Pisciotto, K. A., Ingle, J. C, Jr., von Breymann, M. T, Barron, J., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 127/128, Pt. 1

24. PLIOCENE-PLEISTOCENE FLUCTUATIONS IN COMPOSITION AND ACCUMULATION RATESOF EOLO-MARINE SEDIMENTS AT SITE 798 (OKI RIDGE, SEA OF JAPAN) AND CLIMATIC

CHANGE: PRELIMINARY RESULTS1

M. Dersch2 and R. Stein2

ABSTRACT

Preliminary bulk and clay mineralogy data as determined by XRD as well as accumulation rates of quartz and feldspar at OkiRidge Site 798 give important information about the paleoclimate in Asia and transport mechanisms of the siliciclastic sedimentfraction. The (?uppermost Miocene/) lowermost Pliocene sediments at ODP Site 798 are characterized by high amounts offeldspars, smectites, and additionally abundant sand-sized terrigenous components. This is taken as an indication for turbidity-current-influenced deposition and a source area of the material different from that of the upper Pliocene/Pleistocene sediments.A distinct increase in accumulation rates for quartz and feldspar occurs near the Gauss/Matuyama boundary. We attribute thisprominent shift to higher eolian sediment supply, caused by the increased aridification of Asia, which was probably triggered bythe development of major Northern Hemisphere glaciation near 2.5 Ma. The late Pliocene/Pleistocene interval is characterizedby short-term "Milankovitch-type" cyclic changes in terrigenous sediment composition most probably reflecting glacial/inter-glacial arid/humid climate cycles.

INTRODUCTION

During Leg 128 of the Ocean Drilling Program (ODP), three siteswere drilled in the Japan Sea with the purpose of reconstructing boththe development of the back-arc basin and the origin and genesis ofits sediments in relation to the regional paleoclimatic and paleocean-ographic evolution (Ingle, Suyehiro, von Breymann, et al., 1990). Asshown in Figure 1, the sites were drilled in the Kita-Yamato Troughwithin the larger Yamato Rise (Site 799), in the northern part ofYamato Basin (Site 794) and—to recover a paleoceanographic refer-ence section—on top of Oki Ridge (Site 798).

The major objective of this study is to reconstruct climate-inducedchanges in terrigenous sediment supply from the Asian continent inorder to create an important link between continental loess records inAsia (e.g., Burbank and Jijun, 1985; Kukla, 1987; Tungsheng, 1988)and eolo-marine records in the Pacific Ocean (e.g., Rea and Janecek,1982; Pisias and Leinen, 1984; Hovan et al, 1989). The study is basedon determinations of accumulation rates and composition of terrige-nous matter (bulk and clay mineralogy).

GEOLOGICAL SETTING AND MODERNTERRIGENOUS SEDIMENT SUPPLY AT SITE 798

Site 798 (37.04°N, 134.80°E; 903 m water depth) is located in thesoutheastern Japan Sea, about 160 km north of the western coast ofHonshu in a small sediment-filled graben on top of Oki Ridge (Fig.1). This shallow-water position, well above the modern calcite com-pensation depth (CCD) (1500-2200 m; Ujiie and Ichikura, 1973) andon a structurally isolated height (Fig. 1), was chosen to obtain acarbonate-rich sequence undiluted by coarse-grained gravity-flowsediments common to the basinal areas (Ingle, Suyehiro, von Brey-mann, et al., 1990). At Site 798, a 517-m-thick pelagic to hemipelagicsediment sequence of (latest Miocene/) early Pliocene to Holoceneage was drilled (Ingle, Suyehiro, von Breymann, et al., 1990; L.Burckle, pers. comm., 1990). Diatomaceous ooze, diatomaceous clay,silty clay, clay, and siliceous claystone are the predominant sediments

Pisciotto, K. A., Ingle, J. C , Jr., von Breymann, M. T., Barron, J., et al., 1992. Proc.ODP, Sci. Results, 127/128, Pt. 1: College Station, TX (Ocean Drilling Program).

2 Alfred-Wegener-Institute for Polar and Marine Research, Columbusstrasse, W-2850Bremerhaven, Federal Republic of Germany.

with common foraminifers and calcareous nannofossils in the upper220 m. In addition, this upper 220 m is characterized by rhythmicchanges between dark, laminated, diatom- and organic-carbon-richintervals and light, non-bioturbated to intensely bioturbated, clay-richintervals (see also Föllmi et al., 1990). The intercalated volcanic ashlayers show a maximum occurrence between 0.9 and 0.3 Ma (Ingle,Suyehiro, von Breymann, et al., 1990). Shipboard paleomagnetic andbiostratigraphic data (Ingle, Suyehiro, von Breymann, et al., 1990)give sedimentation rates in the range of 9.2-21.7 cm/1000 yr with anaverage sedimentation rate of 12 cm/1000 yr for the entire sequenceof Site 798.

Terrigenous sediment supply to the Japan Sea is strongly influ-enced by the prevailing westerly winds (Fig. 2; Tungsheng, 1988) aswell as the Pacific drainage system of major rivers from the Asiancontinent and the Japanese Islands. The sources of the transportedsediment components at Site 798 can be derived from distant, aridareas, such as the Gobi desert region, as well as more proximal coastalareas surrounding the Sea of Japan (Fig. 2). Because of its isolatedposition far away from the continent, the modern terrigenous sedi-ment supply at Site 798 is restricted mainly to an eolian input. Thus,the investigation of accumulation rates, composition, and grain sizeof these terrigenous sediment components may provide informationabout the paleoclimate of the source area and intensity of the windsystem (e.g., Sarnthein et al., 1981, 1982; Stein, 1985a, 1985b; Reaand Janecek, 1982). Furthermore, correlation of the deposition ofwind-blown loess in central and eastern China, eolian sediments inthe North Pacific Ocean, and the marine delta18O record (e.g., theglobal climate record) indicates a link between changes in aridity inAsia and global glacial/interglacial climate cycles (e.g., Kukla et al.,1988; Tungsheng, 1988; Hovan et al., 1989), which should be re-corded in the sediments at Site 798.

METHODS

Bulk Mineralogy

After 15 wt% corundum was added as an internal standard, allsamples were ground 15 min using a grinding mill in order to getwell-homogenized samples. X-ray-diffraction (XRD) measurementswere performed on randomly oriented, pressed-powder slides bymeans of a Siemens XRD system (D 501). The diffractometer hascopper Kα radiation combined with a Na Iodide detector.

409

M. DERSCH, R. STEIN

45°N

40°

SEA OF JAPAN

130°E 140°

NW

JS8

20

B20 40 60 80 100 km

Vertical exaggeration = 20 S E

1000 jE,

2000 Q-Q

3000

4000

Figure 1. Location map (top) of DSDP Leg 31 Sites 299-302 and ODP Leg 127 Sites 794-797, and Leg 128 Sites 798 and 799, and 794, which were returned tofor drilling. InteΦreted northwest-southeast single-channel seismic-reflection profile (bottom) showing the location of Hole 798B at the top of Oki Ridge. Therock units are G = granite, V = volcanic rock, B = unknown basement rock, M = Miocene sediments, P = Pliocene sediments, Q = Quaternary sediments (profilefromTanaki, 1988).

For the bulk mineralogy, the samples were measured from 2°to 62° two theta (2θ) at 0.03° steps/s. Quartz and feldspar data,presented in this study, are given in weight percentages of bulksediment with a relative error of <5 wt% (for the method ofquantitative and qualitative determination of bulk mineralogy seeEmmermann et al., 1989).

Clay Mineralogy

After the dissolution of carbonate by 50% acetic acid, the < 2-µmfraction was separated by the Atterberg method (Muller, 1967).

After the addition of 1 wt% MoS2 as an internal standard, thesamples were sucked onto cellulose nitrate filters under vacuumconditions to produce texturally oriented samples. These sampleswere measured by means of a Phillips PWI700 XRD system withcobalt Kα radiation. The measurements were performed on bothuntreated samples and glycolated samples from 2° to 40° 2θ at0.02°-step width/2 s. Additionally, each sample was measured be-tween 28° and 30.5° 2θ at 0.005°-step width/2 s to separate thekaolinite and chlorite more accurately.

Semiquantitative evaluations were based on peak heights, whichwere multiplied by "Biscaye-factors" (Biscaye, 1965): smectite, 17

410

ACCUMULATION RATES AND CLIMATIC CHANGE

14080 90 100 110 120 1

2020

90 100 110 120 130

\yC\ Source Area of Loess Chinese Border

f^\ \ j Deposition Area of Loess ® Site Position

^ ™ ^ * ~ Westerlies

Figure 2. Source region and deposition region of loess and prevailing westerlywinds (after Tungsheng, 1988).

Å glycolated peak height (×l); illite, 10 Å peak height (×4), andchlorite-kaolinite, 7 A peak height (×2). The 7 Å-peak is sepa-rated into a kaolinite and chlorite proportion using the intensityratio of the kaolinite (002)-peak (3.58 Å) and the chlorite (004)-peak(3.54 Å) (for discussion of the method see Gibbs, 1967; Elverh0iand R0nningsland, 1978). The sum of the weighted peak heightsaccounts for 100% of the clay mineralogy of the sample. A moredetailed study of clay (and bulk) mineralogy based on peak areas isin progress (Dersch, unpubl. data).

Calculation of Accumulation Rates

Mass-accumulation rates were calculated according to Van Andelet al. (1975), using physical-property and mean sedimentation ratedata from Ingle, Suyehiro, von Breymann, et al. (1990) and L. Burckle(pers. comm., 1990), as follows:

MAR = LSR (WBD-1.026 PO/100),

where MAR = mass accumulation rate (g cm"2 k.y.~1), LSR = linearsedimentation rate (cm/1000 yr), WBD = wet-bulk density (g/cm3),and PO = porosity (%). The MAR of quartz and feldspar can thenbe calculated by multiplying the MAR by the weight percent valueof quartz and feldspar, respectively. Using these accumulationrates, dilution effects by other components can be excluded. Fur-thermore, the compaction problem can be eliminated and a com-parison of sediment accumulation on both a temporal and spatialbasis is possible, whereas further diagenetic influence has not beentaken into account.

These calculations were performed on the basis of an averagesample spacing of 1.50 m, approximately 12,000 yr, respectively. InCores 798-13H to 798-15H, a time resolution of approximately 3000yr was reached, using sampling intervals of 0.30 m.

RESULTS

Composition of the Siliciclastic Sediment Fraction

In order to get information about the composition of the siliciclas-tic (i.e., terrigenous) sediment fraction the amounts of quartz and

feldspar as well as illite, chlorite, kaolinite, and smectite were deter-mined by XRD (Tables 1 and 2).

The entire sediment sequence at Site 798 is characterized by quartzcontents ranging between 5% and 20% (Fig. 3). At about 290 m belowseafloor (mbsf) (i.e., near the Gauss/Matuyama boundary), a distinctincrease in the amplitude of fluctuations occurs. Thus, the upper 290m (i.e., the last 2.47 Ma) show distinct short-term fluctuations. Asection within the detailed sampling interval between 115 and 143mbsf reveals a 40,000-yr cycle (Fig. 3; for details of the short-term"Milankovitch-type" cycles, deduced from the logging data, seedeMenocal et al., this volume).

The feldspar content varies between 2% and 20% (Fig. 3). Maxi-mum values of 15%-20% are concentrated in the lowermost 80 m(440-520 mbsf), between 120 and 200 mbsf, and in the upper 40 mof the record.

Most of the quartz/feldspar ratios vary between 0.5 and 3 (Fig. 3).Low ratios are typical in the lowermost 80 m of the sediment sequence(ratios of 0.5-1.5) and in the uppermost 100 m (ratios of 0.5-2). Thehigher ratios of >2 are found between 420 and 120 mbsf. Isolatedpeaks with ratios of more than 4 occur at 350, 270, and 150 mbsf.

Clay mineral data are shown in Figure 4. In the upper 413 m, theclay fraction at Site 798 is dominated by illite with values between60% and 88% (mean value: 78%) whereas the lowermost part showsdistinctly lower values between 30% and 60% (mean value: 42%;Fig. 4).

The chlorite content varies between about 5% and 27% (Fig. 4).A short sequence in the lowermost part of the sediment record withvalues between 4% and 14% (500-320 mbsf) is followed by a distinctincrease in both maximum values and amplitude near 320 mbsf

In general, kaolinite content ranges between 0% and 12% (Fig. 4).The lowermost part of the section (500-325 mbsf) is characterized bymore constant values between 3% and 7%, whereas the upper part(325-0 mbsf) displays higher fluctuations with values ranging from0% to 12%.

Smectite values range between 0% and 63% (Fig. 4). The smec-tite record can be divided into two sections. The highest values occurin the lowest part (500^423 mbsf) ranging from 23% to 63%,whereas near 413 mbsf a distinct decrease to values between 0% and22% occurs.

Kaolinite/illite ratios range between 0 and 0.31, and smectite/il-lite ratios show expansion between 0 and 2.1. The highest smec-tite/illite ratios of 0.9-2.1 are typical for the lowermost part of therecord (Table 2.).

Based on first results of peak area evaluation, the trends for illiteand smectite are similar to those based on peak height, althoughdifferences occur in absolute number: illite values vary between 30%and 70%, whereas smectite values vary between 10% and 70%.Chlorite and kaolinite values display variations between 5% and 20%(Dersch, unpubl. data).

Accumulation Rates of Quartz and Feldspars

In order to quantify the terrigenous sediment input at Site 798, theaccumulation rates of quartz and feldspars have been calculated.

The accumulation rates of quartz vary between about 0.5 and 2.5g cm'2 k.y.~1 (Fig. 5). Low-amplitude variations between 0.5 and 1.5g cm"2 k.y.~1 are typical for the early to early late Pliocene, i.e., 5-2.4Ma. If the lowermost part of the sediment sequence of Site 798 is oflate Miocene age (about 6 Ma; L. Burckle, pers. comm., 1990),accumulation-rate values would decrease to 0.35-1.05 g cm"2 k.y."1.The last 2.4 Ma are characterized by increased amplitude of variationsand increased maximum values of quartz-accumulation rates.

The feldspar accumulation rates vary between 0.1 and 2.5 g cm"2

k.y."1 (Fig. 5), whereas the lowermost section from 5 to 4.3 Madisplays the highest rates (range: 0.7-2.21 g cm"2 k.y."1; mean value:1.1 g cm"2 k.y."1).The values would decrease to 0.49-1.55 g cm"2

411

M. DERSCH, R. STEIN

Table 1. Bulk accumulation and accumulation rates of quartz, and feldspar, quartz, and feldspar contentof the bulk mineralogy, and quartz/feldspar ratios.

Hole, core, section,interval (cm)

798A-1H-1, 39-^1798A-lH-2,46-51798A-1H-3, 37-39798A-lH-4,38^5798A-1H-5, 38^0798A-1H-6, 38-44798A-2H-2, 46-51798A-2H-3, 46-49798A-2H-4, 4-9798A-3H-1,71-73798A-3H-2, 49-51798A-3H-3, 49-51798 A-3H-4,49-51798A-3H-5, 53-55798A-3H-6, 49-51798A-4H-2, 29-31798A-4H-3, 29-31798A-4H-4, 59-63798A-5H-2, 80-82798A-5H-3, 49-51798A-5H-4, 86-88798A-5H-6, 33-35798A-5H-7, 7-11798A-6H-2, 49-54798A-6H-3, 7-9798A-6H-3, 18-20798A-6H-3, 28-30798A-6H-3, 35-37798A-6H-4, 4-9798A-6H-6, 84-89798A-7H-2, 130-135798A-7H-3, 51-53798A-7H-4, 50-55798A-7H-6, 65-70798A-7H-7, 66-68798A-7H-8, 55-60798A-8H-3, 32-34798A-8H-5, 61-63798A-8H-7, 64-66798A-8H-8,45-^7798A-9H-1, 54-56798A-9H-2, 29-31798A-9H-3, 55-57798A-9H-4, 54-56798A-9H-5, 54-56798A-9H-6, 53-58798A-10H-2,0-5798A-10H-3, 40-45798A-10H-5, 103-108798A-10H-7, 20-25798A-10H-7, 128-133798A-10H-8, 63-65798 A-UH-1,97-99798A-11H-2, 0-5798A-11H-2, 80-84798A-11H-4, 2-4798A-11H-4, 86-90798A-11H-6, 102-107798A-11H-7, 72-74798A-12H-2, 30-35798A-12H-5,0-5798A-12H-8, 52-54798B-13H-2, 61-100798A-13H-2, 98-103798B-13H-3, 0-30798B-13H-3, 90-120798B-13H-4, 30-60798B-13H-4, 90-120798A-13H-4, 74-79798B-13H-5, 90-120798B-14H-1, 87-89

Depth(mbsf)

0.391.963.374.886.387.88

11.2612.7613.8419.0120.2921.7923.2924.8326.2929.0930.5932.3938.8339.9341.6643.9445.0948.1049.1249.2349.3249.3950.5254.1458.0658.6659.9262.5763.8565.0268.2971.2673.9975.1775.3076.4678.0979.4780.8682.2485.7487.4490.6992.6393.5994.3695.0295.5396.2998.4099.20

102.21103.35105.35108.97113.30115.31115.78116.07116.84117.61118.12118.42119.41123.94

Age(Ma)

0.0030.0160.0280.0410.0530.0660.0940.1060.1150.1580.1690.1820.1940.2070.2190.2420.2550.2700.3240.3330.3470.3650.3740.3980.4070.4070.4080.4090.4180.4470.4850.4920.5050.5350.5490.5610.5980.6300.6600.6730.6750.6870.7050.7200.7410.7710.8390.8660.9130.9270.9350.9400.9450.9490.9540.9700.9760.9971005102010461077109110951097110211081111111411211153

Bulkaccumulation

8.298.097.868.297.957.628.889.919.736.289.508.479.749.459.40

10.5910.2210.1010.538.929.23

10.519.85

10.5410.5010.5010.5010.509.74

10.506.716.438.397.427.187.507.257.898.167.268.968.326.646.553.683.514.304.409.28

10.0011.6911.949.008.009.82

10.509.67

13.4411.0010.419.80

11.0014.0015.5314.0012.5012.0011.5011.4010.5010.89

Quartz

1.271.210.961.090.880.801.121.631.420.731.451.091.761.241.541.441.701.151.020.881.021.380.851.011.071.181.081.001.021.810.830.621.210.750.751.000.530.960.710.690.880.940.900.540.350.320.400.400.931.001.101.210.770.801.111.000.991.790.690.900.931.572.102.981.571.231.551.371.221.561.36

Feldsparg/cm2/k.y.

1.140.720.730.911.380.700.771.090.770.701.800.731.461.171.421.450.8)0.661.500.440.601.160.640.830.950.720.790.790.681.190.790.300.730.710.440.850.000.840.690.580.520.490.440.520.180.220.340.360.900.480.910.570.830.411.100.840.710.980.400.840.890.761.332.420.630.560.820.920.630.671.06

Quartz(wt%)

15.315.012.213.111.110.512.616.414.611.615.312.918.113.116.413.616.611.49.79.9

11.013.18.69.6

10.211.210.39.5

10.517.212.39.7

14.410.110.413.37.3

12.28.79.59.8

11.313.58.29.49.29.49.2

10.010.09.4

10.18.6

10.011.39.5

10.213.36.38.69.5

14.315.019.211.29.8

12.911.910.714.912.5

Feldspar(wt%)

13.88.99.3

11.017.39.28.7

11.07.9

11.118.98.6

15.012.415.113.77.96.5

14.24.96.5

11.06.57.99.06.97.57.57.0

11.311.84.68.79.66.1

11.30.0

10.68.48.05.85.96.67.94.96.48.08.19.74.87.84.89.25.1

11.28.07.37.33.68.19.16.99.5

15.64.54.56.88.05.56.49.7

Quartz/Feldsparratio

1.11.71.31.20.61.11.51.51.91.10.81.51.21.11.11.02.11.80.72.01.71.21.31.21.11.61.41.31.51.51.02.11.71.11.71.2

1.21.01.21.71.92.11.01.91.41.21.11.02.11.22.10.92.01.01.21.41.81.81.11.02.11.61.22.52.21.91.52.02.31.3

412

ACCUMULATION RATES AND CLIMATIC CHANGE

Table 1 (continued).

Hole, core, section,interval (cm)

798B-14H-2, 0-40798B-14H-2.0-40798A-14H-2, 30-34798B-14H-2, 90-120798B-14H-2,116-120798B-14H-2, 120-150798B-14H-3,O-3O798B-14H-3, 75-108798B-14H-3, 89-91798B-14H-3, 116-150798B-14H-4, 39-41798B-14H-4, 60-90798A-14H-4, 62-66798B-14H-4, 90-120798B-14H-4, 120-150798B-14H-4, 120-150798B-14H-5, 0-30798B-14H-5, 30-60798A-14H-6,49-53798B-14H-6, 90-120798B-14H-6, 120-150798B-14H-7, 65-67798B-14H-7, 137-150798B-14H-8.0-30798B-14H-8, 30-60798B-14H-8, 65-67798A-14H-8, 22-26798B-14H-8, 110-134798B-15H-2, 0-30798A-15H-2, 35-39798B-15H-2, 30-80798B-15H-2, 80-120798B-15H-2, 109-111798B-15H-3, 22-24798A-15H-4, 30-34798B-15H-4, 74-76798B-15H-5, 60-90798B-15H-5, 74-76798A-15H-6, 15-19798B-15H-6,0-30798B-15H-6, 74-76798B-15H-7, 30-53798B-15H-7, 90-120798B-15H-7, 120-150798A-15H-8, 29-33798B-15H-8, 75-80798B-15H-8,110-134798B-16X-1,45-47798B-16X-2, 45-50798B-16X-4, 48-53798B-16X-6, 34-39798B-16X-7, 47-^9798B-17X-2, 48-54798B-17X-3, 49-51798B-17X-4, 28-35798B-17X-5, 29-31798B-17X-6, 39-44798B-18X-1,30-32798B-18X-3,43^15798B-18X-5, 130-132798B-18X-6, 25-30798B-19X-1, 59-61798B-19X-2, 60-65798B-18X-9, 30-35798B-19X-3, 59-61798B-19X-5, 51-53798B-19X-6, 60-65798B-20X-1, 39-41798B-20X-2, 40-45798B-20X-4, 103-108798B-20X-5, 46-48798B-20X-6, 60-65

Depth(mbsf)

124.48124.48124.73125.24125.46125.50125.75126.39126.51126.74127.36127.54127.73127.79128.05128.05128.30128.56130.32130.34130.60131.41132.02132.13132.38132.68132.79133.06134.17134.34134.42134.84135.09135.62136.79137.33138.48138.60139.16139.24139.87140.76141.27141.52141.77142.41142.71143.05144.53147.53150.36151.98154.18155.61156.83158.26159.78162.20165.33169.20169.65172.19173.70174.20175.19178.11179.70181.62182.86185.85186.62187.97

Age(Ma)

115711571159116211641164116611711172117311781179118011811183118311841186119911991201120712111212121412161217121912261228122812311233123712451249125712581262126312671274127712791281128612881290130113221343135413701380138913991410142714501478148114991510151415211542155315671576159716031612

Bulkaccumulation

10.8010.8010.8212.5013.1013.0012.5011.0012.9413.0013.2212.5012.2612.2012.5012.5013.0013.0012.8012.8012.8013.2213.5013.5013.2013.199.78

11.9410.009.56

10.0010.5011.3311.709.62

10.5010.509.64

12.1612.0011.028.508.50S.508.50

11.8911.0010.408.728.467.598.007.407.137.687.898.15

11.8410.047.658.00

10.636.338.00

11.7812.5013.7810.6812.868.848.74

11.00

Quartz

1.251.241.071.311.441.551.641.431.821.441.471.031.371.021.241.261.341.051.361.371.601.591.661.771.691.821.090.801.181.021.351.531.681.300.801.550.920.941.911.251.050.751.331.290.781.651.671.080.780.630.600.650.440.450.590.590.641.701.040.450.770.960.450.450.860.951.981.742.070.690.980.36

Feldsparg/cm2/k.y.

0.670.790.911.450.690.771.250.721.761.081.530.680.850.600.910.980.700.830.561.281.051.161.461.070.921.160.822.230.840.360.500.720.880.840.300.970.360.561.410.560.000.340.510.480.221.080.950.430.350.260.000.160.270.000.410.310.501.300.950.240.500.510.410.342.000.951.130.961.480.490.483.04

Quartz(wt%)

11.611.59.9

10.511.011.913.113.014.111.111.18.2

11.28.49.9

10.110.38.1

10.610.712.512.012.313.112.813.811.16.7

11.810.713.514.614.811.18.3

14.88.89.8

15.710.49.58.8

15.715.29.2

13.915.210.48.97.57.98.15.96.37.77.57.8

14.410.45.99.69.07.15.67.37.6

14.416.316.17.8

11.23.3

Feldspar(wt%)

6.27.38.4

11.65.35.9

10.06.5

13.68.3

11.65.46.94.97.37.85.46.44.4

10.08.28.8

10.87.97.08.88.4

18.78.43.85.06.97.87.23.19.23.45.8

11.64.70.04.06.05.62.69.18.64.14.03.10.02.03.70.05.43.96.1

11.09.53.26.24.86.44.2

17.07.68.29.0

11.55.55.5

27.6

Quartz/Feldsparratio

1.91.61.20.92.12.01.32.01.01.31.01.51.61.71.41.31.91.32.41.11.51.41.11.71.81.61.30.41.42.82.72.11.91.52.71.62.61.71.42.2

2.22.62.73.51.51.82.52.22.4

4.11.6

1.41.91.31.31.11.81.61.91.11.30.41.01.81.81.41.42.00.1

413

M. DERSCH, R. STEIN

Table 1 (continued).

Hole, core, section,interval (cm)

798B-21X-2, 77-79798B-21X-3, 59-61798B-21X-4, 39-44798B-21X-6, 59-64798B-21X-7, 77-79798B-21X-8, 19-24798B-22X-2, 30-35798B-22X-4, 17-22798B-22X-6, 90-92798B-22X-7, 42-47798B-22X-8, 77-79798B-23X-2, 30-32798B-23X-5, 120-125798B-23X-6, 61-63798B-23X-8, 34-36798B-24X-2, 49-54798B-24X-4, 49-54798B-24X-5, 50-52798B-24X-6, 43^8798B-24X-7, 22-24798B-25X-2, 68-73798B-25X-3, 68-70798B-25X-4, 63-67798B-27X-1,66-68798B-27X-2, 38-43798B-27X4, 15-20798B-27X-5, 97-99798B-27X-6, 95-100798B-27X-7, 30-32798B-28X4, 31-35798B-28X-5, 29-31798B-28X-6, 27-33798B-29X-1, 31-33798B-29X-3, 30-35798B-29X-3, 31-33798B-29X-5, 31-33798B-30X-2, 100-102798B-30X-3, 100-102798B-30X-5, 100-102798B-30X-6, 100-102798B-31X-2, 30-35798B-31X-4, 55-60798B-31X-6, 30-35798B-31X-7, 86-88798B-31X-8, 30-35798B-32X-2, 40-45798B-32X-3, 20-22798B-32X4, 70-75798B-32X-5, 20-22798B-33X-1,20-22798B-33X4, 60-65798B-33X-5, 19-21798B-33X-6, 30-35798B-34X-1, 90-92798B-34X-2, 84-89798B-34X-4, 5-10798B-34X-5, 39-41798B-34X-6, 24-29798B-35X-3, 39-^1798B-35X-4, 40-45798B-35X-5, 40-42798B-35X-6, 60-65798B-35X-7, 5-7798B-36X-1, 80-82798B-36X-2, 20-22798B-36X-3, 14-16798B-37X-1,60-62798B-37X-2, 46-48798B-38X-1, 17-19798B-38X-2, 40^2798B-38X-4, 31-33

Depth(mbsf)

193.27194.59195.89199.09200.77201.69202.07204.41207.46208.29209.80211.81216.35217.11219.40221.95224.89226.37227.77229.04231.88233.38234.83249.65250.86253.60255.90257.37258.21262.82264.09265.35268.55270.94270.95273.36279.70281.20284.20285.70288.03290.99293.48295.36296.21297.54298.60300.22301.04305.89310.53311.56313.09316.20317.64319.85321.69323.04328.39329.90331.40333.10334.05335.40336.30337.74344.90346.26354.07355.80358.71

Age(Ma)

16501660167417091727173717411766179918081825184718901896191219301951196119711980200020102021212521342153216921792185221822272236225822752275229223372347236823792395241624342447245324622470248824982553260626182635267126872713273427492810282728452864287528902901291729993014310431233157

Bulkaccumulation

10.2510.886.739.035.656.006.525.597.327.028.505.809.30

11.1510.219.519.179.059.84

10.389.259.69

11.3214.1313.658.89

11.4313.378.099.159.317.46

11.307.707.699.25

13.6813.6216.828.948.57

10.139.189.419.339.418.635.135.295.205.665.655.796.145.905.897.097.207.818.248.437.507.118.009.419.109.038.707.247.576.46

Quartz

1.111.130.411.450.420.420.550.541.010.861.300.490.991.481.331.000.750.780.901.140.801.071.422.442.100.901.052.020.000.940.780.551.630.480.520.852.132.113.550.890.390.380.880.880.760.640.540.310.300.360.440.420.350.410.470.470.881.010.700.790.970.990.781.171.191.261.061.170.590.770.63

Feldsparg/cm2/k.y.

0.520.601.181.040.410.230.220.000.650.860.650.260.530.880.590.500.840.750.840.560.311.040.851.611.870.531.231.060.150.590.530.390.980.100.120.541.180.721.720.550.321.280.920.450.710.720.550.120.180.210.300.270.440.490.210.250.430.490.490.630.690.580.430.490.760.580.550.140.410.000.39

Quartz(wt%)

10.810.46.1

16.17.47.08.49.7

13.812.315.38.4

10.613.313.010.58.28.69.1

11.08.7

11.012.517.315.410.19.2

15.10.0

10.38.47.4

14.46.26.79.2

15.615.521.110.04.63.89.69.48.16.86.36.15.77.07.87.56.16.77.97.9

12.414.08.99.6

11.513.211.014.612.613.911.713.58.1

10.29.8

Feldspar(wt%)

5.15.5

17.611.57.33.93.40.08.9

12.37.64.55.77.95.85.39.28.38.55.43.4

10.77.5

11.413.76.0

10.87.91.96.45.75.28.71.31.55.88.65.3

10.26.13.7

12.610.04.87.67.76.42.43.44.05.34.77.68.03.64.36.16.86.37.78.27.76.16.18.16.46.11.65.60.06.1

Quartz/Feldsparratio

2.11.90.41.41.01.82.5

1.61.02.01.91.91.72.22.00.91.01.12.02.61.01.71.51.11.70.91.9

1.61.51.41.74.84.51.61.82.92.11.61.20.31.02.01.10.91.02.51.71.81.51.60.80.82.21.82.02.11.41.31.41.71.82.41.62.21.98.41.5

1.6

414

ACCUMULATION RATES AND CLIMATIC CHANGE

Table 1 (continued).

Hole, core, section,interval (cm)

798B-38X-5, 24-26798B-39X-3, 32-34798B-39X-4, 85-87798B-39X-5, 92-94798B-39X-6, 127-129798B-40X-1,24-26798B-40X-2, 23-25798B-40X-4, 131-133798B-41X-3, 60-65798B-41X-4, 60-62798B-41X-5, 61-66798B-41X-6, 20-22798B-42X-2, 19-21798B-42X-3, 20-25798B-42X-4, 16-18798B-42X-7, 19-24798B-43X-2, 80-85798B-43X-4, 50-55798B-43X-6, 20-25798B-44X-2, 40-45798B-44X-7, 50-55798B-45X-7, 31-33798B-46X-3,110-112798B-46X-4, 66-68798B-46X-5, 43-45798B-46X-6,112-114798B-47X-1,26-28798B-47X-2, 7-9798B-47X-3, 74-76798B-47X-4, 44-^6798B-47X-6, 80-82798B-48X-1,44-46798B-47X-8, 0-5798B-48X-2, 21-23798B-48X-3, 104-106798B-48X-4, 62-64798B-48X-5, 97-99798B-49X-1,63-65798B-49X-2, 110-112798B-49X-3, 76-78798B-49X-4, 112-114798B-49X-6,110-112798B-49X-7, 50-55798B-50X-1, 75-77798B-50X-2, 110-112798B-51X-3, 84-86798B-51X-4,0-5798B-51X-6, 86-88798B-52X-1, 19-21798B-52X-2, 17-19798B-52X-3, 93-95798B-52X-4, 94-96798B-52X-5, 91-93798B-52X-6, 40-42798B-53X-1, 121-123798B-53X-3, 0-5798B-53X-4, 56-58798B-53X-5, 8-10798B-54X-1,54-56798B-54X-2, 56-58798B-54X-3, 123-125798B-54X-4, 131-133798B-54X-5, 92-94798B-54X-7, 0-5798B-54X-8, 55-60

Depth(mbsf)

360.14366.92368.95370.52372.37373.44374.93379.01386.40387.85389.32390.38394.19395.70397.16401.69404.50407.20409.90413.56420.62428.58435.12436.16437.40439.55441.06442.35444.50445.69449.02450.84451.20452.11454.44455.52457.37460.71462.62463.75465.56468.45469.32470.45472.30483.24483.90487.76489.19490.65492.89494.39495.84496.82499.91501.70503.76504.78508.78510.11512.03513.42514.40516.23518.04

Age(Ma)

31733251327432923313332533423389347334903507351935623579359636483680371137423784386439554030404240564081409841134137415141894210421442244251426342844322434443574378441144214434445545804587463246484665469047074724473547704791481448264872488749094925493649574978

Bulkaccumulation

7.416.806.525.136.996.996.876.526.727.368.187.488.879.87

10.0010.559.98

10.229.499.45

10.388.499.789.899.62

10.758.959.308.668.488.27

10.608.009.96

10.1911.0011.1911.759.50

10.269.92

12.1012.0011.7310.7211.3411.4011.9612.1111.5311.7111.5110.9211.0311.5511.3011.4011.5111.2010.2610.9111.6711.6111.5011.50

Quartz

0.770.600.590.530.620.660.640.640.620.750.750.921.071.101.381.301.040.920.890.761.120.690.990.900.840.900.861.000.780.720.931.010.711.041.211.331.161.410.860.870.801.561.921.410.771.331.231.301.271.071.371.231.171.111.160.971.141.050.930.700.961.141.041.121.22

Feldsparg/cm /k.y.

0.370.290.320.230.380.570.000.320.590.540.470.520.440.990.670.740.461.670.600.530.440.441.371.081.440.001.081.000.460.530.890.940.970.751.020.771.142.140.761.060.710.912.211.891.241.751.311.612.010.960.730.871.051.280.751.110.910.921.361.151.010.951.931.180.70

Quartz(wt%)

10.48.89.1

10.38.99.59.39.89.3

10.29.2

12.312.111.113.812.310.49.09.48.0

10.88.1

10.19.18.78.49.6

10.79.08.5

11.29.58.9

10.411.912.110.412.09.18.58.1

12.916.012.07.2

11.710.810.910.59.3

11.710.710.710.110.08.6

10.09.18.36.88.89.89.09.7

10.6

Feldspar(wt%)

5.04.34.94.55.48.20.04.98.87.35.76.95.0

10.06.77.04.6

16.36.35.64.25.2

14.010.915.00.0

12.110.85.36.2

10.88.9

12.17.5

10.07.0

10.218.28.0

10.37.27.5

18.416.111.615.411.513.516.68.36.27.69.6

11.66.59.88.08.0

12.111.29.38.1

16.610.36.1

Quartz/Feldsparratio

2.12.11.92.31.71.2

2.01.11.41.61.82.41.12.11.82.30.61.51.42.61.60.70.80.6

0.81.01.71.41.01.10.71.41.21.71.00.71.10.81.11.70.90.80.60.80.90.80.61.11.91.41.10.91.50.91.31.10.70.61.01.20.50.91.7

415

M. DERSCH, R. STEIN

Table 2. Composition of the clay mineral fraction as relative percentage of illite, chlorite, kaolinite,and smectite, and kaolinite/illite, and smectite/illite ratios.

Hole, core, section,interval (cm)

Depth(mbsf)

Illite Chlorite Kaolinite Smectite Kaolinite/illiteratio

Smectite/illiteratio

798A-1H-3, 37-39798A-1H-5, 2-7798A-2H-1,129-131798C-2H-4, 53-55798C-2H-4, 71-73798C-2H-4, 73-75798C-2H-4, 125-127798A-2H-3, 25-30798A-3H-6, 44-49798A-5H-5,48-50798A-6H-2, 82-87798A-6H-3, 7-9798A-6H-3, 18-20798A-6H-3, 28-30798A-6H-3, 35-37798A-7H-4, 24-26798A-7H-4, 123-127798A-7H-5, 131-133798 A-1 OH-1,44-46798A-10H-3, 83-85798A-10H-5, 39-41798A-10H-7, 35-37798A-11H-3, 94-96798A-11H-4, 2-A798A-11H-3, 146-148798A-12H-8, 0-2798A-12H-8,23-25798A-12H-8, 52-54798A-12H-8, 62-64798A-12H-8, 70-72798A-12H-8, 75-77798A-14H-2, 112-114798A-14H-3,118-122798B-14H-5, 39^1798A-14H-8, 66-68798A-15H-3, 103-107798B-15H-7, 75-77798B-16X-5, 57-59798B-17X-7, 39-41798B-18X-2, 30-35798B-20X-3, 40-42798B-21X-5, 35-37798B-22X-3, 71-73798B-23X-3, 25-27798B-24X-3,49-51798B-25X-5, 60-62798B-27X-3, 38-40798B-28X-2, 27-32798B-29X-7, 35-37798B-30X-4, 100-105798B-3IX-1,30-32798B-32X-1, 129-131798B-33X-3, 79-81798B-34X-3, 87-89798B-35X-1, 15-17798B-37X-3, 44-46798B-38X-3, 31-33798B-39X-1, 14-16798B-42X-5, 140-142798B-44X-1, 60-62798B-44X-1, 105-110798B-45X-5, 132-134798B-46X-2, 131-133798B-47X-5, 4-45798B-48X-2, 28-30798B-48X-4, 62-64798B-49X-3, 131-133798B-49X-7, 23-25798B-50X-3, 132-135798B-51X-6, 21-24798B-52X-6, 128-130

3.376.0210.5911.3811.5711.6012.1612.5526.2443.2248.5649.2749.3949.494915760.4861.6663.2584.8988.3390.8493.7998.0998.6298.63114.30114.57114.90115.02115.11115.17125.84127.41129.66134.33137.04142.79149.18161.71163.70184.79197.35204.47213.60223.50236.30252.38260.52277.74282.70286.73297.59309.53319.17325.15347.74357.21363.74399.90412.45412.93429.24433.94447.23452.18455.52464.45469.34474.02487.11497.79

7982877671687666827585747376767585798077738072807877817880777682848587758885837977778170727979838483688080848374718260724731413943306246423251

1197241215151411251515272424971072315913112214111414181579

714715921810191692121101610011111610981098654654971447

1096099912700110009811701211990988659777611508077009007072199077788855666476036

00008808000000060060007000000004000000007601410000001100001014022134259484946632341446136

0.120.110.070.000.120.120.120.180.090.000.000.150.000.000.000.110.090.140.090.000.160.130.120.110.000.110.100.100.080.070.120.090.090.090.070.150.060.000.100.000.090.100.000.000.120.000.000.090.000.080.310.120.110.000.090.090.090.090.130.110.110.160.140.160.130.120.120.130.000.100.12

0.000.000.000.000.110.110.000.120.000.000.000.000.000.000.000.090.000.000.080.000.000.000.090.000.000.000.000.000.000.000.000.040.000.000.000.000.000.000.000.000.100.080.000.200.140.000.000.000.000.000.160.000.000.000.000.130.190.000.370.180.881.901.181.261.062.130.360.901.061.890.71

416

Quartz (%)

5 10 15 20

Quartz (%

10 20

Feldspar (%) Quartz/Feldspar

30 0 10 20 30 0 2 4

Figure 3. Quartz and feldspar contents and quartz/feldspar ratios of bulk sediment vs. depth at Site 798. Magnetic reversal column as far as Gauss/Matuyama boundary (Ingle, Suyehiro, vonBreymann, et al., 1990).

Illite (%)

0 30 60

100 -

200 -

Q.α>Q 300 -

400

500

Chlorite (%)

90 0 10 20

0

Kaolinite (%)

30 0 10 20

0

30

Smectite (%)

30 60 90

100 -

200 -

300

400

500 500 500Figure 4. Composition of the clay mineralogy referred to 100 wt% clay fraction of Site 798; T = attributed to turbidity currents.

ACCUMULATION RATES AND CLIMATIC CHANGE

k.y. ' and a mean value of 0.77 g cm 2 k.y. ', if the time intervalincreases from 6 to 4.3 Ma). The following unit from 4.3 to 2.4 Marepresents a period of relatively constant, low feldspar accumulationwith a mean rate of 0.51 g cm"2 k.y."1 (range: 0-1.44 g cm"2 k.y."1).The last 2.4 Ma are characterized by a different pattern, revealing atthe base a distinct increase of feldspar accumulation rates (meanvalue: 0.82 g cm"2 k.y."1; range: 0-3.04 g cm"2 k.y."1) with a higheramplitude and a more short-term variation (Fig. 5). Between 2.4 and1.1 Ma, and during the last 0.4 Ma, maximum rates of 1.5-2.5 g cm"2

k.y."1 are typical.

DISCUSSION

In general, the accumulation rates and composition of the silici-clastic material (i.e., quartz, feldspars, and clay minerals) may allowus (1) to reveal different source areas (i.e., the Asian continent and/orthe Japanese islands), (2) to distinguish different transport mecha-nisms (i.e., fluvial or eolian), and (3) to reconstruct the climate of thesource areas (e.g., Thiede, 1979; Sarnthein et al., 1981,1982; Rea andJanecek, 1982; Stein, 1985a, 1985b; Chamley, 1989).

Accumulation rates and composition of the siliciclastic materialas shown in Figures 3-6, demonstrate long-term (i.e., the order of100,000 yr) and short-term (i.e., about 40,000 yr) fluctuations, whichare probably controlled by climate- and oceanographic-inducedchanges as well as changes in transport mechanisms. Furthermore,they seem to fit into orbital-forced processes, as spectral analysesshow significant frequencies at 103, 41, 27, 19, 16, 12, and 10 k.y.(Dunbar et al., this volume; Föllmi et al., this volume).

During (?latest Miocene/) earliest Pliocene, the terrigenous sedi-ment input was characterized by increased flux rates of feldspar, lowquartz/feldspar-ratios, and maximum contents of smectite, which isdistinctly different from the late Pliocene/Pleistocene record. Fur-thermore, in the same interval, well-rounded glauconites grains andcoarse-grained quartz were recorded, which were interpreted asallochthonous components (Ingle, Suyehiro, von Breymann, et al.,1990). These characteristics imply distinctly different transport mecha-nisms and probably different source areas of the siliciclastic materialin comparison to the younger sediments. We attribute these changesto turbidity currents as the major transport mechanism. These sedi-ments were probably deposited prior to significant uplift of thepresent-day Oki Ridge. The uplift of the Oki Ridge during the latePliocene/Pleistocene is indicated by the disappearance of coarse-grained (turbiditic) terrigenous sediments and the decrease in accu-mulation rates (Fig. 5). Between 3.78 and 2.4 Ma a gradual increasein the amounts of illite, chlorite, and kaolinite is documented until at2.4 Ma the values already reach their present-day level. This devel-opment is interpreted as the response to a gradual change to domi-nantly physical weathering due to more arid climatic conditions.

The next distinct change occurs between 2.6 and 2.4 Ma. The latePliocene/Pleistocene record is characterized by distinctly higherquartz and feldspar accumulation rates (Fig. 5). This change is inter-preted as a climate-controlled increase in eolian sediment supply atOki Ridge at that time. The eolian transport mechanism of terrigenoussediments is also corroborated by the dominance of silt-sized quartzgrains as based on smear-slide descriptions (Ingle, Suyehiro, vonBreymann, et al., 1990). Furthermore, this increase is almost synchro-nous with the major expansion of deserts and deposition of loess inChina (Kukla, 1987; Tungsheng, 1988). The increase in eolian sedi-ment supply is accompanied by an increase of (marine) organiccarbon and biogenic opal, due to an increase in surface water produc-tivity (Stein and Stax, this volume). These paleoenvironmentalchanges were probably caused by intensified oceanic and atmos-pherical circulation and aridification in Asia triggered by the devel-opment of major Northern Hemisphere glaciation near 2.5 Ma (e.g.,Shackleton et al., 1984). Similar changes have been recorded in otherparts of the global ocean, such as the tropical/subtropical Atlantic

(e.g., Stein 1985a, 1985b; Ruddiman and Janecek, 1989) or south-western Pacific (e.g., Dersch and Stein, in press).

In general, the upper Pliocene/Pleistocene record at Site 798reflects a depositional sequence characterized by visible long- andshort-term cyclic changes in sediment flux and composition, whereasshort-term (40,000 yr) cycles can be overprinted by the long-termcycles (100,000 yr; Föllmi et al., this volume). Since there was nodetailed δ 1 8 θ record available until now, interpretation of the cyclicityin terms of paleoenvironmental conditions is still highly preliminary.The uplift of the Oki Ridge during the late Pliocene/Pleistocene aswell as the intensified volcanic activity on the Japanese islands sinceabout 1.6 Ma (Ingle, Suyehiro, von Breymann, et al., 1990) must alsobe considered as a possible overprinting factor to the cycles. Lowaccumulation rates of terrigenous matter occur between 0.7 and 0.9,1.3 and 1.4, and 1.7 and 1.9 Ma (Fig. 5), which may suggest a reducedeolian sediment supply resulting from more humid climatic condi-tions. Furthermore, there is an increase in carbonate accumulationsince 1.9 Ma (cf. Stein and Stax, this volume), probably resulting froma gradual emergence of Oki Ridge above the early Pleistocene CCD(Ingle, Suyehiro, von Breymann, et al., 1990). Between 1.7 and 1.9Ma, i.e., during the Olduvai paleomagnetic event, a phase of pro-longed warm and relatively humid climate is also indicated by sedi-mentological and paleobotanical evidence in China, e.g., by a seriesof dense polygenetic soils within the Chinese loess soil sequence(Kukla, 1987). There is also a consistent pattern of loess depositionduring glacial maxima and the formation of soil horizons duringinterglacial periods for the last 1 Ma (Heller and Liu, 1984). Maximain the amounts of terrigenous sediments determined by logging dataare also interpreted as an increased Asian eolian dust supply associ-ated with glacial maxima (de Menocal et al., this volume; withaluminum as proxy). The composition of the clay fraction with illiteand chlorite as dominant clay minerals in the upper Pliocene/Pleisto-cene record at Site 798 reflects dominantly physical weathering dueto more arid climate conditions in the source area of the terrigenousmaterial, too.

The short-term cycles in the Site 798 sediments show two typesof periods: a period equivalent to "Milankovitch-type" cyclicity ofabout 40,000 yr (5-6 m in thickness) (Fig. 3; de Menocal et al., thisvolume) and within these cycles, dark/light subcycles of 1000-12,000yr that are characterized by distinct changes in carbonate and organiccarbon contents (Stein and Stax, this volume; Fig. 6). These subcyclesdemonstrate changes between finely laminated, dark intervals withincreased organic carbon and lighter, homogenous to bioturbatedintervals with lower organic carbon contents (Föllmi et al., 1990;Stein and Stax, this volume). Whereas the "Milankovitch-type" gla-cial/interglacial cycles show differences in terrigenous sediment fluxand composition (Figs. 3 and 5), the investigated dark/light subcyclesdisplay no clear differences in their siliciclastic composition (Fig. 6),indicating formation under similar climatic conditions as well as asimilar source area. The higher kaolinite value in the upper light partof the cycle shown in Figure 6, on the other hand, may suggest adifferent (more southern and tropical) source area for the case of thisspecial paleoenvironmental situation. The 40,000 yr cyclicity seemsto reflect global influences as climatic changes, whereas the dark/lightsubcycles might expose regional paleoenvironmental changes, e.g.,due to hydrodynamic fluctuations. These short-term subcycles, whichare not bound to either glacial or interglacial stages, may be inter-preted as phases within glacial or interglacial periods, when Oceano-graphic factors may have caused changes in paleoenvironmentalconditions (i.e., anoxic vs. oxic condition; cf. Oba et al., 1991; Stein,1991; Stein and Stax, this volume). Nevertheless, shipboard andshore-based investigations of these remarkable dark/light cycles (e.g.,Ingle, Suyehiro, von Breymann, et al., 1990; Dunbar et al., Föllmi etal., this volume) display a complex relationship between stratigraphicsetting, lithology, and the different biogenic and terrigenous sedimentparameters when going into detail. This may indicate, that each cycle

419

Bulk-accumulation rate

(gcm-2ky-1)0 10 20

Quartz accumulation

(gcm-2ky-1)1 2 3

Feldspar accumulation

(gcm-2ky-1)0 1 2 3 4

2.5 Ma

Figure 5. Accumulation rates of bulk sediment fraction, quartz, and feldspar.

ACCUMULATION RATES AND CLIMATIC CHANGE

Biogenic

δ i e 0

4.08

128-798A-6H-3

CaCθ3

14.2

TOC(%)

2.44

3.87 7.5 2.98 * -

2.2 4.15 -fc-

4.12 23.5 1.67

Terrigenous

Quartz Feldspar

10.2 9.0

Clay minerals

Illite Chlorite Kaollnlte

74 15 11

Smectite

0

11.2 6.9 73 27

10.3 7.5 76 24

9.5 7.5 76 24

Figure 6. Dark/light cycle at Site 798 (798A-6H-3, 7-37 cm). Terrigenous and biogenic parameters (TOC = Total Organic Carbon).

probably stands for certain paleoenvironmental conditions, thereforeprecluding one simple model as explanation for the cyclicity. Thismakes obvious, that for a more detailed interpretation of the data interms of paleoenvironment, i.e., to explain the cyclicity, a detailedstable oxygen isotope record and the investigation of additionaldark/light cycles from different climatic (i.e., glacial and interglacial)times are necessary.

CONCLUSIONS

Our preliminary results can be summarized as follows:Sediments from Site 798 are characterized by long- and short-term

fluctuations in composition and flux rates of terrigenous material,probably reflecting paleoenvironmental changes in climate, oceanog-raphy, and transport mechanisms.

The (?latest Miocene/) earliest Pliocene record is characterized byrelatively high amounts of feldspar, low quartz/feldspar ratios, highestsmectite values, glauconites, and coarse-grained quartz grains, attrib-uted to a deposition influenced by turbidity currents.

A distinct increase in quartz and feldspar accumulation as wellas chlorite contents at about 2.5 Ma point to substantial changes inpaleoenvironmental conditions, which is monitored by an increasein both amplitude and period of the fluctuations. This profoundchange, which can be correlated with the onset of major loessdeposition in China, may have been caused by intensified atmos-

pherical circulation and aridification in Asia due to major NorthernHemisphere glaciation.

The late Pliocene/Pleistocene sediment sequence is characterized bydistinct cyclic changes in sediment flux and composition of differentperiods. "Milankovitch-type" cycles are indicated by changes in flux ratesand composition of terrigenous sediments, reflecting different climatic(i.e., humid/arid) conditions in the source area. Several investigateddark/light subcycles with periods of about 1000-10,000 yr show no cleardifferences in terrigenous sediment characteristics, only in biogeniccompounds, reflecting changes in paleoceanic conditions. Further de-tailed investigations of more dark/light cycles from glacial and inter-glacial intervals as well as a stable oxygen isotope record are necessarybefore a more precise interpretation of the data in terms of paleoenviron-ment is possible.

ACKNOWLEDGMENTS

We thank Dr. J. Lauterjung (Giessen University) for cooperationin evaluation of the XRD data. We thank D. Brosinsky, U. Mann, andR. Stax for technical assistance. We also thank Drs. K. Grimm and W.P. Prell for their numerous constructive suggestions for improvementof the manuscript. Financial support by the Deutsche Forschungsge-meinschaft (Grant No. STE 412/3) is gratefully acknowledged. Thisis contribution No. 445 of the Alfred-Wegener-Institute for Polar andMarine Research.

421

M. DERSCH, R. STEIN

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Date of initial receipt: 22 May 1991Date of acceptance: 27 September 1991Ms 127/128B-145