4. PLANKTONIC FORAMINIFER FAUNAL VARIATIONS IN THE ...4. PLANKTONIC FORAMINIFER FAUNAL VARIATIONS IN THE NORTHEASTERN INDIAN OCEAN: A HIGH-RESOLUTION RECORD OF THE PAST 800,000 YEARS

Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 121

4. PLANKTONIC FORAMINIFER FAUNAL VARIATIONS IN THE NORTHEASTERN INDIANOCEAN: A HIGH-RESOLUTION RECORD OF THE PAST 800,000 YEARS FROM SITE 7581

Min-Te Chen2 and John Farrell2

ABSTRACT

We present a high-resolution (6 k.y. sample interval) record of planktonic foraminifer faunal variations for the past 800 k.y. fromODP Site 758 in the northeastern Indian Ocean. The record is examined within the context of a coarse fraction stratigraphy whichis a lithologic index of CaCθ3 preservation, and an oxygen isotope stratigraphy which provides a chronostratigraphy and a recordof climate change. Variations in the relative abundance of 27 planktonic foraminifer species primarily reflect fluctuations in theintensity of CaCθ3 dissolution. CaCθ3 dissolution covaries with climate fluctuations at a cyclicity of about 100 k.y. Glacial-agedsediments are generally well preserved, as indicated by faunal and lithologic indices. Interglacial-aged sediments show poorerpreservation. The -100-k.y. cycles are superimposed upon the long-term Brunhes Dissolution Cycle. This cycle is characterized byan interval of poor preservation centered between 400 and 550 ka and is bounded by good preservation events at 25 and 750 ka.Ecological factors also control variations in the foraminifer fauna. Changes in ecology are inferred from downcore fluctuations inthe relative abundances of foraminifer species that have a similar level of resistance to dissolution. We focus on three species(Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii) with a relatively high resistance to dissolution.The long-term increase in N. dutertrei since at least 800 ka is interpreted as either a gradual decrease in the sea-surface salinity, oran increase in the biogenic productivity of the surface waters. These factors may be controlled by the strength of the monsoon climate.Extremely high abundances of P. obliquiloculata are observed in certain downcore intervals and are interpreted as times when thesurface water conditions in the northeast Indian Ocean were similar to those in the modern western tropical Pacific, but not themodern Indian Ocean.

INTRODUCTION

A major objective of Ocean Drilling Program (ODP) Leg 121is the investigation of Neogene climate and ocean history in theIndian Ocean. A significant portion of this history is recorded inplanktonic foraminifers. Climate and ocean change is recorded byvariations in faunal composition, by fluctuations in the chemicalcomposition of foraminifer skeletons, and by the preservationstate of foraminifers. With faunal information, we can evaluatethe ecological response to climatically-induced changes in surfacewater conditions. Faunal assemblages also reflect the preservationstate of CaCθ3 on the seafloor which documents fluctuations inthe intensity of CaCOß dissolution, and thus changes in abyssalwater chemistry.

The sedimentary section recovered at ODP Site 758, locatedin the northeastern Indian Ocean (Fig. 1), provides an excellentopportunity to study oceanic and climatic change. The Quaternarysection was recovered in multiple advanced piston cores (APC)as a continuous and undisturbed sequence which accumulated atan average rate of 1.6 cm/k.y. (Farrell and Janecek, this volume).The specific objectives of this study are to: (1) document vari-ations in the relative abundance of planktonic foraminifer speciesin late Quaternary sediments (0-800 ka); (2) determine the timingand magnitude of faunal change; and (3) identify the ecologicaland preservational processes which affected the assemblages.

REGIONAL SETTINGSite 758 was drilled at 5°23.05'N, 90°21.67'E in a water depth

of 2924 m (Fig. 1). The site is located at the southernmost end ofthe Bay of Bengal, atop the Ninetyeast Ridge, at least 1000 m

1 Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991. Proc. ODP, Sci. Results,121: College Station, TX (Ocean Drilling Program).

2 Department of Geological Sciences, Brown University, Providence, Rl 02912-1846, U.S.A.

20°

10°

70° 75° 80? 85° 90° 95° 100° 105°

Figure 1. Location of ODP Site 758 in the northeastern Indian Ocean.

above the Bengal Fan. This fan is the world's largest, and it isbuilt of terrigenous sediments carried from the Himalayan regionby the Ganges-Brahmaputra, Irrawaddy, and Salween river sys-tems to the Bay of Bengal (Curray and Moore, 1971; Goldbergand Griffin, 1970). The Quaternary sediments at Site 758 arepredominantly biogenic calcareous ooze with secondary amountsof terrigenous clay and occasional intercalated volcanic ash layers(Shipboard Scientific Party, 1989).

125

M.-T. CHEN, J. FARRELL

The oceanography and climatology of the North Indian Oceanare strongly controlled by the monsoon climate regime which ischaracterized by the seasonal reversal of wind and surface watercirculation. In the northern Bay of Bengal, the surface watersundergo dramatic changes in salinity associated with the mon-soon. During the summer months (May-October), strong south-westerly winds transport large quantities of water vapor to theIndian subcontinent. Much of the water precipitated over land iscarried to the Bay of Bengal by the river systems. In the wintermonths (November-April), dry winds blow from the northeast.The volume of water discharged into the Bay of Bengal from theGanges-Brahmaputra and Irrawaddy river systems during thesummer is five times greater than during the winter (Rodolfo,1967). The variation in fresh water input to the Bay of Bengalproduces a seasonal salinity gradient in the surface waters. Duringthe summer monsoon, a steep salinity gradient, increasing to thesouth, exists along 90°E (Wyrtki, 1973). This gradient formsprimarily in response to riverine input, but is enhanced by theSouthwest Monsoon Current which carries high salinity waterinto the central Bay of Bengal from the equatorial Indian Ocean(Prell et al., 1980). Sea-surface temperature (SST) variations inthe Bay of Bengal are nearly constant over the year when com-pared to the fluctuations observed in the Arabian Sea (Wyrtki,1973). During the winter monsoon, the average temperatures inthe Bay of Bengal range from 26°C in the north, to 28°C in thesouth. During the summer monsoon, the temperatures are slightlyhigher and nearly isothermal, ranging from 28°C to 29°C (Wyrtki,1973). In the southernmost reaches of the Bay of Bengal, the SSTand the salinity show little variation throughout the year. In thisregion, near Site 758, the SST is 28.5±1.0°C and the salinity is33.8±O.3°/oo throughout the year (Wyrtki, 1988). These values aretypical of open ocean tropical regions.

Little is known about monsoon induced variations in biologi-cal productivity in the Bay of Bengal. Only in isolated locationswithin the Andaman Sea have distinct biological responses tomonsoonal induced upwelling been documented (Colborn, 1975).During the late Quaternary, variations in biological productivityin the Andaman Sea have been related to the strength of theNortheast winter Monsoon (Fontugne and Duplessy, 1986). Thevariation of productivity recorded at Site 758 would largely de-pend on the Northeast Monsoon strength and associated equato-rial current intensity.

METHODS AND DATA

Stratigraphy

Since sediment recovery gaps as large as 2.7 m occurredbetween almost every advanced piston core at Site 758, a com-posite depth section was constructed (Farrell and Janecek, thisvolume) by splicing together sedimentary intervals from Holes758A and B based on detailed correlations of high resolutionstratigraphies such as magnetic susceptibility. The compositedepth section provides a continuous record of the sedimentarysequence at Site 758. We used samples from Cores 121-758A-1Hand -2H which had recovery percentages of 102% and 104%,respectively. A sedimentary interval of approximately 1.42 mrepresenting approximately 60 k.y. is missing between the twocores (see Fig. 4 in Farrell and Janecek, this volume). This gap,which contains oxygen isotope stage 10 (using the nomenclatureof Imbrie et al., 1984), is successfully patched with the equivalentsedimentary interval from Core 121-758B-1H.

The chronostratigraphy used in this study is from Farrell andJanecek (this volume) and is primarily based on the correlation ofthe Site 758 oxygen isotope record from the planktonic fo-raminifer Globigerinoides sacculifer (300-355 µm size fraction)to the global average oxygen isotope record, the Spectral Mapping

(SPECMAP) stack (Imbrie et al., 1984; Prell et al., 1986). Agecontrol is also provided by the position of the Brunhes/Matuyamapaleomagnetic chron boundary (at 10.75 meters below sea floor(mbsf) which corresponds to 12.17 m on the composite depthscale). Our chronostratigraphy is consistent with the age con-straints provided by the biostratigraphic datums established on-board (Shipboard Scientific Party, 1989) and with thedisappearance of the pink variety of Globigerinoides ruber at 120ka (Thompson et al., 1979) which occurs between Samples 121-758A-1H-2, 131 cm and 121 cm.

Faunal Analysis

The 134 faunal samples used in this study are aliquots fromthe same samples used to determine %CaCθ3, coarse fraction, andstable isotope composition (Farrell and Janecek, this volume).The samples have an average spacing of 10 cm, which is equiva-lent in time to 6 k.y. The samples span from the core top (0.01 m)down to 13.83 m which corresponds to an age interval of 0-800ka. Preparation of the samples from the initial stages up to andincluding the separation of the >150 µm size fraction is describedin Farrell and Janecek (this volume). Subsequent sample prepara-tion and analysis follows the method outlined in Imbrie and Kipp(1971). Each sample is a split of the >150 µm fraction. Of the 134samples, 124 contain at least 300 whole foraminifers. The remain-ing 10 samples have experienced greater dissolution and thuscontain between 200 and 300 whole foraminifers. The relativeabundances of 27 species of planktonic foraminifers were calcu-lated for each sample and are listed in Table 1. The taxonomy ofplanktonic foraminifers used in this study is based on Parker(1962), Be (1967), and Kipp (1976). This taxonomy was also usedin previous studies of late Quaternary Indian Ocean foraminifers(e.g., Cullen, 1981; Cullen and Prell, 1984).

RESULTS

Relative Abundance of Planktonic Foraminifers

Descriptive statistics (Table 2) show that over the past 800k.y., the foraminifer assemblages are dominated by six specieswhich constitute 76% of the total planktonic foraminifer compo-sition. In order of decreasing mean abundance, the six species are:G. ruber, Pulleniatina obliquiloculata, Neogloboquadrina duter-trei, Globorotalia menardii, G. sacculifer, and Globigerinita glu-tinata. These six species show larger downcore variations thanthe other species, as indicated by the larger standard deviationsof the six.

Variations in the abundance of these six dominant species inthe tropical Indian Ocean have previously been attributed toCaCθ3 preservation and ecological factors (Cullen and Prell,1984). G. ruber and G. sacculifer are major components of surfacesediments underlying the warm waters of tropical oceans and arehighly-susceptible to CaCθ3 dissolution. G. glutinata is moder-ately-susceptible to dissolution, and is a dominant component inassemblages from regions influenced by upwelling, such as thewestern Arabian Sea and the northeastern Bay of Bengal (Cullenand Prell, 1984). The ecological role of this species is not wellunderstood but it may be an ecotone between upwelling andtropical faunal assemblages (Hutson and Prell, 1980), or it maybe associated with the margins of upwelling zones (Prell et al.,1990). P. obliquiloculata, N. dutertrei, and G. menardii are tropi-cal species generally considered to be more resistant to dissolu-tion (Berger, 1968; 1979; Parker and Berger, 1971; Adelseck,1978; Thunell and Honjo, 1981a; b). N. dutertrei and G. menardiiare highly abundant in regions characterized by a relatively shal-low thermocline, intense upwelling, and high productivity (Fair-banks et al., 1982; Thunell and Reynolds, 1984). These speciesare especially abundant in strong current systems near continental

126

PLANKTONIC FORAMINIFER FAUNAL VARIATIONS

margins (Be and Tolderlund, 1971). The ecological niche of P.obliquiloculata is unclear. The high abundance of this species inthe western-central tropical Pacific suggests that it prefers rela-tively warmer SST and/or a depressed thermocline. Nevertheless,in samples from the tropical Atlantic Ocean (Ravelo et al., 1990),this species occurred with N. dutertrei and G. menardii whichprefer a shallow thermocline.

Globigerina bulloides and Globorotalia tumida are relativelyminor but important components of the foraminifer assemblagesbecause respectively, they are strongly linked to ecological andpreservational conditions. G. bulloides is an important componentof subpolar-transitional faunas (Parker and Berger, 1971; Kipp,1976; Hutson and Prell, 1980) and is abundant in low latitudeupwelling regions such as the Arabian Sea (Hutson and Prell,1980; Prell and Curry, 1981; Prell, 1984; Cullen and Prell, 1984).G. tumida is most abundant in samples from the deepest watersand is quite rare in shallow water sites (Cullen and Prell, 1984).This observation is consistent with the conclusion that G. tumidais one of the most dissolution-resistant foraminifer species(Berger, 1968; 1979; Parker and Berger, 1971; Adelseck, 1978;Thunell and Honjo, 1981a; b).

Temporal Variation of Planktonic Foraminifers

The relative percentages of the species in the core top sample(121-758A-1H-1,1 cm) from Site 758 are quite similar to percent-ages observed in nearby surface sediment samples (Cullen andPrell, 1984). Downcore faunal assemblages at Site 758 can beinterpreted with respect to assemblages from modern sediments(Cullen and Prell, 1984; Prell, 1985) because of the similarity incore top results. Fluctuations in relative abundance of eight majorforaminifer species are presented with respect to age (Fig. 2). Theeight species include G. bulloides and G. tumida, in addition tothe six dominant species previously discussed. Particularly note-worthy among the trends shown in Figure 2 are:

1. The pulse of decreased abundances of G. ruber, G. saccu-lifer, G. bulloides, and G. glutinata between 490 and 550 ka, andthe concomitant increase in the abundance of P. obliquiloculata,G. menardii, and G. tumida.

2. A long-term cycle in G. tumida abundance characterized bylow values from 700 to 800 ka and from 0 to 10 ka, and high valuescentered between 400 and 550 ka.

3. Intervals, such as between 490 and 550 ka, in which theabundance of P. obliquiloculata was much greater than the maxi-mum value of 21% observed in surface sediments of the NorthIndian Ocean (Cullen and Prell, 1984).

Temporal Variations in the Resistant Species Ratio(RSP%)

Variations in foraminifer assemblages reflect changes in pres-ervation on the sea floor, as well as ecological changes at the seasurface. The preservation of planktonic foraminifers in the deepsea is controlled by the saturation state of the bottom waters andthe interstitial pore waters with respect to calcium carbonate,which in turn is a function of overall mixing rate and fertility(Berger, 1979). Quantitative (Thunell and Honjo, 1981a; b) andsemi-quantitative (Adelseck, 1978; Berger, 1968; 1979; Parkerand Berger, 1971; Malmgren, 1983; Peterson and Prell, 1985a)field and laboratory studies have established a relative ranking ofthe degree to which planktonic foraminifer species are susceptibleto CaCθ3 dissolution. For example, in surface sediments from theeastern equatorial Indian Ocean, Peterson and Prell (1985a) ob-served a systematic increase in the abundance of foraminiferspecies with thick skeletons, such as G. menardii, G. tumida, andiV. dutertrei, with increasing water depth. The abundance of thedissolution resistant species increases at the expense of less resis-tant species. In deep water samples, the assemblage contains a

reduced number of specimens from the species which are moder-ately-susceptible to dissolution, such as G. glutinata, N.hexagona, G. bulloides, and G. calida, and even fewer specimensof the highly-susceptible species such as G. ruber and G. saccu-lifer.

The foraminifer lysocline (FL) is the water depth which sepa-rates well-preserved from poorly-preserved foraminifer assem-blages (Berger, 1975). The depth of the FL in the Indian Oceanhas been determined quantitatively based on noticeable changesin the resistant species ratio (RSP) (Cullen and Prell, 1984). TheRSP% is defined (Cullen and Prell, 1984) as the sum of thepercentages of the following 14 dissolution-resistant species andvarieties: G. menardii, N. dutertrei, P. obliquiloculata, G. tumida,S. dehiscens, "pachyderma-dutertrei" intergrade, G. crassa-formis, G. menardii neoflexuosa, G. pachyderma right coiling, G.truncatulinoides right coiling, G. pachyderma left coiling, G.inflata, G. truncatulinoides left coiling, and G. humilis. In ourcalculation of the RSP%, we grouped G. menardii neoflexuosawith G. menardii, and we excluded G. pachyderma left coilingand G. humilis because they are absent from the assemblages. TheFL has been equated with the water depth where the RSP level isequal to 30% (Cullen and Prell, 1984). Values greater than 30%are considered to lie beneath the FL. Consistent with the originaldefinition of RSP%, the variations in faunal assemblages withRSP% greater than 30% are considered to be significantly influ-enced by CaCθ3 dissolution. Assemblages with RSP% values lessthan 30% lie above the FL and variations in these assemblages arethought to primarily reflect fluctuations in foraminifer input fromthe surface waters, and thus ecological factors. Nevertheless,some dissolution does occur above the FL (Peterson and Prell,1985a). In the Bay of Bengal, the FL deepens from -2200 m inthe northernmost regions, to 2750 m in the south (Cullen and Prell,1984). According to this estimate, the depth of Site 758 (2924 m)is presently located slightly below the FL. This estimate is con-sistent with the Site 758 core top RSP value of 36%.

In Figure 3, the RSP% of the samples from Site 758 is com-pared to the coarse fraction and to δ 1 8 θ . Coarse fraction is definedas the wt% of a sample >150 mm. Coarse fraction is a reliableindex of relative dissolution intensity in many regions (Berger,1970; Berger et al., 1982). Furthermore, since coarse fraction is alithologic index rather than a faunal index, it is an independentproxy indicator of dissolution. Low coarse fraction values areinterpreted as times of enhanced dissolution. The oxygen isotoperecord is a proxy indicator of global ice volume (Shackleton andOpdyke, 1973; 1976) and is thus used to identify glacial andinterglacial climate stages (Imbrie et al., 1984; Prell et al., 1986).As expected, a negative correlation exists between RSP% andcoarse fraction (Fig. 4) since dissolution is thought to control bothparameters. The correlation is noisy for several reasons. First,RSP% is controlled by ecological factors as well as by dissolution.Second, RSP% is probably more sensitive than coarse fraction tosubtle changes in the corrosiveness of the water. If this is the case,the dissolution response of RSP% would lead the coarse fractionresponse. In addition, the magnitude of the RSP% response is notnecessarily equal to the magnitude of the coarse fraction response.The increase of RSP% in the downcore sample is mainly attrib-uted to the increase of differential solution of foraminifer assem-blages. The most severe dissolution is indicated by the highestRSP% and the lowest coarse fractions. Likewise, the best preser-vation is indicated by the lowest RSP% and the highest coarsefractions. Using the narrow definition of the FL as the 30% RSPlevel, the downcore RSP% results indicate that Site 758 hasremained beneath the FL during most of the past 800 ka. The onlytimes during which the FL deepened to or below the depth of Site758 (2924 m) was during the latter half of glacial stages andduring transitions from glacials to interglacials. For example, the

127


Table 1. Planktonic foraminifer percentage data in the composite depth section from Hole 758A. Foraminiferal codes are listed in Table 2.

Core, sectioninterval (cm)

Compositedepth (m)

Age(Ma) O.unive G.cglob G.ruber G.tenel G.saccu S.dehis G.aequi G.calid G.bullo G.falco G.digit G.rubes G.pac R N.duter

1H-1, 1IH-1, 11IH-1, 21IH-1, 31IH-1, 41IH-1, 51IH-1, 611H-I, 71IH-1, 81IH-1, 91IH-1, 101IH-1, 111IH-1, 121IH-1, 131IH-1, 1411H-2, 11H-2, 111H-2, 211H-2, 311H-2, 411H-2, 511H-2, 611H-2, 711H-2, 811H-2, 911H-2, 1011H-2, 1111H-2, 1211H-2, 1311H-2, 1411H-3, 11H-3, 111H-3, 211H-3, 31IH-3, 411H-3, 51IH-3, 61IH-3, 71IH-3, 81IH-3, 91IH-3, 101IH-3, 111IH-3, 121IH-3, 131IH-3, 1411H-4, 11H-4, II1H-4, 211H-4, 311H-4, 41IH-4, 51IH-4, 611H-4, 711H-4, 81IH-3, 81IH-3, 91IH-3, 101IH-3, 111IH-3, 121IH-3, 131IH-3, 141IH-4, I1H-4, IIIH-4, 21IH-4, 31IH-4, 41IH-4, 51IH-4, 61IH-4, 71IH-4, 81IH-4, 91IH-4, 101IH-4, 111IH-4, 121IH-4, 131IH-4, 141

2H-12H-12H-12H-12H-12H-12H-12H-12H-12H-12H-12H-12H-12H-12H-1

1II2131415161718191101

121131141

2H-2, 1

0.010.110.210.310.410.510.610.710.810.911.011.111.211.311.411.511.611.711.811.912.012.112.212.312.412.512.612.712.812.913.013.113.213.313.413.513.613.713.813.914.014.114.214.314.414.514.614.714.814.915.015.115.215.315.325.425.525.625.725.825.926.026.126.226.326.426.526.626.726.826.927.027.127.227.327.427.437.537.637.737.837.938.038.138.238.338.438.538.638.738.838.93

0.0010.0060.0100.0150.0190.0240.0290.0340.0380.0430.0480.0530.0580.0630.0670.071Ash AAsh AAsh A0.076Ash B0.0790.0800.0890.0990.1070.1140.1220.1260.1310.1350.1420.1490.1570.1640.1710.1770.1830.1890.1940.2050.2110.2180.2230.2280.2320.2370.2400.2440.2470.2520.2570.2630.2680.2690.2780.2870.2950.3010.3060.3110.3160.3210.3260.3310.3340.3380.3410.3490.3570.3650.3730.3810.3880.3960.4040.4050.4190.4340.4430.4520.4600.4690.4780.4910.5130.525Ash C0.5380.5520.5560.559

0.8950.6190.7810.1380.8400.3250.7230.3690.7310.8830.3030.9550.4760.8290.4390.484

0.344

0.2251.5710.5360.0000.5360.5090.0000.3380.9760.5291.0240.8791.8440.4460.6290.8900.3820.2090.7040.3250.0000.0000.5920.3380.4800.5160.9570.4420.8570.0000.6990.1940.3981.5870.0000.0000.0000.2981.0940.9321.5562.3331.6781.1810.9591.3992.3440.9261.4121.6810.6921.1521.0260.4180.0001.5060.7811.2821.7241.6951.0290.6121.0250.5331.3450.000

2.0510.9431.3031.194

4.2512.4740.3120.6882.3532.6021.6272.2141.3162.2971.0611.2740.7141.3032.3390.000

1.375

0.8991.7453.3932.3212.6811.0192.8321.2391.3010.7411.5360.9771.9282.2321.0482.6711.7212.5111.4081.9480.8482.1692.0712.1960.4801.2890.2390.7360.8570.9930.4660.5832.7831.5870.0000.3560.3291.1900.0000.6991.1671.3331.0070.3940.7190.2330.3910.4630.7060.8401.0381.1520.5130.4180.5180.3012.7342.5642.8740.7261.3231.8352.4590.0001.3450.000

0.0000.9432.6060.000

21.92432.16535.15635.48821.34412.52017.72215.49817.25112.54415.90913.05718.80922.51214.32810.896

9.966

15.50624.08412.32113.03611.52810.5267.407

28.60421.62614.18023.55029.19926.15216.51818.86814.24316.82610.67016.6675.520

13.13616.14518.93516.89210.79118.55732.29719.58813.71412.25217.01620.77724.05627.38123.67315.30318.09216.96417.06812.12111.67312.33310.06717.32333.57335.19818.35915.27821.88218.06720.06922.81123.8466.695

12.43517.47010.92821.15419.15719.85521.61818.34914.75416.8003.1397.407

6.66712.26416.28720.597

0.0000.4120.1561.6511.8490.3250.0000.0001.3160.1770.7581.2740.9520.3562.3393.632

1.031

0.4490.3490.8930.8930.0000.3400.8710.3381.6262.4340.3410.9771.6760.8930.8390.5940.7650.8370.2350.0000.6360.4820.2960.3380.9590.0000.2390.1470.8570.9930.4660.7770.7950.3970.6641.7790.3290.8932.1880.4661.1670.0000.3360.0000.2401.1663.1253.2410.4710.0000.0000.6910.2560.0002.5910.0000.0002.1371.1491.2111.4710.3061.0252.4000.0000.463

0.0004.7170.0000.896

15.21315.87610.6258.2536.3878.1305.2445.1665.7024.5946.3643.8225.0008.7686.1404.601

6.873

11.4619.4247.500

11.9647.7755.603

14.37911.59910.8947.302

11.26311.62110.8979.8216.4998.6058.4134.1848.6857.4685.7209.157

10.05910.3042.638

10.56713.87614.1387.1439.934

11.42211.65014.31413.0953.9828.541

10.19710.4177.2212.7975.058

10.3334.027

11.41714.86813.9867.8125.0938.2359.2445.8828.0648.2055.8585.1819.337

15.23411.75213.21812.1077.5009.480

13.5252.4003.1398.333

5.6415.975

15.3098.060

0.8950.0000.0000.0000.3360.1630.1810.1850.0000.0000.1510.6370.0000.3560.1460.484

1.031

0.4490.5240.3570.7140.8040.0000.6540.1130.0000.2120.3410.0980.1680.2230.0000.0000.1910.2090.0000.3250.2120.2410.2960.1690.0000.0000.0000.0000.0000.0000.0000.1940.1990.0000.0000.0000.0000.0000.0000.0000.0000.0000.3360.3940.2400.0000.3910.0000.0000.0000.3460.4610.2562.0921.0361.2050.7810.4270.3830.0000.0000.3060.2051.3330.8970.926

0.0001.2580.6510.896

5.3694.1242.9694.5392.1851.7893.9784.9813.0702.1201.3643.1854.7622.1333.3161.453

1.718

2.6975.2362.6793.3933.2171.5282.8323.7162.1141.5871.8772.9302.0122.0093.3542.9671.5301.8833.5213.2471.0591.9282.0712.8721.4393.3512.1531.0311.2861.6561.6323.3012.7833.1751.1060.3561.6453.5711.7502.5643.1133.6671.3424.3314.0774.8951.9533.2413.0592.5211.7303.2263.3333.7662.5913.9165.4692.9922.8744.1163.3822.1411.0253.2002.6912.778

3.5902.5164.5601.791

1.1191.8561.4061.6512.0171.9511.0852.0292.3391.2370.9091.2742.6191.5402.7780.484

1.031

2.0222.2691.2502.1431.3410.8491.0892.0271.9512.1162.5602.0513.1851.3391.6772.9672.1031.6740.9390.3251.0590.9641.1830.8451.4392.3200.9571.3251.0000.9930.9321.7480.7951.1900.6641.0680.9871.4880.0000.4662.3350.6671.3422.7561.6791.6322.3442.3152.1180.8400.6920.9221.0262.5111.0361.5062.7341.4961.9161.4531.3232.1412.0490.5330.8970.926

0.5131.5721.9540.299

5.8174.9486.5626.1907.2278.1306.691

11.2557.3108.127

10.9099.2365.7145.4504.9717.748

5.498

7.1914.1887.5004.8214.0213.3968.9325.9696.6678.7835.6315.6643.6046.2507.3384.1543.6336.6952.3472.2738.4754.8195.9175.2365.5165.6702.8713.6827.0004.9678.3926.7963.1812.3818.6289.6096.9084.464

10.7229.5577.0045.0005.7053.1503.5973.7302.3448.3332.8243.3617.2662.9954.3591.2555.1814.5184.2975.1282.8743.8747.3533.3642.8694.5333.5872.778

0.5137.2334.5605.373

0.0000.0000.1560.1380.0000.3250.7230.1850.1460.1770.1510.3180.2380.1180.1461.453

0.000

0.4490.3490.8930.0000.0000.1700.0000.3380.6501.1640.5120.1950.1681.3390.2100.2970.0000.6280.0000.3250.6360.2410.0000.1690.7190.7730.0000.1470.8570.9930.4660.3880.5960.0000.4430.0000.0000.0000.8750.2330.0000.0000.0000.0000.0000.0001.5621.8520.9410.8401.0380.0000.5130.0000.0000.0000.0000.8550.5750.2420.2940.0000.0000.0000.0000.463

0.0000.6290.6510.299

0.0000.4120.0000.1380.1680.0000.1810.7380.4390.5300.0000.0000.9520.4740.4390.242

0.344

1.1240.8730.1790.3570.8040.5090.0000.1130.1630.2120.3410.0980.2520.2230.0000.2970.0000.4180.4690.0000.0000.7230.0000.0000.0000.0000.4790.0000.0000.0000.0000.5830.1991.1900.8851.0680.9870.2980.2190.4660.3890.0000.3360.3940.0000.2330.3910.9261.1770.0000.6920.4610.5131.2551.0360.9040.3910.2140.3830.0000.4411.2230.4100.5230.0001.852

0.5130.0000.6510.597

0.0000.0000.0000.0000.0000.3251.2660.0000.4390.3530.7580.6370.4760.2370.7310.726

0.344

0.0000.0000.0000.3570.0000.3400.4360.1130.1630.2120.3410.3910.2520.8931.0481.1870.9560.4180.0000.0000.2120.4820.0000.1691.1990.0000.0000.0001.1431.6561.1661.3590.7950.3971.5491.7790.0000.0000.4380.9320.3890.0000.3360.0000.0000.2330.7811.3890.2350.0001.3840.2300.0000.0000.5180.3010.0000.2140.7660.4840.8820.0000.2050.0000.0000.000

0.0000.6290.3260.000

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1180.0000.000

0.000

0.0000.0000.0000.1790.0000.1700.0000.0000.1630.0000.1710.0000.0000.4460.4190.8901.1470.6280.2350.0000.0000.0000.0000.0000.9590.0000.4790.1470.4290.0000.4660.0000.1990.0001.1060.0000.9870.2980.0000.0000.0000.3330.3360.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1470.3060.0000.0000.0000.463

0.0000.3150.0000.000

16.10710.51614.8447.978

19.32821.46319.34915.31412.86513.60411.6676.688

15.00017.41713.01216.707

14.089

12.8098.551

11.25015.17921.71617.99723.09412.83811.05717.77816.55311.03514.58517.41119.07822.25518.35615.69017.84010.71416.31419.51821.30217.23018.46514.17510.28716.20018.14318.21211.42218.25211.33212.69811.72615.65819.40811.30912.91017.71619.84418.00011.74521.26011.51110.72314.84415.74117.64714.70611.41911.06014.10315.90010.88113.55413.28118.37612.83514.04418.23517.43114.54910.93313.45317.130

16.41014.46511.07510.448

128


Table 1 (continued).

Core, sectionInterval (cm)

1H-1, 11H-1, 111H-1H-1H-1H-1H-1H-1H-1H-1H-

, 21, 131, 41, 51, 61, 71, 81, 91, 101

H-l , 1111H-1, 1211H-1, 131H-1, 1411H-2, 11H-2, 111H-2, 211H-2, 311H-2, 411H-2, 511H-2, 611H-2, 711H-2, 811H-2, 911H-2, 1011H-2, 1111H-2, 1211H-2, 1311H-2, 141IH-3, 1IH-3, 11IH-3, 21IH-3, 31IH-3, 41IH-3, 51IH-3, 61IH-3, 71IH-3, 81IH-3, 91IH-3, 101IH-3, 111IH-3, 121IH-3, 131lH-3,1411H-4, 11H-4, 111H-4, 211H-4, 311H-4, 411H-4, 511H-4, 611H-4, 711H-4, 81IH-3, 81IH-3, 91IH-3, 101IH-3, 111IH-3, 121IH-3, 131IH-3, 1411H-4, 11H-4, 111H-4, 211H4, 311H4, 411H4, 511H4, 611H4, 711H4, 81IH4, 91IH-4, 1011H4, 1111H4, 1211H4, 1311H4, 1412H-1, 12H-1, 112H-2H-2H-2H-2H-2H-2H-2H-2H-2H-2H-2H-2H-

, 21, 31, 41, 1, 61, 71, XI, 91, 101, 111. 121, 131, 141

2H-2, 1

Compositedepth(m)

0.010.110.210.310.410.510.610.710.810.911.011.111.21

.31

.41

.51

.61

.71

.81

.912.012.112.212.312.412.512.612.712.812.913.013.113.213.313.413.513.613.713.813.914.014.114.214.314.414.514.614.714.814.915.015.115.215.315.325.425.525.625.725.825.926.026.126.226.326.426.526.626.726.826.927.027.127.227.327.427.437.537.637.737.837.938.038.138.238.338.438.538.638.738.83S.93

Age(Ma)

0.0010.0060.0100.0150.0190.0240.0290.0340.0380.0430.0480.0530.0580.0630.0670.071Ash AAsh AAshA0.076AshB0.0790.0800.0890.0990.1070.1140.1220.1260.1310.1350.1420.1490.1570.1640.1710.1770.1830.1890.1940.2050.2110.2180.2230.2280.2320.2370.2400.2440.2470.2520.2570.2630.2680.2690.2780.2870.2950.3010.3060.3110.3160.3210.3260.3310.3340.3380.3410.3490.3570.3650.3730.3810.3880.3960.4040.4050.4190.4340.4430.4520.4600.4690.4780.4910.5130.525Ash C0.5380.5520.5560.559

G.cglom G.hexag

4.698 .1193.711 2.4741.875 2.0310.688 3.4392.8572.1143.2556.2734.3863.003

.681

.9S1

.989

.107

.901

.S903.030 2.1214.140 .2744.524 0.4765.569 2.0144.971 .6083.632 0.242

8.591

5.8436.108

.031

.124

.0477.321 0.7145.714 .4296.702 0.5362.886 0.8494.3572.5904.3902.6464.6074.8833.7723.5712.3062.374

.089

.378

.301

.270-.218-.832.760.786.145

..3742.868 0.5742.7209.390

.046

.1747.143 0.3254.873 0.6362.8925.3256.0812.3986.7012.6323.2401.5712.3182.5643.8836.5615.556

.928).888>.027).959).773>. 153.325.143.324

).233.359.392

1.3811.549 0.6642.8474.2766.2502.8452.5643.8919.3333.0205.9054.0773.7301.1721.8521.4128.8244.8443.456

).356.645

>.083).656).466).778.000

).000!.3621.316.865

).000).926*.059.681

).692.152

3.846 0.0000.8371.5544.518

).418.036.506

6.641 0.3913.2054.4063.8742.7945.1993.0742.6671.3452.778

1.5392.201

.068

.341

.453

.323).917).820).533).897).000

).000).629

2.280 0.9772.388 5.687

P.obliq

9.6207.8357.1888.253

12.43713.33311.75418.81919.59126.85518.03021.33820.47614.10014.18111.864

18.557

13.93312.56513.0369.464

10.99214.60110.2405.1809.919

12.69810.5809.4737.963

11.38410.48212.4638.031

18.61917.60617.20813.9839.398

10.05911.31814.62810.56711.24411.78216.71412.91413.0548.544

11.1339.1279.2929.6097.566

15.1799.847

11.42214.39716.33320.47011.0245.5165.3619.375

10.18512.23514.70619.72318.66414.61518.41015.02615.96413.2818.333

12.83514.2869.853

14.37316.59820.53333.63230.093

38.46121.38422.47622.388

G.infla

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

0.000

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000'0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.3970.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1470.0000.0000.0000.0000.000

0.0000.0000.0000.000

G.trn L

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

0.000

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

0.0000.0000.0000.000

G.trn R

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

0.000

o.ooo0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.2960.3380.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000o.oooo.ooo0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.2670.0000.000

0.0000.0000.0000.896

G.crasf

0.0000.0000.1560.0001.0080.8130.1810.5530.8770.8830.4550.0000.2380.2372.4852.179

0.687

0.6741.3960.7143.2141.0722.2070.4360.2250.3252.1161.8770.6841.5090.6700.4190.5941.5301.8830.9395.5202.9662.8920.8883.3784.0772.0621.4351.7672.2863.3111.3992.1363.5793.1753.0972.8473.2902.3813.0631.8650.7781.0002.6850.3940.7190.0001.9532.3150.7060.8400.6920.9220.5130.8370.0001.8071.5621.2820.5750.4840.4411.2231.4341.3330.8970.000

0.5130.0000.9771.194

P-D int

0.4470.2060.0000.1380.8401.1380.7230.1850.5850.5300.1510.0000.7140.2370.7310.726

0.000

0.4490.3490.3571.2502.4131.6981.9610.5631.9511.5870.0000.6841.0062.4552.7252.3742.1031.0461.6431.2993.8140.4820.5921.5202.6382.0620.4791.4731.2862.9800.2330.7770.9940.3973.0971.7793.2901A883.0632.5640.7780.0001.6781.1810.0000.0001.9530.4632.1180.8400.3460.2300.0002.0920.5180.0001.9530.6410.3830.7261.0291.5290.0000.0000.4482.778

0.0000.3150.0000.000

G.hirsu

0.0000.0000.3120.0000.0000.3250.5420.0000.2920.5300.1510.6370.4760.5920.2920.000

0.000

0.2250.1740.1790.1790.0000.0000.0000.2250.3250.0000.0000.3910.0841.3391.0480.0000.1910.2090.7040.0000.0000.0000.0000.1690.0000.2580.9570.2950.0000.0001.1660.0000.0000.3970.0000.3560.0000.0000.2190.0000.0000.3330.0000.0000.9590.0000.0000.9260.0000.0000.0000.2300.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

0.0000.0000.0000.000

G.scitu

0.0000.2060.4690.5500.1680.1630.0000.0000.1460.1770.6060.0000.0000.0000.1460.000

0.000

0.4490.0000.0000.1790.0000.0000.4360.1130.4880.2120.3410.0000.0000.0000.0000.0000.1910.0000.0000.0000.0000.7230.5920.1690.2400.0000.0000.0000.0000.3310.0000.1940.1990.0000.4430.0000.6580.2980.0000.0000.0000.3330.0000.0000.2400.2330.0000.0000.0000.0000.0000.0000.5130.0000.0000.3010.0000.0000.0000.0000.0000.0000.0000.0000.4480.463

0.0000.3150.0000.299

G.menar

7.8303.7112.9693.1646.5559.756

11.93510.5179.357

10.4249.848

18.79012.1437.3469.211

13.317

14.777

11.23610.82012.67911.96417.96219.69411.5474.7307.9688.5717.3385.6648.8018.2599.853

11.86912.42814.01711.97226.94817.58515.42211.53910.47315.58814.9496.2209.131

10.8579.934

15.8517.3796.5617.143

11.50411.74417.10513.98811.37917.01618.28813.33317.4509.8423.1183.030

14.45311.57410.35315.54613.49512.90315.64125.52317.09813.25312.1096.624

10.1537.7488.382

10.09210.04110.66718.8347.407

11.7957.2337.8187.761

Ctumid

0.6710.0000.0000.1380.5041.6260.9041.1070.2921.S901.2121.9111.1900.3561.9013.874

4.467

0.4490.5241.9641.0714.2893.2261.7430.4510.3250.6350.8530.2930.4191.3390.8390.5942.2943.9750.7045.8441.6950.7230.2961.0132.6380.2580.2390.5891.0001.3241.8650.5833.9762.3810.8850.7120.0001.7860.8754.6622.3351.6674.0272.7560.0000.2331.9531.38S0.4711.6811.3841.1521.2829.6239.8454.8194.6881.9231.9162.9062.5002.1413.6885.0679.8659.722

9.7444.4021.9543.582

G.gluti

4.0278.454

12.03116.7819.916

10.7329.9463.5059.6497.774

14.0919.5544.0487.938

13.45015.254

8.247

10.3377.853

14.28610.1791.609

11.8855.664

15.20313.65813.0166.1438.9847.9639.1528.1765.341

13.7679.8332.8173.2476.1448.6756.8056.757

11.7515.1559.809

12.81311.85712.9149.0918.5443.1813.968

15.04414.2352.3035.357

13.56710.4905.0582.667

12.0803.9379.592

12.12112.50011.5748.9413.7826.5748.0645.6412.092

11.9173.3132.7348.3337.6638.7178.5297.034

10.24615.7333.1393.241

2.05110.0633.5838.358

129



Core, sectioninterval (cm)

2H-2, 112H-2, 212H-2, 312H-2, 412H-2, 512H-2, 612H-2, 712H-2, 812H-2, 912H-2, 1012H-2, 1112H-2, 1212H-2, 1312H-2, 1412H-3, 12H-3, 112H-3, 212H-3, 312H-3, 412H-3, 512H-3, 612H-3, 712H-3, 812H-3, 912H-3, 1012H-3, 1112H-3, 1212H-3, 1312H-3, 1412H-4, 12H-4, 112H-4, 212H-4, 312H-4, 412H-4, 512H-4, 612H-4, 712H-4, 812H-4, 912H-4, 1012H-4, 1112H-4, 1212H-4, 1312H-4, 1412H-5, 12H-5, 112H-5, 212H-5, 312H-5, 41

Compositedepth (m)

9.039.139.239.339.439.539.639.739.839.93

10.0310.1310.2310.3310.4310.5310.6310.7310.8310.9311.0311.1311.2311.3311.4311.5311.6311.7311.8311.9312.0312.1312.2312.3312.4312.5312.6312.7312.8312.9313.0313.1313.2313.3313.4313.5313.6313.7313.83

Age(Ma)

0.5630.5660.5690.5720.5760.5810.5850.5900.5960.6070.6170.6230.6280.6340.6410.6480.6550.6620.6690.6760.6820.6870.6929.6970.7030.7080.7140.7190.7230.7270.7310.734Ash DAsh D0.7450.7480.7520.7560.7600.7650.7690.7730.7770.7820.7860.7900.7950.7990.806

O.unive

1.5920.5520.6620.0000.9261.5270.5100.3700.3971.1321.1360.3880.3790.6750.5340.1370.5711.1850.2290.6990.3900.6171.2601.1350.6491.9230.9781.1252.8171.9612.1791.013

1.7601.5600.7181.3670.5400.6410.8950.8440.5000.4840.7730.2200.0000.7330.969

G.cglob

0.5310.7350.3310.7962.3152.2900.5100.7410.3970.7550.5680.1941.7051.1801.9570.9560.0003.3176.1790.2331.3650.7721.1021.8441.2971.2241.0750.6430.9391.7650.8712.703

0.5501.4041.1960.4560.8992.3501.6102.9540.2500.9691.2370.4401.4440.2931.453

G.ruber

25.99517.6477.6165.5704.6309.733

11.39513.33310.7145.283

16.47724.27211.93216.69512.98916.12014.85722.98616.47611.42223.00231.94427.40227.80121.79023.95130.59626.36723.47418.62734.85822.973

32.67330.88921.77015.49020.68314.74428.08618.14316.75016.46519.93828.63420.39719.50125.666

G.tenel

4.2443.1250.0000.0000.0000.0001.3613.1480.3970.0001.4201.9422.4623.8793.2033.5523.4292.1330.0001.8650.0001.0800.9450.7091.6861.2240.1951.1251.6430.7840.2180.000

1.3201.2481.1961.1390.1800.0000.0000.0000.0000.2420.1550.0000.5421.0260.242

G.saccu

5.5707.1698.2786.897

11.1118.2065.4427.4077.540

10.18912.21611.26210.60612.6488.5419.6997.4297.1098.0095.128

10.52610.80312.28311.0647.5237.343

10.06810.2897.5129.6088.932

13.514

14.08113.72919.13913.21210.43211.11110.0188.6508.2508.9599.7379.6924.8747.478

10.412

S.dehis

0.5310.1842.3181.3261.6200.7630.5100.7411.5871.8871.1360.3880.5680.6750.5340.4100.5711.1851.6020.9321.1700.3090.6300.5671.1670.5251.5640.4820.4691.1772.1792.196

0.4400.7800.0000.4560.899

.496

.968

.266

.500..421.237.542.083.906

0.969

G.aequi

1.5922.2065.9604.2444.3983.2441.7012.0371.9843.7742.8412.5242.2732.6983.2032.0493.4293.0811.8312.3314.8732.4693.6225.5322.4642.4482.1504.3413.2864.1182.6144.730

2.6402.6522.6321.3671.7991.4960.8951.4771.2502.4212.6271.1010.7221.0262.179

G.calid

2.3871.4710.6620.5313.2411.9081.0202.2221.1902.6422.8412.7181.8941.1802.3131.7765.1431.4221.1440.6992.7290.4631.2602.2701.2972.7970.9781.1250.9391.7650.8710.845

1.4302.1841.1962.5061.7991.4962.1470.4221.2501.6951.5460.2200.9021.0261.937

G.bullo

4.2444.4123.3113.4489.491

11.8327.9938.3339.9217.925

11.0806.4087.954

10.2879.0755.6018.0003.5555.2637.9258.1876.3277.4027.3765.9666.1198.7006.1097.277

11.3733.9228.784

4.1805.1485.742

12.3019.7129.8299.1238.4397.7507.9907.7287.9308.4848.2118.717

G.falco

0.5310.3680.0000.2650.6940.7630.5100.7410.0001.1320.5680.7771.8941.3491.2460.5460.0000.9480.4582.3310.3900.3090.1580.8510.6491.2240.7820.4820.4690.9800.2180.845

0.4400.6240.7181.5940.8991.9230.3580.2111.0000.0000.3090.4400.9020.1470.726

G.digit

0.0000.1840.3310.7961.1571.3360.1700.1850.7940.7550.5680.3880.5680.3370.7120.4101.7140.7111.3730.2331.3650.0000.0000.1420.2590.1750.0000.1610.2350.1960.2180.676

0.0000.3120.2390.9111.0791.0680.7160.0000.0000.0000.0000.4400.1810.1470.000

G.rubes

1.5920.3680.0000.0000.0000.0000.0000.1850.0000.0000.0000.0000.0000.1690.1780.1370.0000.4740.0000.2330.0000.0000.0000.1421.2972.6220.1951.1252.1130.1960.0000.000

0.0000.9360.9570.0000.0000.4270.3580.2110.5000.0000.4640.4401.6251.4660.726

G.pac R

0.0000.1840.0000.5310.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1780.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.2350.0000.0000.000

0.0000.0000.0000.2280.0000.0000.0000.2110.0000.0000.0000.2200.1810.0000.000

N.duter

14.05821.32426.15916.97614.12017.93915.98623.70414.28613.58515.90912.03917.80311.97316.72615.16412.00012.32215.10318.41514.42510.6488.819

10.92215.56410.1408.993

10.2898.685

10.00010.89310.304

7.4817.4886.9385.923

12.2308.3339.302

11.39216.25010.17012.2107.709

17.27413.3436.053

FL appears to have deepened below the depth of Site 758 duringthe δ 1 8 θ stages 20, 18, and 6 and during the glacial to interglacialtransitions from δ 1 8 θ stages 18 to 17, 16 to 15, 10 to 9, 8 to 7, 6to 5, and 2 to 1.

The relationship among RSP%, coarse fraction, and δ 1 8 θ isgenerally characterized by low coarse fraction values and highRSP% during interglacial stages and high coarse fraction valuesand low RSP% during glacials (Fig. 3). The quasi-periodic 100-k.y. cycles of the late Quaternary are superimposed on a long-termdissolution cycle. The mid-point of this cycle is between 400 and550 ka, where dissolution is greatest as indicated by the highestvalues of RSP% (80%) and by the lowest coarse fraction values.The coarse fraction value of 1.89% at 510 ka (Sample 121-758A-2H-1, 101 cm) is the lowest value of the past 2.9 Ma (Farrell andJanecek, this volume). At the mid-point of the cycle, the increasein the abundances of the dissolution resistant species P. obliquilo-culata, G. menardii, and G. tumida is mirrored by the decreasedabundances of the dissolution susceptible species G. ruber, G.sacculifer, G. bulloides, and G. glutinata. The dissolution trendis clearly observed in the downcore record of G. tumida abun-dance (Fig. 2). At the beginning and end of the cycle, near 25 and750 ka, enhanced CaCθ3 preservation is indicated by high coarsefraction values and low RSP%.

DISCUSSION

Effects of Dissolution on Faunal Assemblages

The relative abundance of planktonic foraminifers distributedon the seafloor is largely determined by the carbonate chemistry

of the waters bathing the foraminifers. CaCθ3 dissolution deter-mines which species will be preserved in the fossil assemblage(Be and Hutson, 1977; Cullen and Prell, 1984). We documentvariations in CaCθ3 preservation based on faunal variations,faunal dissolution indices, and coarse fraction. The relationshipsbetween the coarse fraction values of each sample (a proxy fordissolution) and the abundances of eight major foraminifer spe-cies in the same sample are shown in Figure 5. The trends showthat high coarse fraction is generally associated with high percent-ages of dissolution-susceptible species G. ruber, G. sacculifer, G.bulloides, and G. glutinata. Likewise, low coarse fraction isassociated with high percentages of dissolution-resistant speciesN. dutertrei, G. menardii, P. obliquiloculata, and G. tumida.Good preservation is generally characterized by high abundancesof the dissolution-susceptible species and high coarse fractions.Poor preservation is marked by high abundances of the dissolu-tion-resistant species and low coarse fractions. The scatter aboutthe trends is most likely related to a variety of ecological and/ornon-preservational factors. Variations of %CaC03 at Site 758primarily reflect dilution by terrigenous sediments and are there-fore not a reliable index of dissolution (Farrell and Janecek, thisvolume).

The downcore pattern of dissolution occurs at a quasi-periodiccycle of ~IOO k.y., closely linked to the glacial-interglacial cli-matic record interpreted from the 818O to interglacials. Theseintervals are associated with the lowest RSP% and the highestcoarse fractions.

The atypical faunal composition observed between 490 and550 ka is attributed to a dramatic increase in CaCθ3 dissolution

130



Core, sectionInterval (cm)

2H-2, 1112H-2, 1212H-2, 1312H-2, 412H-2, 512H-2, 612H-2, 712H-2, 812H-2, 912H-2, 1012H-2, 1112H-2, 1212H-2, 1312H-2, 1412H-3, 12H-3, 112H-3, 212H-3, 312H-3, 412H-3, 512H-3, 612H-3, 712H-3, 812H-3, 912H-3, 1012H-3, 1112H-3, 1212H-3, 1312H-3, 1412H-4, 12H-4, 112H-4, 212H-4, 312H-4, 412H-4, 512H-4, 612H-4, 712H-4, 812H4, 912H-4.1012H-4, 1112H-4, 1212H-4, 1312H-4, 1412H-5, 12H-5, 112H-5, 212H-5, 312H-5, 41

Compositedepth(m)

9.039.139.239.339.439 539.639.739.839.9310.0310.1310.2310.3310.4310.5310.6310.7310.8310.9311.0311.1311.2311.3311.4311.5311.6311.7311.8311.9312.0312.1312.2312.3312.4312.5312.6312.7312.8312.9313.0313.1313.2313.3313.4313.5313.6313.7313.83

Age(Ma)

0.5630.5660.5690.5720.5760.5810.5850.5900.5960.6070.6170.6230.6280.6340.6410.6480.6550.6620.6690.6760.6820.6870.6920.6970.7030.7080.7140.7190.7230.7270.7310.734Ash DAsh D0.7450.7480.7520.7560.7600.7650.7690.7730.7770.7820.7860.7900.7950.7990.806

G.cglom

0.0000.9190.9930.7961.8521.3360.6801.1112.7780.3771.9891.5530.1892.0242.3132.0491.7142.3701.6020.6992.7293.5492.3622.2700.9080.8743.6170.3211.4081.3731.9611.689

0.4400.9360.4790.0000.0000.6410.5371.2661.0002.9060.9273.3042.5271.9062.906

G.hexag

1.5920.5520.6621.0611.1570.7630.0000.1852.3811.1321.9891.9421.7050.6751.2461.5030.0000.9480.9150.0001.1700.7721.8902.5530.5191.7481.4661.9290.9390.5880.0002.027

4.7303.4321.9142.2782.3381.4962.6831.4770.7502.4211.7002.6431.0831.1732.663

P.obliq

13.26311.39721.52329.97327.31522.32821.93912.40717.85720.7559.3758.544

11.74212.81613.34520.62817.1438.768

15.56116.08413.6459.877

14.17313.05014.6561S.90910.9488.682

18.07513.33310.89316.216

7.7018.112

16.98621.64019.06526.06818.96219.19818.50023.48715.76513.87713.71821.70117.918

G.infla

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1890.1690.1780.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

0.0000.0000.0000.2280.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

G.trn L

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.2290.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.2180.000

0.0000.0000.2390.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000

G.trn R

0.5311.4710.9931.5920.9260.5735.102.1.2960.7941.8870.2841.3591.1361.1801.2460.6831.1430.7111.1441.1660.1950.0000.1580.5671.0380.8740.1950.6431.6430.7840.8710.000

0.0000.1560.9570.4562.1580.6410.0001.0550.2500.4840.0001.5426.3180.5870.000

G.crasf

0.2651.1031.3240.7960.4631.1450.0000.1852.3811.8870.2840.0000.3790.5060.5340.5460.0001.8961.3730.0000.3900.1540.1580.7090.0000.1750.0000.0000.2350.0000.4360.169

0.1100.0000.2391.3670.1800.0000.0000.0000.0000.0000.0000.2200.3610.1470.484

P-D int

0.5310.0000.3310.2650.0000.1910.3400.1850.0000.0000.0000.0000.1890.0000.1780.1370.0000.0000.0000.0000.0000.1540.1580.4250.3890.1750.1950.3210.4690.1960.0000.000

0.2200.0000.2390.0000.0000.0000.1790.0000.0000.0000.0000.2200.1810.0000.000

G.hirsu

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1610.0000.0000.0000.000

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.1550.2200.0000.2930.000

G.scitu

0.5310.1840.0000.0000.0000.0000.0000.0000.0000.0000.5680.0000.3790.1690.0000.2730.0000.2370.0000.0000.0000.1540.1580.0000.0000.1750.0000.3210.2350.3920.2180.000

0.0000.0000.0000.4560.5400.0000.0000.2110.0000.0000.0000.0000.1810.1470.000

G.menar

6.6317.537

12.25214.3249.9547.4436.8037.778

16.27016.9815.6824.466

12.5008.6007.8296.9678.000

12.32211.89911.6557.6028.1797.5595.2489.728

10.1405.7676.752

10.09413.7259.3686.926

1.5402.6528.373

10.9348.6339.6155.188

10.54911.0006.295

12.2104.4057.0406.3053.874

G.tumid

2.9184.9635.6295.8362.5464.0083.9123.8893.5714.1511.1361.1650.7581.8550.8900.8204.0001.4221.1442.0980.5850.6170.3150.425I.g460.8740.9780.8040.9390.9801.9612.027

0.4400.7800.4791.3670.7191.9230.7161.2660.7500.9691.2371.5421.2631.3201.937

G.gluti

10.87511.9490.6623.9792.0832.672

14.1169.8154.7623.774

11.93217.67010.7958.263

10.8549.836

10.85710.9008.467

15.8515.263

10.8038.1894.3979.2097.343

10.55716.3995.8686.0786.1002.365

17.82214.9777.6554.3285.2164.7016.261

10.76012.50011.62210.04612.99613.71810.11710.170

Table 2. Codes and descriptive statistics for planktonic foraminifer percentage data from Site 758 and from modern Indian Oceansediments.

Foraminifer species

Orbulina universaGlobigernoides conglobatusGlobigerinoides ruberGlobigerinoides tenellusGlobigerinoides sacculiferSphaeroidinella dehiseensGlobigerinella aequilateralisGtobigerina calidaGlobigerina bulloidesGlobigerina falconensisGlobigerina digilataGlobigerina rubescensGlobigerina pachyderma R.Neogloboquadrina dulertreiGloboquadrina conglormerataGtoboquadrina hexagonaPulleniatina obliquiloculataGloborotalia inflataGloborotalia truncatulinoides L.Globorotalia truncatulinoides R.Globorotalia crassaformis"P-D intergrade"Globorotalia hirsutaGloborotalia scitulaGloborotalia menardiiGloborotalia tumidaGlobigerinita glutinata

Code

O.univeG.cglobGruberG.tenelG.saccuS.dehisG.aequiG.calidG.bulloG.falcoG.digitG rubesG.pac RN.duterG.cglomG.hexagP.obliqG.inflaG.trn LG.trn RG.crasfP-D.intG.hirsuG.scituG.menarG.tumidG.gluti

Average

0.81721.2957

18.31110.95079.05410.56672.74791.54966.26460.47030.41590.41130.1008

14.26843.08411.3808

14.72750.00980.00510.36261.03980-73240.12330.1310

10.40662.05718.7150

0-800 ka at Site 758

St. Dev.

0.56730.96467.12161.02933.16580.59991.14040.76482.43250.47710.40810.52160.22583.97492.02420.88735.68870.04840.03390.82241.05990.89340.25570.18634.33562.06683.9905

Minimum

0.00000.00003.13900.00002.40000.00000.35590.00000.51280.0000O.OOCO0.00000.00005.92260.00000.00005.18020.00000.00000.00000.00000.00000.00000.00001.54020.00000.6623

Maximum

2.81696.1785

35.48834.7170

19.13872.42135.96035.1429

12.30072.33101.851g2.62241.1472

26.15899.38974.7305

38.46150.39680.23926.31775.51953.81361.33930.7229

26.94809.8655

17.8218

Modern surface sediments (Prell

Average

0.56801.1826

16.73040.76837.01980.30522.65281.4470

14.70201.60760.56730.64222.07828.35391.36041.07033.60867.12211.21890.43590.14801.65650.22980.3476

10.00061.2848

12.8898

St. Dev.

0.75501.4198

13.31721.12135.83550.66682.16331.4611

15.87172.95091.14361.01636.79096.91102.23791.33893.7212

15.15512.66381.35290.45656.04870.65400.5476

12.39933.0309

10.4878

Minimum

0.00000.00000.00000.00000.00000.00000.00000.00000.29850.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.0000

, 1985)

Maximum

4.70358.1340

57.98327.7128

24.79345.62259.19229.1954

67.771119.718312.77015.2817

71.428635.156312.91147.9625

20.779283.642013.131310.58024.3902

83.33334.43694.6332

65.819223.465741.6667

131


I • • ' I ' I

21% (maximum value in modern Indian Ocean sediments)

0.0 0.4 0.5

Age (Ma)Figure 2. Percentage abundance of the eight major species of planktonic foraminifers plotted vs. age. The shaded region (490-550 ka) identifies atime of enhanced CaCθ3 dissolution characterized by low abundances of G. ruber, G. sacculifer, G. bulloides, G. glutinata, and high abundancesof G. menardii, P. obliquiloculata, G. tumida. The maximum value (21%) of P. obliquiloculata observed in modern sediments from the IndianOcean is marked by an arrow.

132


30% (regional modern lysocline)

0.0

0.0 0.1 0.2 0.3 0.4

Age (Ma)

0.5 0.6 0.7 0.8

Figure 3. Percentage abundance of A. Resistant species ratio (RSP%). Arrow shows the modern foraminifer lysocline as defined by the 30% RSPlevel (see text for explanation). B. Coarse fraction (wt%>150 µm), and C. δ 1 8 θ (%o to PDB of G. sacculifer from the 300-355 µm size range) fromSite 758 plotted against age.

since species of similar susceptibility to dissolution show a com-mon pattern. Between 490 and 550 ka, the relative percentages ofsusceptible species G. ruber, G. sacculifer, G. bulloides, and G.glutinata were significantly reduced while the abundances of theresistant species P. obliquiloculata, G. menardii, and G. tumidaincreased. This time interval is considered the mid-point of along-term dissolution trend. The trend is also clearly observed inthe RSP%, the coarse fraction record, and in the downcore abun-dance record of G. tumida. The dissolution trend is the "BrunhesDissolution Cycle" first described in sedimentary records fromthe equatorial Pacific (Adelseck, 1977). This cycle has beenobserved in other Indian Ocean records in water depths rangingfrom 533 m (Cullen and Droxler, 1990; Droxler et al., 1990) toover 4000 m (Peterson and Prell, 1985b). The cause of the cycleis unknown but has been related to forces both internal (Pisias andRea, 1988) and external (Jansen et al., 1986) to the climate system.

Ecological Factors Influencing Faunal Assemblages

The relative abundance of planktonic foraminifers distributedon the seafloor is determined not only by dissolution, but also bythe ecology of the overlying surface waters. Numerous studieshave documented the great utility of planktonic foraminifers inpaleoecological reconstructions. In sediments which have not

been significantly altered by CaCθ3 dissolution, variations inforaminifer faunal assemblages have been related to surface waterconditions (Imbrie and Kipp, 1971; Cullen, 1984). The mostimportant factors controlling foraminifer paleoecology includesea-surface temperature, salinity, and biological productivity.

As previously discussed, the SST in the Bay of Bengal is nearlyconstant on an annual basis. In addition, the SST during the lastglacial maximum (LGM), ~ 18,000 years ago, was nearly the sameas it is today. Reconstructions indicate a cooling of SST at theLGM of no more than 1°C in this area (CLIMAP, 1976; 1981;Prell et al., 1980; Cullen, 1981). The low seasonality in modernSST, and the stable response of SST to glacial-interglacial cyclessuggests that SST was probably not a major influence on faunalvariation at Site 758.

Past changes in the salinity gradient in the Bay of Bengal havebeen reconstructed along north-south transects based on plank-tonic foraminifer isotopes (Duplessy, 1982) and on faunal vari-ations (Cullen, 1981). The faunal proxy of the salinity gradientwas reconstructed along 90°E from 5°S to 20°N at three timeintervals: the LGM; the middle of the transition (MT) from theLGM to the Holocene near 9 ka; and the Holocene (Cullen, 1981).During the LGM, the salinity gradient was weakened by anincrease in salinity in the northern reaches of the Bay of Bengal.During the MT, the gradient was enhanced due to a freshening of

133


30^

2 0 -

10-

0 10 20 30 40 50

RSP%60 70 80

Figure 4. Scatter diagram of RSP% vs. coarse fraction. Enhanced dissolutionis characterized by high RSP% values which are associated with low coarsefraction values.

waters in the north. In the Holocene, the salinity gradient wassimilar to that observed in the modern Bay of Bengal. The changesin the north were attributed to fluctuations in monsoonal precipi-tation and riverine input of fresh water (Cullen, 1981). The highsalinity during the LGM was attributed to a reduction of mon-soonal precipitation and riverine input. Likewise, the low salinityduring the MT was attributed to an increase in precipitation andriverine input. Except for a brief period near the MT, the salinityin the southern Bay of Bengal (~5°N) has remained relativelystable at values between 33.5%o and 34.O%o. Between the LGMand the MT, the salinity of the waters above Site 758 increased toabout 34.5%o (Figure 20 in Cullen, 1981). This somewhat highersalinity in the central Bay of Bengal was attributed to an increasein the eastward advection of high salinity water carried by theSouthwest Monsoon Current, which in turn was related to astronger summer monsoon (Cullen, 1981). The largest change insalinity occurred in the northern part of the Bay of Bengal.Salinity at Site 758 was probably not directly affected by riverineinput as the site is located far from the coasts. Changes in intensityof the Southwest Monsoon Current, however, may have had aneffect on the salinity of the waters above Site 758. The degree towhich the salinity may have changed over Site 758 and theecological response of the foraminifers are discussed below.

If biological productivity at Site 758 changed significantlyover the past 800 k.y., it most likely changed in response tovariations in the upwelling of nutrient-rich waters. Upwelling atSite 758 is presently minimal. The site is away from the upwellingregions associated with the equatorial divergence and with coastalupwelling zones in the Andaman Sea and off the coast of Sumatra.In Holocene and LGM sediments, variations in productivity re-lated to upwelling intensity have been identified in the AndamanSea and the northeastern Bay of Bengal (Fontugne and Duplessy,1986). Greater organic carbon accumulation during the LGM wasinterpreted as indicating greater productivity and upwelling,which in turn was related to intensification of the Northeastmonsoon. Only minor amounts of siliceous microfossils, whichare considered an indication of upwelling and enhanced produc-tivity, occur in the Quaternary sediments at Site 758 (ShipboardScientific Party, 1989).

Ecological Change at Site 758

Having demonstrated that the late Quaternary faunal change atSite 758 is mainly controlled by the variation of CaCC«3 dissolu-tion, a dissolution-buffered strategy has to be used to extract theecological signal from the foraminifer abundance data. To mini-mize the effects of dissolution, which would increase the relativeabundance of dissolution-resistant species at the expense of thesusceptible species, we followed the method of Cullen (1981) andrecalculated the percentage of three dissolution-resistant speciesN. dutertrei, P. obliquiloculata, and G. menardii (Fig. 6). Thismethod allows us to examine ecological variations in abundanceof N. dutertrei, P. obliquiloculata, and G. menardii on a dissolu-tion-buffered basis because these three species have a similarresistance to dissolution.

In the recalculated abundances we observe large variations(10%-60%) in N. dutertrei which are superimposed on a long-term, but subtle increase of about 10% since at least 800 ka (Fig.6A). The high amplitude fluctuations, which occurred on a timescale of 10-50 k.y., were most common between 0 and 600 ka.We attribute these patterns to ecological factors which may reflectenvironmental change in the surface waters above Site 758. Twoecological factors have been associated with the abundance of N.dutertrei in surface sediment samples. In the North Indian Ocean,high abundances of N. dutertrei appear closely linked to lowsea-surface salinity (Cullen, 1981). Alternatively, high abun-dances of N. dutertrei in the eastern tropical Pacific (Berger,1973) and in the Arabian Sea (Cullen, 1981) have been interpretedas indicating a shallow thermocline, a high nutrient level, and thushigh productivity. To remove the dissolution imprint from themodern faunal abundance data (Prell, 1985) and to extract theecologically controlled pattern of N. dutertrei, we recalculated theN. dutertrei percentage within three dissolution-resistant speciesP. obliquiloculata, G. menardii, and N. dutertrei (Fig. 7). Thisrecalculation was identical to that performed on the downcoresamples from Site 758. The core top values shown in Figure 7 arethose in which the original sum of P. obliquiloculata, G.menardii, and N. dutertrei is greater than 30%. This level waschosen to avoid core tops with low abundances of these threespecies. Low abundances result in relative proportions which arenot statistically significant.

High abundances of N. dutertrei in core tops are observedbeneath waters with lowest salinities (<34%o) in the northern Bayof Bengal, in the Andaman Sea, and off Sumatra Coast. Lowestabundances of N. dutertrei were observed in the Arabian Seawhere salinity exceeds 36%o. Based on this observation, a simpleinterpretation of the increase in the abundance of N. dutertrei withtime is a long-term decrease in salinity at Site 758. Likewise, thelarge amplitude and high frequency fluctuations in the abundanceof N. dutertrei may reflect relatively large and abrupt changes insalinity. Due to the great distance between Site 758 and the coast,it is unlikely that river run-off into the northern Bay of Bengaldirectly affected the salinity of the surface waters in the southernBay of Bengal. A reduction in the salinity near Site 758 may haveresulted from an increase in the advection of low salinity watersfrom the east through the Lombok Strait in response to a strength-ening of the Southeast Asian Monsoon (Murray and Arief, 1988).Alternatively, there may have been a decrease in the advection ofhigh salinity waters from the west by the Southwest MonsoonCurrent due to a weakening of the Southwest Monsoon. Finally,the salinity at Site 758 may have changed in response to regionalfluctuations in the ratio of evaporation to precipitation.

We also observe a high concentration of N. dutertrei in coretops from regions where the thermocline shoals and upwelling ofnutrient-rich water occurs. N. dutertrei is numerous in the central

134


Dissolution-susceptible species

A

+ + ÷ + -H- +

5 10G. bulloides%

B

15 0 10G. gluünata%

Dissolution-resistant species

10 20 30P. obliquiloculata%

40 0G. tumida%

Figure 5. Scatter diagrams of coarse fraction vs. percentages of dissolution-susceptible species: A. G. ruber. B.G. sacculifer. C. G. bulloides. D. G. glutinata. Dissolution-resistant species: E. N. dutertrei. F. G. menardii.G. P. obliquiloculata. H. G. tumida. The lines are simple linear regressions. These plots show the relationshipsbetween foraminifer abundances and dissolution intensity. Strong dissolution is characterized by low coarsefraction and low percentages of dissolution-susceptible species (A-D). Enhanced preservation is characterizedby high coarse fraction and low percentages of dissolution-resistant species (E-H).

135


i i i

1034% (maximum value in modern Indian Ocean sediments)

0.4

Age (Ma)0.5 0.6 0.7 0.8

Figure 6. Recalculated percentage abundances of three dissolution-resistant species A. N. dutertrei. B. G. menardii. C. P. obliquiloculata plotted againstage. The line in A is a simple linear regression which shows a 10% increase in N. dutertrei since 800 ka. For comparison to downcore values, the arrowin C shows the maximum surface sediment abundance (34%) of P. obliquiloculata from Prell (1985) in terms of recalculated percentage abundances(see text for discussion). Samples at 43 ka, 538 ka, and 765 ka contain anomalously high P. obliquiloculata abundances and are marked with a "*".

and south tropical Indian Ocean underneath the North and SouthEquatorial Current where there is a strong equatorial divergence(Fig. 7). High concentrations of N. dutertrei are also observed inthe regions of coastal upwelling in the Andaman Sea and SumatraCoast (Fig. 7). The intensity of this coastal upwelling may becontrolled by the strength of the Northeast Monsoon (Fontugneand Duplessy 1986). Based on this observation, changes in theabundance of N. dutertrei overtime at Site 758 could be explainedby fluctuations in productivity. These changes may be linked tothe intensity of the Northeast Monsoon or the strength of theequatorial divergence.

A second intriguing ecological pattern observed in the Site 758record is the anomalously high abundances of P. obliquiloculata.Several downcore samples from Site 758 have abundances of P.obliquiloculata that are much greater than the abundances ob-

served in all (290) core top samples from the Indian Ocean (Prell,1985). The downcore samples have greater abundances whencalculated either as a percentage of all 27 species (Fig. 2) orrecalculated as a percentage of the three dissolution resistantspecies (P. obliquiloculata, G. menardii, and N. dutertrei) (Fig.6C). This indicates that CaC03 preservation is not controlling theanomalously high abundances. For example, the highest abun-dance of P. obliquiloculata in core tops from the Indian Oceanoccurs off the Sumatra Coast where values reach 21% (databasefrom Prell, 1985) (Table 2). When recalculated as a percentage ofonly the dissolution resistant species, the P. obliquiloculata per-centages in surface sediments are as high as 34%. In the recordfrom Site 758, the percentage of P. obliquiloculata with respectto the entire fauna exceeds the surface sediment values duringinterglacial δ 1 8 θ stages 3, 13, 14, 15, and 21 (Fig. 2). Likewise,

136

40° 50c 60° 70° 80c


90° 100°

N. dutertrei%

%>50

%>40

%>30

20°

10°

10c

20°

Figure 7. Spatial distribution of the recalculated abundance of N. dutertrei in core top samples from the northIndian Ocean (database from Prell, 1985). High abundances are distributed in the Andaman Sea, off the SumatraCoast, and beneath strong equatorial current systems of the tropical Indian Ocean.

when examined on the dissolution-buffered basis, the recalculatedpercentage of P. obliquiloculata abundances in Site 758 oftenexceeds the recalculated core top value of 34% (Fig. 6C). Highabundances off. obliquiloculata in downcore samples at Site 758are, however, similar to the high abundances observed in core topsfrom the western tropical Pacific (Cullen and Prell, 1984).

We compared the faunal compositions of the core tops fromthe tropical Indo-Pacific with three representative downcore sam-ples from Site 758 which have anomalously high abundances ofP. obliquiloculata (samples from δ 1 8 θ stage 3, 14, 21 that aremarked with an " * " in Fig. 6C). First, we recalculate the core topdatabase (Prell, 1985) in terms of the three dissolution-resistant(P. obliquiloculata, G. menardii, and N. dutertrei) species. Sec-ond, we calculated the dissimilarity coefficient (using a squaredchord distance) between the core top samples and the three down-core samples. In Figure 8, we observe a low dissimilarity betweenthe core top samples from the western tropical Pacific and thedowncore samples from Site 758. We assume that similar faunasinhabit similar environments. Based on this assumption, the simi-larity of the core tops from the Pacific with the downcore samplessuggests that surface water conditions in the modern westerntropical Pacific were analogous to those in the northeastern IndianOcean at certain times in the past. One possible explanation forthe similar conditions during these times, is an increase in theadvection of warm surface waters through the Indonesian Archi-pelago and into the northeastern Indian Ocean. This contention issupported by the observation of generally higher percentages ofP. obliquiloculata during interglacials, when the throughflowbetween the tropical Indian and Pacific Oceans was probablygreatest. During glacial times, low stands of the sea most likelyrestricted the exchange of water between these two regions. Al-ternatively, local Oceanographic and climatic factors, such as thedepth of the thermocline and the ratio of evaporation to precipi-tation, may have produced surface water conditions in the north-

eastern Indian Ocean that were similar to those presently foundin the western tropical Pacific.

CONCLUSIONS

1. Over the past 800 k.y., variations in the relative abundanceof planktonic foraminifers at Site 758 were predominantly con-trolled by fluctuations in CaCθ3 dissolution.

2. Dissolution varied at a cyclicity of -100 k.y., in step withglacial/interglacial fluctuations. CaCθ3 preservation was gener-ally good during glacial intervals and poor during interglacials.Greatest preservation occurred on climate transitions from gla-cials to interglacials.

3. The -100 k.y. cycles are superimposed upon a long-termBrunhes Dissolution Cycle. The mid-point of this cycle, wheredissolution was greatest, is centered between 400 and 550 ka andis bounded by strong preservation at 25 and 750 ka.

4. Variations in the dissolution-buffered abundance of N. du-tertrei are attributed to ecological factors, most likely a decreasein sea-surface salinity or an increase in productivity.

5. The abundances of P. obliquiloculata in several downcoresamples from Site 758 are greater than the abundances observedin core top samples from the Indian Ocean, but are similar to thosefound in core tops from the western tropical Pacific. This suggeststhat surface water conditions in the modern tropical Pacific weresimilar to conditions in the northeastern Indian Ocean at certaintimes in the past.

ACKNOWLEDGMENTS

The authors are grateful to A. Martin for assistance withplanktonic foraminifer taxonomy and to W. Kang and T. Saha forhelp with sample preparation. We thank R. C. Thunell and T.Patterson for constructive reviews. Our work has greatly benefit-ted from discussions with J. L. Cullen, W. Howard, D. W. Murray,W. L. Prell, and Kuo-Yen Wei. This research was supported by

137


40° 60c 80° 1000 120° 140° 160° 180° 160° 140°

I I I I

22

m •

43 ka (δ 1 8 θ stage 3)

538 ka (518O stage 14)

765 ka (δ 1 8 θ stage 21)

20°

10°

0°

10°

20°

20°

10°

0°

10°

20°

20°

10°

0°

10°

20°

160° 180° 160° 140°

Dissimilarity coefficient(squared chord distance) xlOO

<5 <IO

Figure 8. Spatial distribution of core top samples from the Indo-Pacific which contain foraminifer assemblages that are similar to the assemblages in

three downcore samples (at -43 ka, -538 ka, and -765 ka) from Site 758. The core top samples which are most similar to the three downcore samples

are from the tropical western Pacific and are considered modem analogues of the downcore samples from Site 758. The modem analogues are indicated

by the lowest dissimilarity coefficients and are mapped as the shaded regions.

grants from JOI/USSAC (TAMRF-20244) (J. W. Farrell and T.Janecek), NSF/OCE 8911874 (D. W. Murray and J. W. Farrell),and a Brown University Teaching Assistantship (1989-1990)awarded to Min-Te Chen.

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