25. QUARTZ CONTENT OF NORTHWEST PACIFIC HOLE 576A AND IMPLICATIONS FORCENOZOIC EOLIAN TRANSPORT1

Margaret Leinen, University of Rhode Island2

ABSTRACT

The amount and the accumulation rate of quartz were measured in 33 samples from Hole 576A. The amount andsource of mineral aerosol being deposited in the northwest Pacific during the Cenozoic are evaluated using these data.When Hole 576A is compared to a Cenozoic record in the central North Pacific, a strong uniformity in the compositionof the mineral aerosol across the North Pacific is seen. The data suggest that Hole 576A entered the influence of thewesterlies about 15 m.y. ago and that since that time the rates of sediment deposition have increased. Only the dramaticchange in quartz accumulation 2.5 m.y. ago can be clearly related to a climatic event, but a gradual increase in quartzaccumulation through the Miocene and early Pliocene is probably a result of increasing Northern Hemisphere aridityand intensified atmospheric activity associated with global cooling during the interval.

INTRODUCTION

Since quartz does not precipitate authigenically in thedeep sea, variations in its accumulation rate in unlithi-fied oceanic sediments must result from changes in de-trital sedimentation. At sites remote from land and pro-tected from turbidite deposition, the quartz content ofsediments reflects eolian transport of terrigenous miner-al aerosol to the oceans (Rex and Goldberg, 1958; Rexet al., 1969; Windom, 1969, 1975). The distribution ofquartz in surface sediments mirrors present-day atmo-spheric circulation patterns (Heath et al., 1973; Kollaand Biscaye, 1977; Molina-Cruz and Price, 1977; Thiede,1979), and its variation in Pleistocene sediments has beenshown to reflect changes in wind intensity, wind-belt po-sition, and continental weathering in areas removed fromthe influence of continental runoff and transport downthe continental slope in areas closer to shore (Kolla andBiscaye, 1977; Pisias and Leinen, 1983; Schramm, 1983).

Only one long, continuous, and unequivocally eolianpre-Pleistocene record of deep-sea quartz content hasbeen available previously to study Cenozoic eolian depo-sition in the Pacific Ocean (Core LL44-GPC3, Leinenand Heath, 1981). Without others it has been difficultto evaluate the relative importance of changing rates ofeolian input and changing composition of mineral aero-sol from changes in deposition due to migration of thesite into and out of wind belts (caused by rotation of thePacific plate).

By examining the long, continuous red-clay sedimentrecord from Hole 576A, we can constrain the previousmodels of Cenozoic eolian deposition in the NorthPacific. Site 576 is presently located at 32°21.4'N,164° 16.5'E, beneath the core of the Northern Hemi-sphere westerlies (Fig. 1). About 53 m of pelagic clayoverlying about 13 m of interbedded Late Cretaceouscarbonate turbidites and pelagic clay were recovered from

Heath, G. R., Burckle, L. H., et al., Init. Repts. DSDP, 86: Washington (U.S. Govt.Printing Office).

2 Address: Graduate School of Oceanography, University of Rhode Island, Narragan-sett, RI 02882.

Hole 576A. A companion hole, Hole 576, terminatedon chert at 76 m. The uppermost clays are yellowishbrown and slightly silty. Below about 12 m the clays aredark brown, finer grained, and somewhat "slick" whencut. The lithologic column from Hole 576A is very simi-lar to, but thicker than, that recovered in a long pistoncore, Core LL44-GPC3, from the central North Pacific.Thus, the quartz content of Hole 576A can be used tostudy Cenozoic eolian deposition in the western NorthPacific.

METHODS

The quartz content of 33 samples from Hole 576A was measuredusing a modification of the techniques of Till and Spears (1969) andEllis and Moore (1973). Freeze-dried samples were leached with aceticacid buffered to pH 5.6 with sodium acetate to remove calcium car-bonate. This procedure and subsequent washing of the sample in de-ionized water also removed sea salt from interstitial pore water in thesample. An amount of alpha A12O3 equal to one-half the sample weightwas added to each sample as an internal standard. The samples wereground for 2 hr. in an automatic mortar and pestle to achieve a uni-form grain size, dried, and fired at 1000°C in alumina crucibles for 24hr. to eliminate interference caused by clay minerals. The samples weresubsequently ground briefly in alumina mortars under acetone and thedried, powdered samples were pressed into duplicate X-ray planchettesusing 40-mesh granulated zinc as a backing medium. The samples were X-rayed on a Phillips diffractometer using Ni-filtered, monochromatizedCuK alpha radiation at 45 kV and 35 raA. Each sample was scannedfrom 25.11° to 25.94° 20 at 0.004° 20/s (200 s) over the 3.48-Å (012)alumina peak and from 26.17° to 27.00° 20 at the same rate and forthe same time interval over the 3.34-Å (101) quartz peak. To correctthe X-ray peaks for background, 200-s background counts were madeat 25°, 26°, and 27° 20. The total X-ray counts over the 200-s intervalswere collected digitally by an interfaced microprocessor and used tocalculate peak areas of alumina and quartz. The ratio of quartz/alu-mina peak areas was converted to weight percent quartz in the sampleby reference to a calibration curve determined by standard additionsof quartz to a central North Pacific red clay from 2500 to 2550 cmdepth in Core LL44-GPC3 that has been homogenized and is used asa standard for geochemical and mineralogical analyses in our laboratory.

RESULTS AND DISCUSSION

Quartz Content of Hole 576A SamplesThe quartz contents of Hole 576A samples are listed

in Table 1. The average of the duplicate determinationsof quartz concentration is plotted versus depth in Fig-

581

M. LEINEN

60 N -

20'

3 0 S -

1 2 0 E 140° 160" 180° 160° 140° 120° 100° 80° W

Figure 1. Location of DSDP Hole 576A and Core LL44-GPC-3. DSDP Sites 305, 310, and 503 are alsoshown. The backtrack paths along which the sites traveled in response to seafloor spreading are alsoshown. Dots along these paths correspond to 5-m.y. time intervals. 20°N latitude is the approximatepresent-day boundary between the Northern Hemisphere westerlies and northeast trade winds.

Table 1. Quartz content of Hole 576A samples.

Core-Section(interval in cm)

1-1, 21-251-3, 81-851-6, 80-832-3, 47-503-1, 47-503-3, 70-733-4, 117-1203-5, 127-1303-6, 110-1134-1, 50-534-2, 103-1064-3, 98-1014-4, 104-1074-5, 20-234-6, 60-635-1, 87-905-2, 40-435-3, 30-335-5, 137-1405-6, 130-1335-7, 25-286-1, 90-936-2, 47-506-3, 93-966-4, 11-146-5, 12-156-5, 127-1306-6, 17-206-7, 22-257-1, 137-1407-2, 22-257-3, 17-207-3, 35-38

Sub-bottomdepth (cm)

23383831

12191869219123892549268128213025*316933253391358138093911*4051445946014646476148695065*513252835399543955935759579359395946

1

16.9418.3915.5717.7913.058.46

11.209.07

14.0510.918.626.025.814.365.165.171.682.644.722.856.223.924.354.746.295.464.545.534.034.185.487.153.51

Quartz (wt.

2

16.3117.8215.4816.9113.069.27

12.118.97

15.2310.938.525.605.914.155.475.151.842.634.632.916.144.004.064.875.955.364.545.053.804.145.515.77

<Vo)

Average

16.6318.1115.5217.3513.068.87

11.669.02

14.6410.928.575.815.864.255.315.161.762.644.672.886.183.964.214.816.125.414.545.293.9!4.165.506.463.51

Note: All sub-bottom depths were calculated by G. R. Heathon the basis of correlation with Holes 576 and 576B.Samples with asterisks are from intervals that show someevidence of flow-in.

ure 2. The sample spacing in the upper portion of thecore was intentionally larger than that in the middle andlower portion of the core because it was anticipated thatsedimentation rates for the upper (late Neogene) portionof the sedimentary record would be much greater thanthose for earlier time intervals.

Comparison of Hole 576A and Core LL44-GPC3Quartz Content

Also plotted for comparison on Figure 2 is the quartzcontent of samples from Core LL44-GPC3, the centralNorth Pacific (30°19.9'N, 157°49.9'W) red clay core thathas been used by Leinen and Heath (1981) to infer Ce-nozoic eolian deposition. The age at the bottom of CoreLL44-GPC3 is roughly equivalent to that at the base ofHole 576A.

The stratigraphy for Core LL44-GPC3 has been re-vised and additional quartz-content determinations havebeen made since it was first published. The original stra-tigraphy was based on paleomagnetic reversal stratigra-phy in the upper 4 m (2.47 m.y. ago) of the core (Princeet al., 1980) and ichthyolith stratigraphy for the remain-ing 20 m of the core (Doyle and Riedel, 1979). Addition-al ichthyolith age determinations (Doyle, 1980) and iden-tification of an Ir anomaly associated with the Creta-ceous/Tertiary boundary (Kyte and Wasson, in press) haveresulted in a new stratigraphy that is discussed in detailin Leinen and Heath (in press) and is used in this chap-ter. The stratigraphy for Hole 576A is based on correla-tion with that of Holes 576 and 576B. Holes 576 and576B were dated using paleomagnetic reversal stratigra-phy (Heath et al., 1983; Heath, Rea, and Levi, this vol-ume), ichthyolith stratigraphy (Doyle, 1983; Doyle and

582

CENOZOIC EOLIAN TRANSPORT

Quartz content(wt. %)

5 10 15 20 25

12

18

-p 24

30

Q 36

42

48

54

60 •-

Quartz content(wt. %)

5 10 15 20 25

10

15

20

25 L

Quartz content

(wt. %)

5 10 15 20 25 0

Quartz content(wt. %)

5 10 15 20 25

Figure 2. Quartz contents of (A) Hole 576A and (B) Core LL44-GPC-3in weight percent plotted versus depth in core. (All data are calci-um carbonate and biogenic opal-free values.) Because of differ-ences in sedimentation rates, the plots are essentially age equiva-lent (i.e., bottom of both cores are approximately time equivalent).

Riedel, this volume), and nannofossil stratigraphy for theinterbedded pelagic clay and carbonate turbidite section.

The remarkable similarity of the two North Pacificquartz records is less striking when they are plotted ver-sus age rather than depth in the core (Fig. 3). Plottedversus age, the absolute amount of quartz in the sedi-ments and the time series of quartz variation are verysimilar for all but the Miocene and early Pliocene (2.5-22 m.y. ago) portions of the record. A major differencein the stratigraphies of the two sediment columns is thelack of a stable magnetic signal for paleomagnetic rever-sal stratigraphy below about 4 m (2.47 m.y. ago) in CoreLL44-GPC3. This has been attributed to a decrease inthe ratio of stable detrital magnetic minerals to unstableauthigenic iron oxide and oxyhydroxides (Prince et al.,1980; HeatH et al., 1983; Heath, Rea, and Levi, this vol-ume). Thus, although high-quality magnetic-reversal stra-tigraphy is available back to the Miocene in Holes 576and 576B, it ends in the early Pliocene in Core LL44-GPC3. This factor is important in evaluating the differ-ences between the temporal quartz record at the twosites during the Miocene and Pliocene epochs. Ichthyo-lith age determinations for Holes 576 and 576B are quitedifferent from paleomagnetic ages in the Miocene sedi-ments (Doyle, 1983; Doyle and Riedel, this volume). Toevaluate the effect of this difference on the quartz curvesin Figure 3, a second temporal quartz record is plotted(dotted line) based on paleomagnetic stratigraphy to 2.47m.y. ago (Heath, Rea, and Levi, this volume) and onichthyolith ages for the rest of the hole. Obviously, thesuperior paleomagnetic data indicate this alternate stra-

10-

i2 0 H 1

30 -*

J. 40-

<50-

6 0 -

7 0 -

80

Figure 3. Quartz content of (A) Hole 576A and (B) Core LL44-GPC-3in weight percent plotted versus age. All data are calcium carbon-ate and biogenic opal-free values. Dotted line on the curve forHole 576A indicates age determinations for the Miocene Epochbased solely on ichthyolith stratigraphy.

tigraphy is wrong. However, we can see from this curvethat the differences between the two sites during the Mi-ocene are at least partly a function of stratigraphy.

In spite of uncertainties in the Miocene, a compari-son of the quartz-content curves from Hole 576A andCore LL44-GPC3 suggests long-term changes in the abun-dance of mineral aerosol being carried to the North Pa-cific. In order to determine whether any of these changesare solely caused by the position of the sites with respectto the major wind belts, the sites have been rotated alongthe paths they have followed as a result of seafloor spread-ing from Pacific spreading centers. The rotation modelused was developed by Prince (1978) and Prince et al.(1980) and is very similar to that of van Andel et al.(1975). The backtrack paths are indicated on Figure 1.Because the sites are at nearly the same latitude, theirtemporal positions relative to the margin of NorthernHemisphere westerlies are very similar.

Leinen and Heath (1981) interpreted the large changein quartz content of Core LL44-GPC3 at roughly 23 m.y.ago as a result of rotation of that core site into the west-erlies. They also found a difference in the chemistry ofthe sediments (quartz/Al ratio) and in the mineralogy(quartz/illite) before and after the change in quartz con-tent. They attributed both to a change in source materi-al from quartz-poor, more basic dust transported fromthe developing western North American cordillera by thetrade winds before the Miocene to more quartz-rich,acidic material from the Asian continent after the site ro-tated into the westerlies. Although the general featuresof this model are confirmed by Hole 576A, the detailsremain unclear because of the stratigraphic uncertainty.The Neogene increase in quartz content from 5 to 14%

583

M. LEINEN

in Hole 576A occurs gradually between 8 and 18 m.y.ago, 5 m.y. after the dramatic increase in quartz contentin Core LL44-GPC3 23 m.y. ago. Both stratigraphiessuggest that the gradual increase in quartz content inHole 576A is correlated with the gradual increase be-tween 8 and 22 m.y ago, not with the sharp increase at23 m.y. ago in Core LL44-GPC3. The sites are at similarlatitudes, and their angle of rotation along the back-track paths is very flat relative to lines of latitude. There-fore, the sites must have traveled north at a rate of about1° latitude/5 m.y. during the Neogene period. The south-ern margin of material deposited from the westerlies isvery sharp (Moore and Heath, 1978). The difference intiming of the major quartz increase (23 m.y. ago in CoreLL44-GPC3 vs. 18 m.y. ago in Hole 576A) could there-fore be because of the small difference in their latitudes(1°1.5'). Thus, the rotation data do not help to con-strain the ambiguities in age of the large increase inquartz content at the sites. Both records are very similarprior to 30 m.y. ago and suggest great uniformity in thesource and nature of mineral aerosol being depositedacross the North Pacific during the early Tertiary.

Implications of Quartz Content for DepositionalProcesses

It has been assumed that the mineral aerosol injectedinto the atmosphere has a complicated vertical distribu-tion and undergoes complex mixing and depositional pro-cesses near its site of injection (Windom, 1975). It hasfurther been assumed that these mixing and deposition-al processes continue over continents and the ocean/continent boundary (Windom, 1975; Pewe et al., 1981).This is confirmed by a growing number of studies ofpresent-day dust storms (e.g., Gillette, 1981; McCauleyet al., 1981; Pewe et al., 1981). Once injected into thestratosphere and transported over the oceans, the miner-al aerosol is assumed to achieve some sort of equilib-rium grain-size distribution (Windom, 1975; Rea andJanecek, 1982; Janecek and Rea, in press); the grain sizeof material deposited from this equilibrium assemblageis representative of the average atmospheric circulation"intensity" or "vigor" (e.g., Parkin, 1974; Parkin andPadgham, 1975). These assumptions have implicationsfor the mineralogy of the aerosol: since clay mineralsare concentrated in the smaller size fractions and silt-sized particles are enriched in quartz and feldspars (e.g.,Griffin et al., 1968), changes in grain size along a duststorm or dust transport trajectory should result in changesin the mineral assemblage along the trajectory. Thus,only the assumption of an "equilibrium" grain-size as-semblage allows the direct comparison of quartz con-tents from cores at different distances from the conti-nent. This assumption is especially important of paleo-climatic studies of Cenozoic sediments, such as those ofRea and his co-workers (Rea and Janecek, 1982) sincethey were forced to compare grain-size data from a smallnumber of widely separated drilling locations, all of whichhad incomplete sedimentary records.

Recent work by Dauphin (1983) has suggested thatthe assumption of a uniform "equilibrium" grain-sizedistribution across the central Pacific wind belts is in-

correct. He analyzed the grain size of only the quartz inthe surface sediments to insure that he was not lookingat material formed authigenically or transported to thesample sites by other means. His analyses show a lineardecrease in quartz grain size from 10.3 m to 7.3 µm across7000 km in the North Pacific. These results suggest anonequilibrium mineral aerosol and imply that mineral-ogy might also change with distance. Although fine-grained, the quartz was concentrated in the silt range(2-20 µm) rather than in the clay range (< 2 µm). Stud-ies by Rea and his co-workers and studies of presentlyaccumulating mineral aerosols (Rea and Janecek, 1982;R. A. Duce, personal communication, 1984) suggest thatthe average size of North Pacific dust is in the clay range.Unless all of this material is deposited as aggregates,Dauphin's (1983) grain-size data may not be representa-tive of the bulk of the mineral aerosol. If particles aretransported as aggregates, techniques that include grain-size analysis of clay particles that have become disaggre-gated are not representative.

The uniformity of quartz concentration between Hole576A and Core LL44-GPC3 (Fig. 2) suggests a very ho-mogeneous mineral aerosol composition across 5000 kmof the North Pacific. Although this might also be in-ferred from the fairly uniform quartz and illite contentof surface sediments in samples analyzed by Heath andhis co-workers (Moore and Heath, 1978; Heath and Pi-sias, 1979), the two cores discussed here established thatthis has been the case for most, if not all of the Cenozo-ic Era.

Quartz Accumulation Rates

Changes in the quartz content of deep-sea sedimentsin the North Pacific have been attributed primarily tochanges in the mineral assemblage resulting from differ-ent source regions (Leinen and Heath, 1981). The massaccumulation of quartz in the sediments is influencedby source regions, but is also influenced by the proxim-ity of source regions, the amount of material availablefor transport, and the vigor of long-distance transport.The last factor is probably a complex function of themean atmospheric circulation intensity and the intensityof the large storms that transport the bulk of mineralaerosol carried to the oceans (e.g., Duce et al., 1980;Gillette, 1981). If differences caused by the proximity ofa depositional site can be evaluated, the remaining rec-ord should contain a paleoclimatic signal. The two sitesdiscussed here, because of their similar source regionsand latitudinal positions, provide a unique opportunityto evaluate the differences in quartz accumulation causedby distance along a transport path.

Mass sediment-accumulation rates for the two sedi-ment columns have been constructed using the paleo-magnetic reversal stratigraphies and ichthyolith stratig-raphies, together with bulk-density data (Table 2). Themass sediment-accumulation rate is R — pS where p isthe dry-bulk density in g/cm3 and S is the linear sedi-mentation rate between two datums in cm/m.y. Quartzaccumulation rates (Table 3, Fig. 4) were calculated fromthe mass sediment-accumulation rates and the analyticalquartz percentages. The quartz accumulation rate record

584

CENOZOIC EOLIAN TRANSPORT

Table 2. Mass sediment-accumulation rates of Hole 576A samples andparameters used for their calculation.

Sample depth a

(cm)

245452727754767

102511111165124313521554162219522076228824342602293930093065324734433529393740364102420242784338446149974560472048214965513653695422571058726009607062776361

Water content8

(%)

60.4762.4162.5563.6466.8969.0467.2166.1066.1064.1664.9163.5060.1654.3452.6154.5454.7559.5157.9860.9461.6957.4556.5258.6858.6855.9653.4952.6151.4653.0555.1655.1659.1856.9057.2654.5554.5532.4336.3133.7741.8635.9034.2133.77

Dry-bulkdensity'3

(g/cm5)

1.0351.0081.0220.9780.9110.8580.8850.9360.9360.9680.9720.982

.1551.247.308.245.226.073.105.094.050.166.191.145.153.189.288.313.350.254.220.238.122.134.188.250.277.838.745.795.634.750.816.788

Sedimentationrate c

(cm/m.y.)

1377137711941194794794794794794499407322318167167175767676767641.541.541.541.541.541.541.541.541.541.541.541.541.541.541.541.5

Mass accumulationrate d

(g/cm^/m.y.)

14251388122011681088681703743743768485400372397218218

938284838048494848495355565251514747495253

Carbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbidites

a Dadey, 1983.b Calculated from water contents and wet-bulk density data (Dadey, 1983):

PD = PW(I--*ΛV) / 1 0°

when p p is the dry-bulk density, pyj is the wet-bulk density, and Xyj is the water con-tent.

c Heath et al., 1983.d Calculated from dry-bulk densities and sedimentation rates, see text.

in Hole 576A shows low and nearly constant accumula-tion rates until the middle Neogene Period. About 15m.y. ago, quartz accumulation began to increase slight-ly. Two large increases in accumulation took place about5 and 2.5 m.y. ago. The timing of the increases is verysimilar to increases in quartz accumulation in Core LL44-GPC3 (Fig. 4, Leinen and Heath, in press). The princi-pal difference between the two sites is in the absolutevalue of the accumulation rate. In Hole 576A quartzhas accumulated at roughly four times the rate in CoreLL44-GPC3 for most of the Pliocene and Pleistoceneepochs (5 m.y. ago to present).

The initial increase in quartz accumulation from thelow mid-Tertiary values in Core LL44-GPC3 occurredabout 20 m.y. ago. Because age determinations are nec-

essary for the accumulation-rate calculation, the valuesare dependent on stratigraphy. Correlation of ichthyo-lith ranges suggests that the increase in quartz accumu-lation in Core LL44-GPC3 is actually coincident withthat in Hole 576. These increases could be caused bychanges in the positions of the sites with respect to thewesterlies (i.e., source mineralogy) or changes in theamount or efficiency of transport from land, but theirassociation with changes in the quartz content and claymineralogy (see Leinen and Heath, 1981; Lenotre et al.,this volume) argues strongly that they reflect the passageof the core sites into the influence of the westerlies rath-er than solely a change in transport efficiency.

During the late Neogene, the quartz accumulation ratein Hole 576A increased markedly between 4 and 6 m.y.

585

M. LEINEN

Table 3. Quartz accumulation rates of Hole 576A sam-ples.

Quartz accumulation rate (g/cm /m.y.)

Core-Section(interval in cm)

1-1, 21-251-3, 81-851-6, 80-832-3, 47-503-1, 47-503-3, 70-733-4, 117-1203-5, 127-1303-6, 110-1134-1, 50-534-2, 103-1064-3, 98-1014-4, 104-1074-5, 20-234-6, 60-635-1, 87-905-2, 40-435-3, 30-335-5, 137-1405-6, 130-1335-7, 25-286-1, 90-936-2, 47-506-3, 93-966-4, 11-146-5, 12-156-5, 127-136-6, 17-206-7, 22-257-1, 137-1407-2, 22-257-3, 17-207-3, 35-38

Sampledepth(cm)

23383831

121918692191238925492681282130253169332533913581380939114051445946014646476148695065513252835399543955935759579359395946

Quartz

16.6318.1115.5217.3513.068.87

11.669.02

14.6410.928.575.815.864.255.315.161.762.644.672.886.183.964.214.816.12?.414.545.293.914.165.506.463.51

Agea

(m.y.)

0.260.530.871.322.563.594.395.867.609.44

12.1314.0316.3517.9425.0028.0031.0034.0044.0047.0048.0051.0054.0058.0060.0064.0066.00

Quartzaccumulation

rate13

(g/cm^/m.y.)

237.0254.0166.0129.047.033.037.0

8.314.010.07.24.82.72.02.62.50.81.32.41.43.01.92.02.43.22.82.4

Carbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbiditesCarbonate turbidites

a Determined by interpolation between paleomagnetic datumsfor 0-24.76 m and by interpolation between ichthyolith da-tums for 24.76-54.0 m.

° Mass accumulation rates for sample depths were interpolatedfrom data in Table 2.

ago. Similar increases in eolian accumulation over thisinterval have been calculated by Rea and Janecek (1982)from DSDP Sites 305 and 310 (Fig. 1). They attributedthe increase to an influx of volcanic ash and pumice as-sociated with the late Miocene to early Pliocene volcanicepisode (Kennett and Thunell, 1975; Kennett, 1977). Thereis, however, no significant ash component in sedimentsof that age in either Core LL44-GPC3 or Hole 576A.Therefore, the increase cannot be attributed to volcanicactivity in these cores.

It is likely that there is aliasing in the temporal recordof quartz concentration. Two Neogene samples have amuch lower quartz content than those above and belowthem. If these points are ignored, the late Neogene rec-ord simply records an accelerating increase in mineral-aerosol accumulation with increasing Miocene and Plio-cene aridity (Wolfe and Hopkins, 1967) caused by globalcooling (Savin, 1977; Kennett, 1977; Woodruff et al.,1981) or movement of the core site into the influence ofthe westerlies.

The Hole 576A quartz accumulation does highlightthe dramatic increase in mineral-aerosol accumulationthat occurred after the onset of Northern Hemisphereglaciation 2.5 m.y. ago (Shackleton et al., 1983; Prell,1982). Quartz accumulation increased from about 50 to

B

20-^

£ 4 0 -

6 0 -

Quartz accumulation rate (g/cm /m.y.)4 8 12

, I i I i I

Figure 4. Quartz accumulation rate in Hole 576A and Core LL44-GPC-3. Filled circles are Core LL44-GPC-3 data; open circles areHole 576A data. A. Neogene rates, B. Pre-Pleistocene rates plot-ted on an expanded scale.

130 g/cm2/m.y., and total sediment accumulation in-creased from about 400 to 700 g/cm2/m.y. at that time.Late Pleistocene quartz accumulation rates are about 250g/cm2/m.y., five times their preglacial rates. The rathersmall changes in grain size of both the total eolian min-eral assemblage in Hole 576 (Janecek, this volume) andthe quartz in Core LL44-GPC3 (Janecek and Rea, inpress) suggest that these changes are due primarily tochanges in climate, which made more material availablefor transport, rather than to changes in wind "vigor."

CONCLUSIONS

The quartz record in Hole 576A reflects changes inthe source and amount of mineral aerosol being depos-

586

CENOZOIC EOLIAN TRANSPORT

ited in the northwest Pacific with time. When the Hole576A quartz record is compared with other long recordsof mineral-aerosol deposition (Core LL44-GPC3), astrong uniformity in the composition of the mineral aero-sol across the North Pacific and an equilibrium mineralassemblage in the aerosol are indicated. The temporalrecord of quartz content and accumulation rate suggeststhat Hole 576A entered the influence of material trans-ported by the westerlies about 15 m.y ago and that ratesof sediment deposition have increased since then. Onlya dramatic change in quartz accumulation about 2.5m.y. ago can be clearly related to a climatic event, but itis likely that the gradual increase in quartz accumula-tion through the Miocene and early Pliocene epochs is aresult of increasing Northern Hemisphere aridity and in-tensified atmospheric activity associated with global cool-ing and an increase in the equator-to-pole temperaturegradient.

ACKNOWLEDGMENTS

The careful X-ray diffraction analyses for this study were performedby Tammy King Walsh and Sandra Furda. The work was supported bythe Subseabed Disposal Program of the Department of Energy (Con-tract No. SAN 26-6610) administered by Sandia National Laborato-ries. I would like to thank Les Shephard for sampling the cores and G.Ross Heath for providing the paleomagnetic reversal stratigraphy be-fore publication and for reviewing the manuscript. The manuscriptwas typed by Kathy Hazard. It benefited from reviews by Thomas Ja-necek and Ted Moore.

REFERENCES

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Date of Initial Receipt: 3 November 1983Date of Acceptance: 14 June 1984

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