Wei and Wise, this volume). Geotechnicalsubunits are also shown.

boundary). This is earlier than the general increase in South-ern Ocean biogenic silica deposition at ~5 Ma correlated witha 300 km northward expansion of the Polar Front (Brewster,1980).

The early and middle Miocene sequence at Site 751 ischaracterized by high, relatively constant, carbonate accumu-lation with variable preservation in specific zones. The largechanges in percent calcium carbonate and the presence of twosignificant hiatuses within this interval (at —109 mbsf and— 131 mbsp argues for erosion by Circumpolar Deep Water orcorrosive intermediate waters in response to the strengtheningof circumpolar circulation patterns. It is difficult to rule outcarbonate productivity changes as being responsible for largechanges in carbonate content, but we feel that other mecha-nisms (e.g., erosion and dissolution) are more likely a result ofthe magnitude and apparent rapidity of many of the shifts. Thepresence of subtropical faunas, and the high percentage ofcarbonate observed in early to middle Miocene samples,indicates that the Polar Front was not established until the lateMiocene over the Kerguelen Plateau (as a high gradientoceanic barrier). The accumulation of siliceous microfossilswas nearly constant throughout this period, although some

100 -

1101 10 100Accumulation rates vs. abundance of R. perplexa

Figure 16. Logarithmic plot of absolute accumulation rates of carbon-ate (open rectangle) and noncarbonate (filled rectangle) in g/m2/k.y.and percent relative abundance of R. perplexa (filled oval) vs. depthfor Site 751; for the depth interval between 40 and 110 mbsf.Geotechnical subunits are also shown.

temporally restricted increases in siliceous accumulation arenoted in association with carbonate reductions. Microfabricinvestigations were used to aid in the interpretation of disso-lution events when possible.

The major change in facies from dominantly nannofossilooze to dominantly siliceous ooze at Site 751 at 40.1 mbsf isinterpreted to represent the establishment of a high-gradientpolar front north of this site in the latest Miocene. Thedecreased rates of carbonate accumulation and increasedsiliceous accumulation across this boundary reflect the cool-ing of the surface waters and strengthening of zonal circula-tion patterns over the plateau. A further decrease in bulkaccumulation in the late Pliocene corresponds with a secondnorthward movement of the Polar Front toward KerguelenIsland.

Fluctuations in physical properties of a high-latitude pe-lagic sedimentary sequence primarily records shifts in thedominance of siliceous and calcareous microfossil assem-blages with depth. These dominance shifts correspond in turnto Oceanographic and climatic changes controlled by bothregional and global forcing mechanisms. The northwardmovement of the Antarctic Polar Front and its associatedwater masses, in response to deteriorating climatic conditionsacross the Kerguelen Plateau during the late Miocene andearly Pliocene, is illustrated on the seafloor by a decreased

193


diversity of microfossil assemblages and an increased domi-nance of siliceous sedimentation.

The evolution and subsequent history of zonal sedimenta-tion patterns on the ocean floor of the southern Indian Oceanis assumed to reflect (1) the strengthening/weakening of theAntarctic Circumpolar Current system; (2) the expansion orcontraction of the geographic area influenced by Antarctic/Subantarctic surface waters and their associated faunas; (3)the changing production of intermediate and deep waters overtime; and (4) shifts in the distribution and volume of ice onAntarctica. Site 751 in the Raggatt Basin preserves the recordof these changes during the Neogene.

ACKNOWLEDGMENTSWe deeply appreciate the assistance of Lisa Donaghe of the

Texas A&M Electron Microscopy Center in conducting ourSEM analyses. The manuscript was greatly improved thanksto discussions with Jack Baldauf (ODP) and reviews by B.Charlotte Schreiber and R. H. Wilkens. This project receivedUSSAC funding under Grant No. 20241.

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Bachman, R. T., 1984. Intratest porosity in foraminifera. J. Sediment.Petrol., 54:257-262.

Barker, P. F., Kennett, J. P., et al., 1988. Proc. ODP, Init. Repts.,113: College Station, TX (Ocean Drilling Program).

, 1990. Proc. ODP, Sci. Results, 113: College Station, TX(Ocean Drilling Program).

Barron, J. A., and Baldauf, J. G., 1989. Tertiary cooling steps andpaleoproductivity as reflected by diatoms and biosiliceous sedi-ments. In Berger, W. H., Smetacek, V. S., and Wefer, G. (Eds.),Productivity of the Ocean: Present and Past. New York (Wiley-Interscience), 341-354.

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Date of initial receipt: 20 February 1990Date of acceptance: 1 November 1990Ms 120B-145

195


Appendix AAbsolute Accumulation Rates, Site 751.

Core, section,interval (top)

120-751A-

1H-1, 301H-1, 701H-1, 1151H-2, 301H-2, 481H-3, 302H-1, 1152H-2, 302H-2, 702H-2, 1152H-3, 302H-3, 702H-3, 1152H-4, 303H-l,203H-l,703H-1, 1153H-2, 303H-2, 703H-2, 1153H-3, 703H-3, 1153H-4, 303H-4, 703H-4, 1153H-5, 303H-5, 703H-5, 1154H-2, 1104H-3, 304H-3, 674H-3, 1134H-4, 304H-4, 674H-4, 1124H-5, 304H-5, 674H-5, 1134H-6, 304H-6, 704H-6, 1135H-1, 305H-1, 705H-1, 1155H-2, 305H-2, 705H-2, 1155H-3, 305H-3, 705H-3, 1155H-4, 305H-4, 705H-4, 1155H-5, 305H-5, 705H-5, 1155H-6, 306H-l,706H-1, 1156H-2, 306H-2, 706H-2, 1156H-3, 306H-3, 706H-3, 1156H-4, 306H-4, 706H-4, 1156H-5, 306H-5, 706H-5, 1156H-6, 306H-6, 706H-6, 115

Depth(mbsf)

0.300.701.151.801.983.305.856.506.907.358.008.408.859.50

14.4014.9015.3516.0016.4016.8517.9018.3519.0019.4019.8520.5020.9021.3526.3027.0027.3727.8328.5028.8729.3230.0030.3730.8331.5031.9032.3333.5033.9034.3535.0035.4035.8536.5036.9037.3538.0038.4038.8539.5039.9040.3541.0043.4043.8544.5044.9045.3546.0046.4046.8547.5047.9048.3549.0049.4049.8550.5050.9051.35

Age(m.y.)

0.000.100.200.450.500.901.902.202.402.802.842.902.933.103.353.383.403.443.603.633.683.733.803.823.853.893.913.934.184.214.224.244.254.274.294.324.334.354.394.414.434.474.494.514.524.544.554.574.594.614.634.654.674.694.705.305.375.595.635.695.735.775.815.855.895.935.965.996.036.076.106.136.166.19

CaCO3

(shore)

15.3411.%12.6814.8517.471.4662.6522.3830.7427.555.18

35.821.630.000.000.000.140.400.290.640.72

10.516.65

13.799.097.04

11.168.720.001.170.111.430.000.65

14.850.58

24.395.670.260.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00

72.4974.3567.9571.1359.9263.0278.0777.8771.5767.5976.0877.2976.8184.3470.9167.1879.5367.2171.07

CaCO3

(ship)

11.00

15.2071.30

31.20

35.20

1.40

0.70

0.40

0.80

13.10

11.20

0.10

0.20

0.30

23.70

0.20

0.20

0.10

0.10

0.10

0.10

74.5067.10

60.10

71.20

74.70

68.20

64.10

SiO2

19.247.3

63.8

69.173.476.1

66.472.1

58.6

69.167.7

63.753.369.061.2

62.1

64.7

64.5

51.3

61.9

45.4

13.5

13.3

12.7

12.98.0

19.0

11.2

LSR

3333333333

20202020202020202020202020202020202020202020202020202020202020202020202020202020202020202010101010101010101010101010101010101010

DBD

0.320.450.350.330.510.760.640.450.440.570.250.520.290.201.450.330.300.370.380.380.370.380.430.400.350.330.370.340.390.270.260.390.310.380.440.400.510.350.380.400.290.350.220.340.300.390.410.390.370.410.360.440.450.500.550.770.870.890.920.800.790.920.890.890.810.870.890.930.970.810.790.830.800.94

BulkAAR

0.961.351.050.991.532.281.921.351.321.715.00

10.405.804.00

6.606.007.407.607.607.407.608.608.007.006.607.406.807.805.405.207.806.207.608.808.00

10.207.007.608.005.807.004.406.806.007.808.207.807.408.207.208.809.00

10.0011.007.708.708.909.208.007.909.208.908.908.108.708.909.309.708.107.908.308.009.40

CaCO3AAR

0.1470.1610.1330.1470.2661.6291.2030.3020.4060.4710.2593.7250.0950.0000.0000.0000.0080.0300.0220.0490.0530.7990.5721.1030.6360.4650.8260.5930.0000.0630.0060.1120.0000.0491.3070.0462.4880.3970.0200.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0005.5826.4686.0486.5444.7944.9797.1826.9306.3705.4756.6196.8797.1438.1815.7445.3076.6015.3776.681

SiO2AAR

0.2590.624

6.635

2.764

5.023

4.9145.480

4.336

5.9435.416

4.2043.9444.6924.774

3.229

4.917

6.579

4.104

4.828

3.995

1.040

1.051

1.130

1.1220.7121.767

1.053

Non-CaCO3AAR

0.8131.1890.9170.8431.2640.6510.7171.0480.9141.2394.7416.6755.7054.000

6.6005.9927.3707.5787.5517.3476.8018.0286.8976.3646.1356.5746.2077.8005.3375.1947.6886.2007.5517.4937.9547.7126.6037.5808.0005.8007.0004.4006.8006.0007.8008.2007.8007.4008.2007.2008.8009.000

10.00011.0002.1182.2322.8522.6563.2062.9212.0181.9702.5302.6252.0812.0212.1571.5192.3562.5931.6992.6232.719

196


Appendix A (continued).Absolute Accumulation Rates, Site 751.


6H-7, 307H-2, 307H-2, 707H-2, 1157H-3, 307H-3, 707H-3, 1157H-4, 307H-4, 707H-4, 1157H-5, 307H-5, 707H-5, 1157H-6, 307H-6, 707H-6, 1157H-7, 308H-3, 308H-3, 708H-3, 1158H-4, 308H-4, 708H-4, 1158H-5, 308H-5, 708H-5, 1158H-6, 30S>H-1,309H-1, 709H-1, 1159H-2, 309H-2, 709H-2, 1159H-3, 309H-3, 709H-3, 1159H-4, 309H-4, 709H-4, 1159H-5, 309H-5, 709H-5, 1159H-6, 309H-6, 709H-6, 1159H-7, 309H-7, 70

10H-1, 7010H-1, 11510H-2, 3010H-2, 7010H-2, 11510H-3, 3010H-3, 7010H-3, 11510H-4, 3010H-4, 7010H-4, 11510H-5, 3010H-5, 7010H-5, 11510H-6, 3010H-6, 7010H-6, 11510H-7, 30HH-1,3011H-1,7O11H-1, 11511H-2, 3011H-2, 7011H-2, 11511H-3, 30HH-3,7011H-3, 11511H-4, 30

Depth(mbsf)

52.0054.0054.4054.8555.5055.9056.3557.0057.4057.8558.5058.9059.3560.0060.4060.8561.5065.0065.4065.8566.5066.9067.3568.0068.4068.8569.5071.5071.9072.3573.0073.4073.8574.5074.9075.3576.0076.4076.8577.5077.9078.3579.0079.4079.8580.5080.9081.4081.8582.5082.9083.3584.0084.4084.8585.5085.9086.3587.0087.4087.8588.5088.9089.3590.0090.5090.9091.3592.0092.4092.8593.5093.9094.3595.00

Age(m.y.)

6.226.256.289.509.539.569.599.629.649.679.709.739.759.779.799.819.84

10.0110.0310.0510.0710.0910.1110.1310.1510.1710.1910.2910.3110.3310.3510.3710.3910.4110.4310.4510.4810.5010.5210.5410.5610.5810.6010.6210.6410.6610.6810.7110.7310.7510.7710.7910.8210.8410.8610.8810.9010.9210.9510.9710.9910.0211.0411.0611.0811.1111.1311.1511.1711.1911.2111.2311.2511.2711.29

CaCO3

(shore)

67.2880.5880.7186.8986.2885.4990.8487.3183.1984.7382.7677.8184.6987.9188.0575.0678.7757.0270.9569.2966.2042.9256.4246.9542.9451.3550.5865.4169.2565.7062.2351.5054.0348.3956.6666.0964.0180.3182.1879.5376.3577.5967.5370.2571.6370.3071.1879.7982.5781.0880.3684.0183.5284.8383.7674.3273.9179.6881.3680.1883.9881.5979.3781.3583.5881.0580.3390.8188.3386.5886.7091.7288.3185.1083.25

CaCO3

(ship)

78.90

83.50

81.60

74.60

85.60

76.60

69.20

42.00

42.30

69.10

51.10

55.50

79.80

76.6076.0568.4769.4058.5668.8169.8080.0081.7280.5579.2082.8883.5584.6082.5574.0572.7078.4779.3080.1083.7282.2280.0581.5582.2280.8080.1089.4688.7286.0085.1392.1387.2085.5582.55

SiO2

12.920.0

LSR DBD

10101020202020202020202020202020202020202020202020202020202020202020202020202020 (202020 (20 (20 (2020 (20202020 (20 (20 (2020 (20 (20 (20 (20 (

3.823.913.913.973.993.981.063.973.963.973.913.873.941.051.133.823.803.703.763.743.743.593.703.643.633.753.673.683.753.753.723.653.683.643.703.813.803.993.923.903.883.883.803.813.81

3.843.973.923.943.883.893.91.04

3.983.843.813.913.95

20 0.8720 (20 (20 (20 (20 (20 (20 (2020202020

3.983.94).873.92).94).913.89.03.04.00.07.07

20 0.992020 (

.033.94

BulkAAR

8.209.109.10

19.4019.8019.6021.2019.4019.2019.4018.2017.4018.8021.0022.6016.4016.0014.0015.2014.8014.8011.8014.0012.8012.6015.0013.4013.6015.0015.0014.4013.0013.6012.8014.0016.2016.0019.8018.4018.0017.6017.6016.0016.2016.20

16.8019.4018.4018.8017.6017.8018.2020.8019.6016.8016.2018.2019.0017.4019.6018.8017.4018.4018.8018.2017.8020.6020.8020.0021.4021.4019.8020.6018.80

CaCO3

AAR

5.5177.3337.345

16.85717.08316.75619.25816.93815.97216.43815.06213.53915.92218.46119.89912.31012.6037.983

10.78410.2559.7985.0657.8996.0105.4107.7036.7788.896

10.3879.8558.9616.6957.3486.1947.932

10.70710.24215.90115.12114.31513.43813.65610.80511.38111.604

11.95815.47915.19315.24314.14314.95415.20117.64516.41712.48611.97314.50215.45813.95116.46015.33913.81014.96815.71314.75114.29918.70718.37317.31618.55419.62817.48517.53115.651

SiO2

AAR

2.3223.520

Non-CaCO3

AAR

2.6831.7671.7552.5432.7172.8441.9422.4623.2282.9623.1383.8612.8782.5392.7014.0903.3976.0174.4164.5455.0026.7356.1016.7907.1907.2976.6224.7044.6135.1455.4396.3056.2526.6066.0685.4935.7583.8993.2793.6854.1623.9445.1954.8204.596

4.8423.9213.2073.5573.4572.8462.9993.1553.1834.3144.2273.6983.5423.4493.1403.4613.5903.4323.0873.4493.5011.8932.4272.6842.8461.7722.3153.0693.149

197




11H-4, 7011H-4, 11511H-4, 13511H-5, 3011H-5, 7011H-5, 11511H-6, 3011H-6, 7011H-6, 11511H-7, 3012H-l,3012H-1, 7012H-1, 11512H-2, 3012H-2, 7012H-2, 11512H-3, 3012H-3, 7012H-3, 11512H-4, 3012H-4, 7012H-4, 11512H-5,3012H-5, 7012H-5, 11512H-6, 3012H-6, 7012H-6, 11512H-7, 3013H-1.7013H-1, 11513H-2, 3013H-2, 7013H-2, 11513H-3, 3013H-3, 7013H-3, 11513H-4, 3013H-4, 7013H-4, 11513H-5, 3013H-5, 7013H-6, 3014H-1, 13014H-2, 3014H-2, 7014H-2, 11514H-3, 3014H-3, 7014H-3, 13014H-4, 3014H-4, 7014H-4, 11514H-5, 3014H-5, 7014H-5, 13014H-6, 3014H-6, 7014H-7, 3014H-7, 7015H-l,3015H-l,7015H-1, 11515H-2, 3015H-2, 8715H-2, 11515H-3, 3015H-3, 7015H-3, 11515H-4, 3015H-4, 7015H-4, 11515H-4, 3015H-5, 7015H-5, 115

Depth(mbsf)

95.4095.8596.0596.5096.9097.3598.0098.4098.8599.50

100.00100.40100.85101.50101.90102.35103.00103.40103.85104.50104.90105.35106.00106.40106.85107.50107.90108.35109.00109.90110.35111.00111.40111.85112.50112.90113.35114.00114.40114.85115.50115.90117.00120.00120.50120.90121.35122.00122.40123.00123.50123.90124.35125.00125.40126.00126.50126.90128.00128.40128.50128.90129.35130.00130.57130.85131.50131.90132.35133.00133.40133.85134.50134.90135.35

Age(m.y.)

11.3111.3311.3511.3711.3911.4111.4411.4711.5011.5311.5611.5911.6211.6511.6811.7111.7411.7711.8011.8311.8611.8911.9211.9511.9712.0012.0312.0612.1014.8014.9515.1715.3215.4715.7015.8516.0216.2216.2516.2816.3216.3516.4116.5616.5916.6216.6516.6916.7216.7616.7916.8216.8516.8916.9216.9516.9817.0117.0617.0817.0817.0917.1217.1517.1717.2017.2417.2717.3017.3517.3817.4117.4517.4817.51

CaCO3

(shore)

78.0669.1359.1772.5082.3884.8270.5072.5351.2253.6659.9442.9051.6645.8646.1256.2362.4867.4360.0470.8666.0771.0463.7869.9760.4755.2159.1049.8025.9074.9674.0978.4859.7379.8787.8384.3981.8274.5145.5124.2587.9592.1291.4846.2091.64

90.4992.7086.3690.9594.4888.1284.2781.8572.4351.6569.43

47.5362.0367.56

74.4681.9172.1970.8571.8874.0677.7785.0684.7688.9986.8485.3588.97

CaCO3

(ship)

77.5069.1461.8972.0580.2083.6370.6472.3050.7554.2261.0642.9051.4047.7346.7057.8162.5667.3060.8970.9065.7071.0663.9769.6059.4856.6458.6049.1526.1674.6073.8977.2258.5079.3087.1383.6080.6374.9745.3024.9088.0591.1092.3044.2392.05

89.6391.9683.6090.2193.7187.5084.8876.8970.9050.7365.64

48.1561.2067.31

74.0581.9771.3090.7161.6474.1075.9793.5584.60

84.90

SiO2 LSR DBD

20202020202020202020202020202020202020

10.2 2013.7 20

202020202020202020202020202020202020 (20 (20202020 (20202020202020202020

0.933.840.643.773.913.953.783.843.793.713.803.643.693.653.643.683.803.853.813.933.983.98).951.01).84).83).89).793.641.02).981.05).801.04.21.19.15.11

).71).43.20.12.20

).59.10.12.11.16.05.12.16.06.05.09

20 0.9620 0.9120 0.8420 0.8320 0.7220 0.8520 0.8520 0.9020 0.9020 120 1

.00

.0320 0.9620 0.9320 0.94

3.5 20 0.975.4 20 0.98

20 120 1

.00

.1220 0.9920 0.9720 1 .03

BulkAAR

18.6016.8012.8015.4018.2019.0015.6016.8015.8014.2016.0012.8013.8013.0012.8013.6016.0017.0016.2018.6019.6019.6019.0020.2016.8016.6017.8015.8012.8020.4019.6021.0016.0020.8024.2023.8023.0022.2014.208.60

24.0022.4024.0011.8022.0022.4022.2023.2021.0022.4023.2021.2021.0021.8019.2018.2016.8016.6014.4017.0017.0018.0018.0020.0020.6019.2018.6018.8019.4019.6020.0022.4019.8019.4020.60

CaCO3 SiO2

AAR AAR

14.51911.6147.574

11.16514.99316.11610.99812.1858.0937.6209.5905.4917.1295.9625.9037.6479.997

11.4639.726

13.180 1.89712.950 2.68513.92412.11814.13410.1599.165

10.5207.8683.315

15.29214.52216.4819.557

16.61321.25520.08518.81916.5416.4622.085

21.10820.63521.9555.452

20.161

20.08921.50618.13620.37321.91918.68117.69717.84313.9079.400

11.664

6.84410.54511.485

13.40316.38214.87113.60313.37013.92315.087 0.67916.672 1.05816.95219.93417.19416.55818.328

Non-CaCO3

AAR

4.0815.1865.2264.2353.2072.8844.6024.6157.7076.5806.4107.3096.6717.0386.8975.9536.0035.5376.4745.4206.6505.6766.8826.0666.6417.4357.2807.9329.4855.1085.0784.5196.4434.1872.9453.7154.1815.6597.7386.5142.8921.7652.0456.3481.839

2.1111.6942.8642.0271.2812.5193.3033.9575.2938.8005.136

7.5566.4555.515

4.5973.6185.7295.5975.2304.8774.3132.9283.0482.4662.6062.8422.272

198




15H-6, 3015H-6, 7015H-6, 11516H-1, 3016H-1, 7016H-1, 11516H-2, 3016H-2, 7016H-2, 11516H-3, 3016H-3, 7016H-3, 11516H-4, 3016H-4, 7016H-4, 11516H-5, 3016H-5, 7016H-5, 11516H-6, 3016H-6, 7016H-6, 11516H-7, 3016H-7, 7017H-l,3017H-2, 3017H-2, 7017H-2, 11517H-3, 3017H-3, 7017H-3, 11517H-4, 11517H-5, 3017H-5, 7017H-6, 3017H-6, 7017H-6, 11517H-7, 3018H-1, 3018H-1, 7018H-1, 11518H-2, 3018H-2, 7018H-2, 11518H-3, 3018H-3, 7018H-3, 11518H-4, 3018H-4, 7018H-4, 11518H-5, 3018H-5, 7018H-5, 11518H-6, 30

Depth(mbsf)

136.00136.40136.85138.00138.40138.85139.50139.90140.35141.00141.40141.85142.50142.90143.35144.00144.40144.85145.50145.90146.35147.00147.40147.50149.00149.40149.85150.50150.90151.35152.85153.50153.90155.00155.40155.85156.50157.00157.40157.85158.50158.90159.35160.00160.40160.85161.50161.90162.35163.00163.40163.85164.50

Age(m.y.)

17.5517.5817.6117.6917.7217.7517.7917.8217.8517.8917.9217.9517.9918.0318.0618.1018.1318.1618.2018.2318.2618.3018.3318.3418.4218.4518.4818.5218.5618.5918.6718.7118.7418.8118.8418.8718.9118.9518.9819.0119.0419.0719.1019.1319.1619.1919.2219.2519.2819.3219.3519.3819.50

CaCO3

(shore)

88.6786.1389.3087.6387.1485.7284.5386.7790.3991.1192.4490.6992.6190.9991.7892.3491.1591.7287.5988.4488.1088.3990.0490.5791.2989.9788.3482.9984.2487.5182.0287.9189.8476.0282.2266.7579.5877.1279.8883.5678.4384.3782.4087.4793.1878.3884.6380.2886.1988.4570.2585.7675.92

CaCO3

(ship)

85.40

86.00

85.60

91.80

90.20

91.00

87.60

88.10

88.40

80.90

78.10

84.10

92.90

80.90

SiO2 LSR

202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020

7.3 206.9 20

202020202020

DBD

:

.08

.01

.031.01.00

0.951.010.93

:

::

:

.05

.05

.07

.07

.08

.07

.11

.10

.00

.08

.07

.05

.09

.04

.06

.06

.091.031.060.930.980.960.861.001.030.900.940.950.990.920.900.900.900.930.990.991.080.851.000.991.021.010.771.010.88

BulkAAR

21.6020.2020.6020.2020.0019.0020.2018.6021.0021.0021.4021.4021.6021.4022.2022.0020.0021.6021.4021.0021.8020.8021.2021.2021.8020.6021.2018.6019.6019.2017.2020.0020.6018.0018.8019.0019.8018.4018.0018.0018.0018.6019.8019.8021.6017.0020.0019.8020.4020.2015.4020.2017.60

CaCO3AAR

19.15317.39818.39617.70117.42816.28717.07516.13918.98219.13319.78219.40820.00419.47220.37520.31518.23019.81218.74418.57219.20618.38519.08819.20119.90118.53418.72815.43616.51116.80214.10717.58218.50713.68415.45712.68215.75714.19014.37815.04114.11715.69316.31517.31920.12713.32516.92615.89517.58317.86710.81917.32413.362

SiO2 Non-CaCO3

AAR AAR

2.4472.8022.2042.4992.5722.7133.1252.4612.018

.867

.618

.992

.596

.928

.825

.685

.770

.7882.6562.4282.5942.4152.1121.9991.8992.0662.4723.1643.0892.3983.0932.4182.0934.3163.3436.3184.0434.2103.6222.9593.8832.9073.4852.4811.473

1.241 3.6751.380 3.074

3.9052.8172.3334.5812.8764.238

Notes: The table given above contains the data used to calculate the accumulation rates for this study. Interval (top) = interval(cm) from top of core section; Depth (mbsf) - sub-bottom depth; Age (m.y.) = age from shipboard biostratigraphy (in Ma);CaCO3 (shore) = shore-based measurement of percentage CaCO3; CaCO3 (ship) = shipboard measurement of percentageCaCO3; SiC>2 = wt% biogenic silica (determined by D. DeMaster, NCSU); LSR = linear sedimentation rate (from shipboardbiostratigraphy); DBD = dry-bulk density (in g/cm3); Bulk AAR = bulk absolute accumulation rate (in g/m2/k.y. org/mm2/k.y.); CaCO3 AAR = absolute accumulation rate of carbonate (same units as Bulk AAR); SiO2 AAR = absoluteaccumulation rate of biogenic silica (same units as Bulk AAR); Non-CaCO3 AAR = noncarbonate absolute accumulation rate(same units as Bulk AAR).

199


APPENDIX B

Microfabric Analysis of SEM Samples1. Sample 120-751A-13H-6, 30-32 cm (117.0 mbsf)

Age: middle Miocene (14-15 Ma)Microfabric: The SEM revealed this sample to be characterized by

well-preserved, intact, scattered foraminifers surrounded by a matrixof intact nannofossils and disassociated nannofossil crystallites. Onlya few severely corroded radiolarian fragments are present and com-pletely filled with the matrix material. We observed a minimum ofopen pore space, as interparticle space was filled, and pores onmicrofossil tests, where present, were closed or filled.

2. Sample 120-751A-12H-4, 115-117 cm (105.4 mbsf)Age: middle Miocene (12-13 Ma)Microfabric: The SEM revealed this sample to be similar to the

underlying sample, different only in that siliceous microfossils areslightly more abundant and extremely corroded; and the nannofossilsare somewhat corroded and disassociated into crystallites. Only avery few diatom fragments are present in the matrix of predominantlynannofossils and nannofossil fragments. Radiolarians, where present,are completely filled with matrix material, and the pores of radiolarianand diatom tests are filled as well. Few large skeletons or skeletalfragments are present in this sample.

3. Samples 120-751A-8H-5, 70-72 cm (68.4 mbsf), and -8H-5, 30-32cm (68.0 mbsf)Age: late Miocene (9-10 Ma)Microfabric: The SEM showed these samples to be similar, character-

ized by concentrations of robust centric diatom valves, heavily silicifieddiatom girdle bands, and the heavily silicified central part of the valve. Someelongate pennate diatoms are present. The larger skeletal elements are incontact with one another, and in general there appears to be comparativelylittle matrix material. Where present, the matrix consists of diatom andnannofossil fragments; few intact nannofossils are present. Both silica andcalcite appear to be corroded. Overgrowths are present on some siliceousskeletons, and pores on some of the siliceous skeletons are filled, althoughpore space is generally open on the robust diatom valves. Some diatomvalves and girdle bands appear to be articulated, and some girdle bandsappear to be nested. The centric diatom valves show an indication ofpreferred orientation, and many appear to be lying flat when viewed fromabove. Some of the centric valves have become broken into smallerfragments, possibly as a result of dissolution.

4. Samples 120-751A-6H-3, 70-72 cm (46.4 mbsf) and -6H-1, 70-72cm (43.4 mbsf)Age: late Miocene (5-6 Ma)Microfabric: SEM analysis showed the siliceous material in these

samples to be highly corroded; few pennate diatoms are present,although the heavily silicified elements of pennate tests are apparentlyconcentrated. Some articulated robust centric diatoms and girdlebands are present. Pore spaces generally are filled with matrixmaterial, which consists of nannofossils (intact and disassociatedcrystallites) and centric and pennate diatom fragments.

5. Sample 120-751A-5H-5, 70-72 cm (39.9 mbsf)Age: early Pliocene (to late Miocene?) (4.5-6 Ma)Microfabric: The SEM revealed that this sample was different from

all the others in that the siliceous skeletal remains were pervasivelycoated; EDAX analysis indicated that the coating material is entirelysiliceous (? opal-A') Few elongate pennate diatoms are present; largevalves are scattered in the matrix of siliceous fragments. Pores onsiliceous skeletons are generally filled by the matrix and/or coatingmaterial.

6. Samples 120-751A-4H-6, 30-32 cm (31.5 mbsf) and -4H-4, 67-69cm (28.9 mbsf)Age: early Pliocene (4-5 Ma)Microfabric: The SEM showed these samples to be composed

predominantly of elongate, essentially intact pennate diatoms; al-though there is evidence of slight dissolution, the surfaces of thesetests are comparatively pristine. In places the preferred orientation ofthe long axes of these tests is apparent. There are isolated instances ofactual laminae of clustered, oriented elongate pennate diatom valves.The matrix is composed of fragments of diatoms; some centric valvesappear to be disassociating into smaller fragments in place. Radiolar-ians appear to be clustered in the diatom debris; nassellarian radio-larian tests, open at one end, are generally filled with matrix materialfiner grained than that surrounding the test. Large skeletal fragmentsare scattered in the matrix and are not generally in contact (evenwhere clustered, as with the radiolarians). Some centric diatoms havetwo valves and the girdle band articulated. Where present, foramini-fers are corroded. The lower of these two samples is distinguishedfrom the upper by some surface corrosion of siliceous material, somebreakage of delicate skeletal elements (e.g., spines), more abundantsilicoflagellates, and shorter pennate valves (although some elongatevalves are present).

200


V

/

•A

1

- J V *5K M K 3 S β

.. . - ; 'j

5 0 µ m 4 4 6 7 1 β

Plate 1. Scanning electron photomicrographs of Sample 120-751A-4H-4, 67-69 cm (Unit I). This sample represents the most highly biosiliceousinterval of Hole 751A (as does Sample 120-751A-4H-6, 30-32 cm, shown in Plate 2). 1. Centric diatom valve (c) and radiolarian (r), along withlarge, elongate pennate diatom valves (p) supported by matrix of pennate diatom fragments. 2. Slightly corroded nassellarian radiolarian skeleton(center) in a matrix predominantly composed of pennate diatom fragments; note that material inside radiolarian is finer than the surroundingmatrix. 3. Articulated centric diatom valves (center) and girdle band. 4. Spumellarian radiolarians (left-hand side); pennate diatom valves(right-hand side) appear to show preferred orientation along the vertical axis of the photo (= bedding direction?). 5, 6. Enlarged detail of alignedpennate diatom valves shown in Figure 4.

201


et ~

\ , ! . . _ . - . : . - -

- • •

. : ; : •

XT*. 5 0 0

3 6Plate 2. Scanning electron photomicrographs of Sample 120-751A-4H-6, 30-32 cm (Unit I). This sample (along with Sample 120-751A-4H-4,67-69 cm, shown in Plate 1) represents the most highly biosiliceous interval of Hole 751A. 1. Siliceous ooze consisting primarily of pennatediatom fragments; note preferred orientation suggested by alignment of centric diatom valves (c) and spongodiscid radiolarians (r) at upper left.2. Centric diatom valve and girdle band (center) in contact with broken silicoflagellate (s). 3. Detail of Figure 2, showing pore structure and fairlywell preserved silica. 4. Articulated centric diatom valve (c) and nassellarian radiolarian (r) in matrix of pennate diatom fragments andsilicoflagellates. 5. Centric diatom valves (c) in matrix of pennate diatom fragments and silicoflagellates. 6. Detail of a centric diatom valveshowing silica to be slightly corroded (i.e., surface layer removed?); pore spaces are mostly open.

202


.. i:

' ;t ' ; x **• >.I Θ K M X T " , S Θ Θ

* #

1 M• m

* • . . .

s>37003

4 j

- ' s " •• •

i f ; , - . 1 • •• J •

*

3 6Plate 3. 1-3. Scanning electron photomicrographs of Sample 120-751 A-5H-5, 70-72 cm (Unit I), -20 cm above the major hiatus at 40.1 mbsf.(1) Siliceous ooze composed of centric diatom valves and pennate diatom fragments; note authigenic overgrowths and closure of pore spaces.(2) Centric diatom valve (c) surrounded by matrix of siliceous fragments; note that some pores are filled with siliceous debris. (3) Detail of centricdiatom in Figure 2 showing authigenic overgrowths; note that some pore ornamentation is unobscured. 4-6. Sample 120-751A-6H-1, 70-72 cm,mixed siliceous-calcareous ooze, comparatively more calcareous (Unit II). (4) Centric diatom valve (center) with girdle band in growth position(although broken) surrounded by a matrix of nannofossils and nannofossil crystallites and pennate diatom fragments; note that pores of diatomare largely blocked by matrix elements. (5) Detail of matrix; note that siliceous elements are typically corroded. (6) Further detail of matrix inFigure 5.

203


•

. - . • <

' - ; . -. \

1SKM X3S® S 5 7 & & <5

Plate 4. 1-3. Scanning electron photomicrographs of Sample 120-751A-8H-5, 30-32 cm, mixed siüceous-calcareous ooze, comparatively moresiliceous (Unit II). (1) Large skeletal elements, which typically are in contact rather than being supported by the matrix, include centric diatoms(c), centric diatom girdle bands (g), and pennate diatoms; the matrix consists of centric and pennate diatom fragments and nannofossüs. (2) Detaüof area immediately to the left of center in Figure 1; note good preservation of nannofossüs (n) and corrosion of siliceous fragments); note alsodetrital (?) mineral grain (m). (3) Robust, heavüy süicified diatom valves and girdle bands showing evidence of corrosion; note specimen to upperleft of center (arrow) reduced to the ornamented part of the valve; silicoflagellates and nannofossüs are present in the matrix. 4-6. Sample120-751A-8H-5, 70-72 cm, mixed siliceous-calcareous ooze, comparatively more siliceous (Unit II). (4) Robust centric diatom valves, havingpossible preferred orientation (flat lying in plan view). (5) Heavily süicified centric diatom valves showing evidence of corrosion. (6) Detail ofmatrix, which consists of nannofossü and diatom fragments; note centric diatom valve disassociating into fragments (arrow).

204


; . T *?

. • . . . - • .

Plate 5. 1-3. Scanning electron photomicrographs of Sample 120-751A-12H-4, 117-119 cm, mixed siliceous-calcareous ooze, comparatively morecalcareous (Unit II). (1) Matrix of nannofossils and nannofossil crystallites surrounding larger skeletal elements, such as the corrodedspumellarian radiolarian to the upper left (r). (2) Detail of radiolarian in Figure 1; note that the test is broken and partially filled with matrixmaterial. (3) Detail of matrix. 4-6. Sample 120-751A-13H-6, 30-32 cm, mixed siliceous-calcareous ooze, comparatively more calcareous (UnitII). (4) Matrix of nannofossils and nannofossil crystallites surrounding larger skeletal elements, such as the foraminifers shown here. (5) Noteradiolarian skeleton (r) almost completely filled with matrix material. (6) Detail of matrix.

205

13. sediment microfabric and physical properties record of late

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