Prell, W. L., Niitsuma, N., et al , 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 117

28. U-SERIES DISEQUILIBRIUM, PARTICLE SCAVENGING, AND SEDIMENT ACCUMULATIONDURING THE LATE PLEISTOCENE ON THE OWEN RIDGE, SITE 7221

Graham B. Shimmield2 and Stephen R. Mowbray2

ABSTRACT

High resolution sampling of the upper 10 m of Hole 722B has provided information on the extent of 2 3 4U/2 3 0Th dis-equilibrium in Owen Ridge sediments accumulating over the last 250 k.y. High biological productivity in surface watersis sustained by seasonal upwelling driven by the Southwest Monsoon. Evidence from the Owen Ridge suggests that therehas been significant variation in this productivity over the late Pleistocene, and that greater biological productionoccurred during interglacial episodes. The resulting organic matter flux drives a variety of redox-related reactions in thesediment, including the reduction and fixing of U. High precision measurements of U/Th trace the paleoproductivityrecord of enhanced organic matter input during the interglacials. Sedimentary 230Th, produced by 2 3 4U decay in thewater column, is a sensitive indicator of water column scavenging by particulates, and sediment winnowing and focus-ing. The decay-corrected flux of excess 2 3 0Th at the Owen Ridge is substantially less than that recorded at other upwell-ing margins. The historical record of 230Th flux suggests that erosion and winnowing of fine sediment has occurred atthe ridge site by active intermediate water circulation, and that this was most prevalent during mid isotope Stage 5 toStage 3.

INTRODUCTION

Under conditions of high biological productivity that occurat continental margins where upwelling is intensified, enhancedpaniculate scavenging of reactive trace metals and radionuclidesis known to be manifest (Bacon et al, 1976; Turekian, 1977;Spencer et al., 1981; DeMaster, 1981; Anderson et al; 1983a, b;Mangini and Diester-Haass, 1983; Shimmield et al; 1986). Thehigh production of biogenic paniculate matter, and recycling ofredox-sensitive metals through the associated oxygen minimumzone, sustains a substantial flux to the seafloor of the scavengedmetals (e.g., Th, Pa, Pb), together with increased diagenetic en-richment of redox-sensitive metals (U, Mo) in the reducing sedi-ment (Veeh, 1967; Yamada and Tsunogai, 1984; Shimmield andPedersen, in press).

Complete, long records of sediment accumulation under pro-ductive continental margin waters record a variety of chemicaltracers attributable to variation in productivity and upwelling(Boyle, 1983; Mangini and Diester-Haass, 1983; Shimmield andMowbray, this volume), but often the chronology of the sourcefunction (production history) is poorly constrained. However,disequilibrium within the natural 238U decay series is particu-larly interesting in that the production of particle reactive 230Thfrom dissolved parent 234U has remained almost constant overthe late Pleistocene due to the long residence time of U in sea-water (Ku et al., 1977). As U is found at constant compositionand isotopic ratio (234U/238U = 1.14 ± 0.01) in normal salinitywaters, it is possible to calculate the theoretical production rateof daughter 230Th. This isotope is very particle reactive with ashort residence time of 41 ± 3 yr (Anderson et al., 1983a) sothat radioactive decay within the water column is negligible (t1/2

= 75,200 yr). Consequently, 230Th possesses unique characteris-tics as a chemical tracer in that its production rate and theoreti-cal flux to the seafloor is well constrained.

In the northwest Arabian Sea the seasonally-reversing sum-mer Southwest Monsoon sustains active oceanic upwelling and

high biological productivity through Ekman transport of thesurface waters (Qasim, 1982; Slater and Kroopnick, 1984; Prell,1984a, b). From several studies of proxy-indicators of upwellingintensity and Monsoon wind strength (pollen type, Van Campoet al., 1982; foraminiferal species, Prell, 1984a, b; stable δ 1 8θsignatures of benthic and planktonic foraminifera, Prell, 1984a, b;Duplessy, 1982; inorganic geochemistry, Shimmield and Mowbray,this volume) the history of the Southwest Monsoon and associ-ated variations in paleoproductivity have been inferred.

In the study presented here we have taken the opportunity ofthe complete, high resolution record of sedimentation obtainedat Site 722 on the Owen Ridge (Fig. 1) to investigate the varia-tion in 230Th activity and flux over the last 250 k.y. We comparethe observed input of particle reactive 230Th to both the theoreti-cal production rate, and to that found in other upwelling areas(northwest Africa, Mangini and Diester-Haass, 1983; Baja Cali-fornia, Shimmield et al., 1986). Our study, the first on 2 3 4U/

1 Prell, W. L., Niitsuma, N., et al., 1991. Proc. ODP, Sci. Results, 117: Col-lege Station, TX (Ocean Drilling Program).

2 Department of Geology and Geophysics, University of Edinburgh, WestMains Road, Edinburgh, EH9 3JW, Scotland, U.K. Figure 1. Location of Site 722 in the Northwest Arabian Sea.

465

G. B. SHIMMIELD, S. R. MOWBRAY

2 3 0Th disequilibrium from the northwest Arabian Sea, suggeststhat surprisingly low fluxes of excess 2 3 0 Th occur and that sig-nificant erosion and winnowing of fine material may have oc-curred on the Owen Ridge. However, the distribution of redox-sensitive U within the sediments appears to record faithfully theinferred variation in productivity at the Oman Margin.

SAMPLING AND METHODS

Site 722 was selected as an ideal site for high resolution stud-ies of U-series disequilibrium in late Pleistocene sediments as itsOwen Ridge top location (2028 m depth) is free from effects oflateral sediment input. In effect, the excess 2 3 0 Th activity that wemeasure here should result from vertical particle scavenging inthe overlying water (with possible removal after deposition bybottom currents). In Hole 722B the upper (I) lithologic unit wassampled and is composed of foraminifer-bearing nannofossilooze (5%-25% foraminifers). On vertical sectioning of the core,20 cm3 plugs were taken at 20 cm intervals and sealed in po-lythene bags. Samples were kept at 4°C until processing. Watercontents were estimated from weight loss on drying at 60°C. Us-ing an average grain density of 2.6 g cm~3 (Shipboard ScientificParty, 1989) dry bulk density values were calculated. Sea saltcontribution was obtained assuming a pore water salinity ofnormal seawater. All data presented in Appendix A are cor-rected for sea salt dilution.

After drying and grinding to a fine powder the sedimentswere completely digested in a mixture of HF, HNO 3 , and HC1O4

acids together with a mixed 2 2 8 Th/ 2 3 2 U tracer of known activity.Following di-isopropyl ether extraction of dissolved Fe and ionexchange chromatography under HC1 and HNO 3 media to sepa-rate U and Th isotopes, the nuclides were electro-deposited ontostainless steel planchettes from (NH4)SO4 media. Estimates oferror are given in Appendix B and are calculated from propa-gated analytical errors and lσ counting errors. All activities arequoted in units of disintegrations per minute per gram dryweight (dpm g~1) following standard oceanographic conven-tions. Data on the concentration of U and Th (in ppm units) aregiven in Appendix A of Shimmield and Mowbray (this volume)and are calculated from 2 3 8 U and 2 3 2Th activities and their rele-vant isotopic abundance.

RESULTS

In order to present the U-series data obtained from this studyon a geochronological basis, and to correct for ingrowth and de-cay (see below) of the relevant nuclides, a detailed age model forthe late Pleistocene in Hole 722B is required. We are grateful toS. Clemens and W. Prell (see this volume) for providing such anage model based on δ 1 8 θ stratigraphy of the planktonic fora-minifer G. sacculifer. Figure 2 presents the uncorrected data forparent 2 3 4 U and daughter 2 3 0 Th with age (depth in the core) to-gether with lσ error bars. From interpretation of the δ 1 8 θ stra-tigraphy the appropriate isotope stages are also indicated. Closeexamination of the 2 3 0 Th profile indicates that the observed ac-tivity is generally less than the 2 3 4 U parent in the upper (youn-ger) portion of the core sampled, and that with depth (age) the2 3 0Th approaches secular equilibrium with its parent 2 3 4U. Sucha situation in deep-sea sediments is relatively unusual (normallylarge excesses of daughter 23OTh are observed) and is due to thecomparatively shallow water depth and high organic matter fluxpromoting active U diagenesis. These points are addressed fur-ther below. The average uncorrected 2 3 0 Th activity of 2-3 dpmg~1 appears to be slightly higher in interglacial episodes, and isaccompanied by rather greater changes in 2 3 4 U activity.

Figure 3 presents the variation between U and Th on aweight ratio basis. Th (calculated from 2 3 2 Th activity) is almostexclusively controlled by the detrital noncarbonate phases in thesediment. On the other hand, U is strongly redox sensitive, oc-

curring as a dissolved uranyl carbonate species ( + 6 oxidationstate) in oxygenated seawater, but becomes reduced ( + 4 oxida-tion state) and fixed within anaerobic sediments. Since there isoften an association between redox state and organic mattercontent of the sediment, and U is often bound to organic matter(see Shimmield and Pedersen, in press, for review), then thestrong agreement between elevated U/Th ratios and productiv-ity indicators (such as Ba/Al) is to be expected (Fig. 3). It isclear that during interglacial episodes there were substantial in-creases in surface water productivity in the northwest ArabianSea (see Shimmield and Mowbray, this volume) promoting en-hanced flux and accumulation of phytodetritus in the sedi-ments. It is probable that subsequent shallowing of the U redox-cline within the sediment allowed substantial reduced U fixationwithin the sediment, although variation in sediment accumula-tion rate is also an important variable. Once U has becomefixed, there appears to be little post-depositional remobiliza-tion.

The distribution of 2 3 0Th observed in Hole 722B can be at-tributed to three main sources: (1) detrital 2 3 0 Th deposited inequilibrium with parent 2 3 4U, (2) ingrown 2 3 0Th from authigenic2 3 4U, and (3) excess 2 3 0 Th (2 3 0Thx s) scavenged from the water col-umn (which decays with a half-life of 75,200 yr). In Figure 4 thetotal 2 3 0Th measured has been corrected for each of these sources(see Appendix A for methods of calculation and further expla-nation) to obtain an estimate of the initial activity (AQ) of2 3 0 Th x s at the time of deposition. The calculation of initial activ-ity is very sensitive to the estimation of detrital 2 3 4 U and 2 3 0 Th.Rather than use a global average figure, we have performed ra-diochemical analysis on surficial samples from 21 box-cores inthe northwest Arabian Sea (Shimmield, unpubl. data). Leastsquares regression of this data set suggests the average initial ac-tivity of 2 3 4 U is 1.03 × 232Th activity. Figure 4 indicates that thehighest initial activities occur in the Holocene (Stage 1), earlyStage 5, and mid-Stage 6. Generally, lower initial activities oc-cur within the glacial sediments. There is a broad correspon-dence between the A0

230Thxs activity and productivity indicators(Ba/Al, Fig. 2) from the beginning of Stage 5 to the present sug-gesting a link between the activity of the nuclide in the sedimentand paleoproductivity. It is the nature of this link, and the mod-ification of the sedimentary record of 230Thxs which we discussin the following section. The lack of correspondence betweenA0

230Th and Ba/Al before Stage 5 is due to increased error inthe calculation (as 23OTh approaches transient equilibrium with234U) and uncertainties in the age model.

DISCUSSIONThe depositional flux of 230Thxs if) through time may be cal-

culated from:

/ = S A0230Thxs p

where S is the linear sediment accumulation rate (cm k.y.~1),A0

230Thxs is the age corrected initial 230Thxs activity (dpm g~1)and p is the dry bulk density (g cm"3). The flux thus calculatedis displayed in Figure 5. At Site 722 the expected water columnflux of 23OThxs is assumed to be equal to its rate of production inthe water column from decay of 234U and therefore depends onthe water depth (z). Hence, the predicted flux (fpTed) is:

/pred = 0.00263 × z

giving a value of 5.3 dpm cm"2 k.y. ~l (shown by vertical dashedline in Fig. 5). It is clear that this flux is exceeded during theHolocene, early Stage 5 and Stage 6, whilst lower-than-pre-dicted fluxes occur from mid-Stage 5 to Stage 3. Clearly, the de-position rate of 230Thxs has varied with time.

466

234U (dpm/g)

100-

<

3 0 0 -

400 J

1 ' ' • ' ' ' . ' . . ' . ! . ' . ' . . ' . ' . ' • ' ' • 1 ' 1

9

10

U/Th DISEQUILIBRIUM, PARTICLE SCAVENGING, SITE 722

230Th (dpm/g)1

: : 2

3

4

'200-

cn<

3 0 0 - ..,,.-,.:,.T..r,-

400 J

9

10

Figure 2. The distribution of 2 3 4U and 2 3 0Th activity (uncorrected except for salt dilution) with age in Hole722B. The error bars are calculated from lσ counting statistics and propagated analytical uncertainties. Thenumbered isotope stages are derived from the δ 1 8 θ stratigraphy of planktonic foraminifers (see Fig. 3).

U/Th (wt.rαtio) Bα/AI × 1 Cf4 6180 (%o)0 1 2 3 4 0 500 1000 I 0 - 1 - 2

' i i i • • I • • • • i • • • • I • • • • I • i i i i i • l ' ' ' i • ' . I . » . ! . ! . 1 . 1 . 1 • i . ' • ' ; . ' 1

234

Φ

<

9

10

Figure 3. The distribution with time of U/Th (weight ratio), Ba/Al (× 10~4) and δ 1 8 θ in G.sacculifer (Clemens and Prell, this volume).

As described in the introduction, continental margins areknown to have high rates of 2 3 0Th scavenging (Anderson et al.,1983b; Mangini and Diester-Haass, 1983; Shimmield et al.,1986). From these studies, and sediment trap work in the Sar-gasso Sea (Bacon et al., 1985), we can compare the results fromHole 722B with areas identified as important sinks for 2 3 0Th(Fig. 6). Interestingly, although there is a significant positivecorrelation between sediment accumulation rate and Ao23OThxs

flux at Site 722, the magnitude of the flux for a given accumula-tion rate is less than that observed off northwest Africa (Manginiand Diester-Haass, 1983) and much less than that off Baja Cali-

fornia (Shimmield et al., 1986), even accounting for the deeperwater depths at these localities (see Fig. 6).

As can be seen in Figure 5, a significant proportion of themeasured 2 3 0 Th x s flux is in deficit when compared to water col-umn production rates. We suggest that the generally low 2 3 0 Th x s

fluxes and the marked deficit from Stage 5 to Stage 3 are a resultof winnowing and current erosion removing the fine fraction ofthe sediment. Cochran and Osmond (1976) identified sedimentredistribution patterns in the Tasman Basin where excesses of2 3 0 Th occurred in the basins and deficits on the ridges, but gavean overall balance of the 2 3 0Th flux for the basin. More recently

467

G. B. SHIMMIELD, S. R. MOWBRAY

23CJ0ThX9 (dpm/g)0.0 0.8 1.6 2.4 3.2 4.0

230Th x s

0.0(dpm/cm — ky)

5.0 10.0 15.0

100-

'200-^

0)CJ

<

300-

400 J

Figure 4. The age distribution of excess 2 3 0 Th corrected for detrital com-ponent, ingrowth from parent authigenic 2 3 4 U, and radioactive decay(see Appendix A for details of calculation) in Hole 722B, measured indpm g~1.

Mangini et al. (1987) and Mangini and Kuhnel (1987) have identi-fied erosion events from the Pacific (Peru Basin and Clarion-Clip-perton fracture zone, respectively). In both studies they attributethe enhanced deep water circulation to "periods of climaticchange, representing both climatic amelioration and deteriora-tion" occurring at 11, 70-79, 117-127, and 173-193 k.y.BP inthe Pacific. At Site 722 the stratigraphy of these erosion eventsis dissimilar, although the Owen Ridge site will be subject tochanges in Indian Ocean Intermediate Water flow rather thanAntarctic Bottom Water suggested by Mangini and co-workers.In another study in this volume (Clemens and Prell, 1990) it isapparent that rather different late Pleistocene accumulation ratesexist between ridge sites. They indicate that sedimentation atSite 722 is some 25% higher than at an adjacent ridge site sam-pled by piston core RC27-61. Nevertheless, chemical (CaCO3

wt.%) and grain size variation are preserved between the twocores. They suggest that the strong, coherent eolian depositionalsignal is preserved despite fine fraction winnowing. We suggestthat winnowing is also present at Site 722. Clearly, the 230Thxs

activity calculated will have been modified by this winnowing,masking the productivity signal attributable to enhanced parti-cle scavenging. We have studied other box and piston cores col-lected from a previous cruise to the Oman Margin in order toconstruct a regional history of 230Th scavenging at the OmanMargin (Shimmield, unpubl. data). Nevertheless, the paleore-dox signal (identified from the U/Th ratio) is preserved andclosely correlates with other indicators of paleoproductivity(Fig. 3; Shimmield and Mowbray, this volume). This is probablydue to the diagenetic process of U fixation in the more organic-rich sediments, rather than scavenging onto fine particles in the

100-

' 2 0 0 -

en<

300-

400 J

Figure 5.The flux of 2 3 0 T h x s (dpm cm~ 2 k.y."*) with time in Hole 722B.See text for details of flux calculation. The vertical line represents thepresent day expected flux of 5.3 dpm c m " 2 k.y.~1.

water column which can be easily transported away by bottomcurrents.

CONCLUSION

Sediments collected from the crest of the Owen Ridge at Site722 record 250 k.y. of 234U and 230Th flux variation. During pe-riods of high biological productivity in surface waters (intergla-cial episodes) substantial fluxes of biogenic fallout occurred andmay be traced within the sediment record (see Shimmield andMowbray, this volume). Comparison of the U/Th ratio, mea-sured by α-spectrometry, with indicators of paleoproductivity(Ba/Al) clearly indicate that enrichment of redox-sensitive Uoccurs in the more organic-rich sediment. The U is reduced(from 6 + oxidation state to 4 + state) and fixed within the sedi-ment by diagenetic processes, preserving a paleoredox signalover the late Pleistocene.

From measurements of 230Th, and estimation of detrital con-tribution, ingrowth from authigenic 234U, and decay of initialunsupported 23OTh, the past record of 230Thxs has been estab-lished (using δ 1 8θ stratigraphy from planktonic foraminifers).From the estimate of 230Thxs flux, and comparison with fluxmeasurements from other highly productive upwelling areas(northwest Africa, Baja California), it is apparent that the ex-pected high 230Thxs flux to the Owen Ridge site is not found.Comparison with expected production from the overlying 2028m water column suggests that the paleoproductivity signal of230Thxs (caused by intensified particle scavenging) is modified byerosion and winnowing. We suggest that particularly enhancedintermediate water circulation from mid-Stage 5 to Stage 3 canbe identified. This hypothesis remains to be confirmed by de-tailed grain-size analysis. At Site 722 the late Pleistocene sedi-

468

U/Th DISEQUILIBRIUM, PARTICLE SCAVENGING, SITE 722

Bαjα- California

40.0 -

Production rate

Baja-CaliforniaNorthwest Africa

Site 722

Sediment accumulation rate (cm/k.y.)

Figure 6. A plot of measured 2 3 0Th x s flux (dpm cm" 2 k.y.~1) againstsediment accumulation rate (cm k.y.~1, derived from δ 1 8 θ stratigraphyof planktonic foraminifers) in Hole 722B (crosses). Also shown are thedata from Baja California (dots, Shimmield et al., 1986) and the regres-sion line and approximate field of data for glacial sediment off north-west Africa (Mangini and Diester-Haass, 1983). The dashed horizontallines represent the water column production rates for the three areas.Site 722 clearly has lower 2 3 0Th x s fluxes than the other two upwellingareas.

ment accumulation rate is lower than at other Owen Ridge sitesdue to this current winnowing.

ACKNOWLEDGMENTS

We wish to thank the staff and crew of the Ocean DrillingProgram and the JOIDES Resolution for the opportunity to un-dertake this study. We are especially grateful to Warren Prell,Kay Emeis, Brian Price, Tom Pedersen, and Steve Clemens forhelpful discussions on this project. Dr. R. Anderson and ananonymous reviewer provided helpful comments on this manu-script. GBS acknowledges the support of NERC Grant GST/02/315 from the ODP Special Topic fund.

REFERENCES

Anderson, R. R, Bacon, M. P., and Brewer, P. G., 1983a. Removal of230Th and 231Pa from the open ocean. Earth Planet. Sci. Lett., 62:7-23.

, 1983b. Removal of 2 3 0Th and 2 3 1Pa at ocean margins. EarthPlanet. Sci. Lett., 66:73-90.

Bacon, M. P., Huh, C.-A., Fleer, A. P., and Deuser, W. G., 1985. Sea-sonality in the flux of natural radionuclides and plutonium in thedeep Sargasso Sea. Deep-Sea Res. Part A, 32:273-286.

Bacon, M. P., Spencer, D. W., and Brewer, P. G., 1976. 21oPb/226Ra and2 1 0Po/2 1 0Pb disequilibria in seawater and suspended particulate mat-ter. Earth Planet. Sci. Lett., 32:277-296.

Boyle, E. A., 1983. Chemical accumulation variations under the PeruCurrent during the past 130,000 years. J. Geophys. Res., 88:7667-7680.

Clemens, S. C , and Prell, W. L., 1990. Late Pleistocene variability ofArabian Sea summer-monsoon winds and dust source-area aridity:an eolian record from the lithogenic component of deep-sea sedi-ments. Paleoceanography, 5:109-145.

Cochran, J. K., and Osmond, J. K., 1976. Sedimentation patterns andaccumulation rates in the Tasman Basin. Deep-Sea Res. Oceanogr.Abstr., 23:193-210.

DeMaster, D. J., 1981. The supply and accumulation of silica in the ma-rine environment. Geochim. Cosmochim. Acta, 45:1715-1732.

Duplessy, J. C , 1982. Glacial to interglacial contrasts in the northernIndian Ocean. Nature, 295:494-498.

Ku, T.-L., Knauss, K. G., and Mathieu, G. G., 1977. Uranium in openocean: concentration and isotope composition. Deep-Sea Res.Oceanogr. Abstr., 24:1005-1017.

Mangini, A., and Diester-Haass, L., 1983. Excess Th-230 in sedimentsoff northwest Africa traces upwelling in the past. In Suess, E., andThiede, J. (Eds.), Coastal Upwelling, its Sediment Record: New York(Plenum), 455-471.

Mangini, A., and Kuhnel, U., 1987. Depositional history in the Clarion-Clipperton Zone during the last 250,000 years—230Th and 2 3 1Pamethods. Geol. Jahrb., D87:105-121.

Mangini, A., Stoffers, P., and Botz, R., 1987. Periodic events of bot-tom transport of Peru Basin sediment during the Late Quaternary.Mar. Geol., 76:325-329.

Prell, W. L., 1984a. Monsoonal climate of the Arabian Sea during thelate Quaternary: a response to changing solar radiation. In Berger,A. L., Imbrie, J., Hays, J., Kukla, G., and Saltzman, B. (Eds.). Mi-lankovitch and Climate (Pt. 1): Dordrecht (D. Reidel), 349-366.

, 1984b. Variation of monsoonal upwelling: a response tochanging solar radiation. In Hansen, J. E., and Takahashi, T. (Eds.),Climatic Processes and Climate Sensitivity. Am. Geophys. Union,Maurice Ewing Series, 5:48-57.

Quasim, S. Z., 1982. Oceanography of the northern Arabian Sea. Deep-Sea Res. Part A, 29:1041-1068.

Shimmield, G. B., Murray, J. W., Thomson, J., Bacon, M. P., Ander-son, R. F., and Price, N. B., 1986. The distribution and behaviourof 230Th and 2 3 1Pa at an ocean margin, Baja California, Mexico.Geochim. Cosmochim. Acta, 50:2499-2507.

Shimmield, G. B., and Pedersen, T. F., in press. The geochemistry of re-active trace metals and halogens in hemipelagic continental marginsediments. CRC Crit. Rev. Mar. Sci.

Shipboard Scientific Party, 1989. Site 722. In Prell, W. L., Niitsuma,N., et al., Proc. ODP, Init. Repts., 117: College Station, TX (OceanDrilling Program), 255-318.

Slater, R. D., and Kroopnick, P., 1984. Controls on dissolved oxygendistribution and organic carbon deposition in the Arabian Sea. InHaq, B. U., and Milliman, J. D. (Eds.), Marine Geology and Ocean-ography of Arabian Sea and Coastal Pakistan. New York (Van Nos-trand Reinhold), 305-313.

Spencer, D. W., Bacon, M. P., and Brewer, P. G., 1981. Models of thedistribution of 2 1 0Pb in a section across the north equatorial Atlan-tic Ocean. J. Mar. Res., 39:119-138.

Turekian, K. K., 1977. The fate of metals in the oceans. Geochim. Cos-mochim. Acta, 41:1139-1144.

Van Campo, E., Duplessy, J. C , and Rossignol-Strict, M., 1982. Cli-matic conditions deduced from a 150-kyr oxygen isotope-pollen re-cord from the Arabian Sea. Nature, 296:56-59.

Veeh, H. H., 1967. Deposition of uranium from the ocean. EarthPlanet. Sci. Lett., 3:145-150.

Yamada, M., and Tsunogai, S., 1984. Postdepositional enrichment ofuranium in sediment from the Bering Sea. Mar. Geol., 54:263-276.

Date of initial receipt: 27 September 1989Date of acceptance: 13 April 1990Ms 117B-169

469

G. B. SHIMMIELD, S. R. MOWBRAY

APPENDIX A

Calculation of initial excess 2 3 0 Th activities

1. Correct all activity data for salt dilution.2. Calculate detrital 2 3 0 Th activity: Assume 2 3 0 Th is in equilibrium with detrital parent 2 3 4 U. From multiple regression of box-core core top data

(Shimmield, unpubl. data) from the Oman Margin with 2 3 4 U / 2 3 8 U ratios = 1.0 ± 0.05,

234Udetrital = 1 03 2 3 2 T h

(All 2 3 2 Th is assumed to be detrital). Hence,

230xu _ 234T T1 "detrital ~ u detrital

3. Calculate ingrowth from authigenic 2 3 4 U parent:

23Orα, _ 234T T . ( _ p - λ t \1I1ingrown ~ u authigenic •̂1 c >

• - 23OTU _ /234Tr _ 234TT . \ . (\ _ e-0.009217 KI.e., Aningrown ~ V utotal udetrital^ U e '

4. Calculate initial excess 2 3 O Th:

A 230™ - ^βtota l - ( ^hjngrown +•̂ O l n x s ~ .-0.009217 t

Where: all isotopes are expressed in activity units (dpm g l)λ is the 2 3 0 Th decay constant ( ~ k y )t is the age of the sample (k.y.)

470

U/Th DISEQUILIBRIUM, PARTICLE SCAVENGING, SITE 722

APPENDIX BCores 722B-1H and 2H, corrected for salt contribution and dilution.

Core, section,interval (cm)

117-722B-

1H-1H-1H-1H-1H-1H-1H-1H-1H-1H-1H-1H-1H-1H-1H-

I, 6-8, 16-18, 26-28, 36-38, 46-48, 56-58

I, 66-68, 76-78, 86-88, 96-98, 106-108, 116-118, 126-128, 136-138, 146-148

1H-2, 6-81H-2, 16-181H-2, 26-281H-2, 36-381H-2, 46-481H-2, 56-581H-2, 66-681H-2, 76-781H-2, 86-881H-2, 96-981H-2, 106-1081H-2, 116-1181H-2, 126-1281H-2, 136-1381H-2, 146-1481H-3, 6-81H-3, 16-181H-3, 26-281H-3, 36-381H-3, 46-481H-3, 56-581H-3, 66-681H-3, 76-781H-3, 86-881H-3, 96-981H-3, 106-1081H-3, 116-1181H-3, 126-1281H-3, 136-1381H-3, 146-1481H-4, 6-81H-4, 16-181H-4, 26-281H-4, 36-381H-4, 46-481H-4, 56-581H-4, 66-681H-4, 76-782H-1, 06-082H-1, 16-182H-1, 26-282H-1, 36-382H-1, 46-482H-1, 56-582H-1, 66-682H-1, 76-782H-1, 86-882H-1, 96-982H-1, 106-1082H-1, 116-1182H-1, 126-1282H-1, 136-1382H-1, 146-1482H-2, 6-82H-2, 16-182H-2, 26-282H-2, 36-382H-2, 46-482H-2, 56-58

Depth(mbsf)

0.070.170.270.370.470.570.670.770.870.97

.07

.17

.27

.37

.47

.57

.67

.77

.87

.972.072.172.272.372.472.572.672.772.872.973.073.173.273.373.473.573.673.773.873.974.074.174.274.374.474.574.674.774.874.975.075.175.275.625.675.775.875.976.076.176.276.376.476.576.676.776.876.977.077.177.277.377.477.57

aAge(k.y.)

6.07.59.0

10.211.512.814.015.016.117.219.221.222.325.527.729.831.934.036.038.140.142.144.146.248.250.352.354.356.358.460.462.565.167.770.173.276.279.284.189.093.598.0

103.2107.5113.1116.8119.8122.8124.1126.0127.5129.2132.0135.8137.1138.2140.5142.6144.1145.2147.6149.4151.2152.7154.2156.2158.0159.5161.4163.2165.0166.8168.7170.7

238U(dpmg~ 1 )

3.084

2.684—

1.901

2.893

3.1422.3712.6561.9042.0211.9411.9802.9692.1552.1942.3552.163

2.9102.6382.7433.0562.2052.2342.4342.3862.0661.2892.0251.8693.5332.7202.8542.917

—2.696

—2.2412.652

2.124—

3.559

3.3162.4912.019

2.495

2.496—

2.040

2.133

2.1132.0952.2852.5942.4262.0042.9021.9892.468

2.0522.461

2.383—

±

0.140

0.126—

0.101

0.133

0.1530.1220.1410.1030.1010.0960.0970.1360.1090.1090.1170.104

0.1360.0720.1290.1440.0740.0800.1200.1170.1090.0620.1000.0800.1570.1330.1280.132

—0.138

—0.1190.123

0.101—

0.170

0.1510.1320.127

0.147

0.141_

0.128

0.108—

0.100.100.110.110.120.090.130.100.15

0.110.12

0.08—

234U(dpmg~ 1 )

3.486

3.000

2.058

3.235

3.4022.5343.0182.1032.0872.1742.1493.1742.3792.3742.4852.387

3.1952.8813.0283.5692.4812.2872.7452.6132.2621.3292.2862.1053.7992.9133.1933.206

2.846

2.4482.928

2.414

3.822

3.6832.3992.108

2.843

2.707—

2.137

2.288

2.2212.2272.4272.6812.4952.1073.0762.1742.623

2.0582.782

2.588—

±

0.156

0.139

0.108

0.147

0.1640.1290.1570.1110.1040.1050.1040.1440.1190.1170.1230.113

0.1470.0770.1400.1650.0810.0820.1330.1260.1160.0630.1090.0880.1680.1410.1410.143

0.144

0.1290.134

0.113—

0.180

0.1660.1280.131

0.162

0.149—

0.132

0.113

0.1040.1100.120.120.120.100.130.110.15

0.110.13

0.08—

2 3 2Th(dpm g x)

0.337

0.554—

0.554—

0.580

0.6300.5750.6160.6110.5900.5940.6700.5650.5820.3870.6600.493

0.5020.5730.5530.5590.5360.5320.5700.5040.6520.5930.6960.5220.7620.8510.5590.440

—0.587

_0.4440.576

0.377—

0.532

0.3760.7150.634

0.561

0.576—

0.597—

0.568—

0.8490.8640.7380.7340.7180.7500.7430.8400.683

0.6900.591

—0.664

—

±

0.027

0.037—

0.038—

0.039

0.0460.0440.0450.0460.0410.0400.0520.0370.0400.0300.0470.034

0.0800.0260.0390.0450.0330.0270.0380.0350.0420.0330.0790.0310.0460.0520.0360.031

—0.039

_0.0330.038

0.028—

0.036

0.0320.0490.061

—0.054

—0.054

—0.057

—0.063

—0.0510.0500.050.040.050.040.050.050.06

0.060.05

_0.03

—

230Th(dpm g~1)

2.052—

1.831—

1.737—

1.933—

2.0621.6911.8691.6771.7081.7241.6812.1601.7661.7902.0441.756

_2.0712.1262.2112.373

.885

.8041.974J.OOO1.859.365.867

1.6432.8792.3812.5542.548

—2.441

—2.0552.631

—1.978

—3.469

—3.2392.4602.130

_2.448

—2.300

—2.060

_1.845

—2.2262.1542.2622.5092.3382.0203.0162.1232.489

2.1902.332

_2.406

—

±

0.100—

0.091—

0.088—

0.096—

0.1090.0930.0990.0930.0900.0890.0980.1040.0930.0920.1090.086

—0.1830.0630.1090.1260.0690.0610.0990.0980.0860.0600.1410.0700.1330.1190.1170.118

—0.120

—0.1090.123

—0.095

—0.157

—0.1570.1270.250

—0.142

—0.133

—0.130

—0.127

—0.110.110.110.110.120.090.210.110.14

—0.230.12

—0.08

—

471

G. B. SHIMMIELD, S. R. MOWBRAY

Appendix B (continued).

Core, section,interval (cm)

117-722B-(Cont.)

2H-2, 66-682H-2, 76-782H-2, 86-882H-2, 96-982H-2, 106-1082H-2, 116-1182H-2, 126-1282H-2, 136-1382H-2, 146-1482H-3, 06-082H-3, 16-182H-3, 26-282H-3, 36-382H-3, 46-482H-3, 56-582H-3, 66-682H-3, 76-782H-3, 86-882H-3, 96-982H-3, 106-1082H-3, 116-1182H-3, 126-1282H-3, 136-138

Depth(mbsf)

7.677.777.877.978.078.178.278.378.478.578.678.778.878.979.079.179.279.379.479.579.679.779.87

aAge(k.y.)

172.8175.2177.6180.0182.0184.1186.2187.7189.7191.1193.2197.6200.8204.3208.0211.5215.0218.5222.1225.8229.8233.4237.0

2 3 8U(dpmg"1)

2.145—

3.068—

2.265—

2.096

1.704—

2.037—

2.335—

1.351

2.390

3.121—

2.700

1.799

±

0.11—

0.14—

0.12—

0.12

0.06—

0.10—

0.10

0.05

0.11

0.16—

0.14

0.13

2 3 4U(dpmg"1)

2.290—

3.253—

2.408—

2.327

2.137—

2.123—

2.447—

1.475

2.470

3.376—

2.850

2.016

±

0.11—

0.15—

0.13—

0.13

0.07—

0.11—

0.10—

0.07

0.11

0.17—

0.15

0.14

232Th(dpmg !)

0.622—

0.755—

0.589—————__

0.313—

0.592

0.664—

0.636—

0.524—

0.337

±

0.06—

0.08—

0.05_————_—

0.02—

0.03

0.04

0.04—

0.04

0.03

23ftTh(dpm g"1)

1.998—

3.216—

2.283—_———_—

2.320—

1.567

2.496

2.524—

2.781

2.010

±

0.12—

0.19—

0.13———————

0.10—

0.06

0.10

0.13—

0.13—

0.09

a Stratigraphy from Clemens and Prell, pers. comm., March 1989.— Sample not analyzed.

472