A High-Resolution Hydrographic Contrast Between the East China Sea and the Japan Sea Based on Forantiniferal .Isotopic Records for the Late Holocene

High-resolution foraminiferal isotopic records obtained from cores from the East C hina Sea and Japan Sea reveal a pattern of contrasting hydro­ graphies for the last 6000 years. The first core, retrieved from the upper continental slope of the East China Sea, was analyzed for 8180 and 813C in both Neogloboquadrina dutertrei (planktonic) and Uvigerina spp. (ben­ thic) foraminifera and, hence, provides a record of paleoceanographic change through time. The relatively reduced amplitude of the planktonic 8180 sig­ nal relative to the benthic samples suggests some modification resulting from changes in the prevailing Kuroshio Current. A progressive depletion in both the 8180 and 813C records of N. dutertrei over the last 2000 years suggests a warmer, more humid climate around the East China Sea. Isotopic records of the second core collected from the Japan Sea show a comparable temporal resolution to those from the East C hina Sea, but they exhibit larger ampli­ tudes presumably due to the more restricted hydrographic setting and greater sensitivity to environmental changes in that area. In addition to the temper­ ature differences revealed by oxygen isotopes, the N. dutertrei 813C records for these two cores also provide clues as to the evolution of the Kuroshio C urrent during the late Holocene. words:


INTRODUCTION
The Kuroshio Current plays a major role in modulating the climate of the Northwestern Pacific through the meridional transport of watermass, heat and freshwater. This current is largely modified both quantitatively and qualitatively by its interaction with shelf waters as it passes through_ eastern Taiwan and flows into the East China Sea (Nitani, 1972;Ichikawa and 1 Institute of Marine Geology,. National Sun Yat-Sen Un iversity, Kaohsiung, Taiwan, R.O.C. 2 Institute of Earth Sciences, Academia Sinica, P. 0. Box 1-55, Nankang, Taiwan, R.O.C. 225 226 TAO, vrol. 7, No.2, June 1996 . Beardsley, 1993). The Tsushima Warm Current is one of the Kuroshio branches which enters the Japan Sea via the Tsushima Strait (Toba et al., 1982). Situated at the boundary between the open ocean and the Eurasian continent, the Kuroshio Current is inevitably affected by the prevailing monsoon system. Thus, paleoceanographic records from this area offer the potential of providing insight into both the evolution of the regional climate and hydrographic conditions in the western Pacific. Here a new high-resolution isotopic record with AMS 14 C age control from the East China Sea that provides a record of paleoceanographic change for the last 6000 years is presented. In addition, a comparable resolution record from the Japan Sea offers a the opportunity to contrast the hydrographic settings between these two sites through time. Although many studies regarding the paleoceanographic change in the Japan Sea have bee. n carried out (e.g., Oba et al., 1991;Keigwin and Gorbaren�o, 1992), even the ODP Leg 127 and 128 in 1989 were both devoted to the evolution of the Japan Sea; also the time-scales in those studies were much larger than one millennium.

MATERIALS AND METHODS
Core samples for this study were obtained during the ROC-Russia Joint Expedition, R /V Akademik Aleksandr Vinogradov Cruise V23/KM92 from July 10 to August 5, 1992 (Chen and Bychkov, 1992). The 525 cm long piston core V23/KM92-E25a (E25a) was re trieved from the East China Sea (29°07.6'N, 127°48.2�E) at a water depth of 1090 m ( Figure   1). The 540 cm long piston core V23/KM92-J3 (J3) was collected from the southern Japan Sea (35°53.S'N, 130°15.0'E) at a water depth of 1400 m. A detailed carbonate stratigraphy of core J3 has been published by Gorbarenko et al. (1995). However, only the results from the top 3.5 m in core J3, covering the last 6000 years, are discussed here .. Because the original sampling strategy was designed fo14 organic geochemistry analysis, samples were taken that integrate 5, and sometimes e\1en 10 cm intervals, for each downcore sample onboard ship.
That is, all the data reported here represent average values covering 5 to 10 cm intervals rather than the. more discrete sample intervals usually taken. Although it is likely that such broad sampling has smoothed original signals, the extreme high sedimentation rates at both locations still provide a high resolution view of events over the past 6000 years.  Stuiver and Polasch ( 1977).
Sediment texture, especially the particle size distribution in deep sea sediments, is an important parameter in unde. rstanding the sedimentary environments and previous transport processes. The mean size of the non-cohesive silt fraction (10-63 µm) has been used here as an indicator of relative current strength since bottom currents size-sort coarse silt during ev·ents of resuspension and ensuing deposition (e.g., Haskell et al., 1991;McCave et al., 1995). In this study, particle size analyses for bulk sediments from core E25a were performed on a laser diffraction grain size analyzer (Coulter LS 100). The downcore size distribution provides information regarding changes in current intensity around this area.

AMS 14C Dates and Sedimentation Rates
The AMS 14 C dates generated from monospecific samples of N. dutertrei for both cores 13 and E25a are listed in Table 1. Age models were developed on the basis of linear interpolation between adjacent radiocarbon dates. The calculated sedimentation rates in the Japan Sea core J3 are between 50 and 100 cm/kyr for the last 6000 years. The sedimentation rates in core E25a are much higher, ranging between 75 and 140 cm/kyr. Even when 14c ages are converted into calendar ages prior to the calculation based on the program CALIB 3.03C (Stuiver and Reimer, 1993), the sedimentation rates are still over 100 cm/kyr for most sections of core E25a. For comparison with other referenced data, all ages in this paper are expressed in radiocarbon years. According to the age model, the age for the base of core E25a is estimated to be around 5500 radiocarbon years old. Each sample (5 cm interval samples) in the top 100 cm of core E25a represents an average of 70 years of deposition, while data points (10 cm interval samples) from below 100 cm downcore represent around 70rv90 years. In the Japan Sea core J3, data points from the upper 200 cm represent about 70 years, but expand to around 100 to 150 years as the sampling inter\rals increased to 10 cm below 200 cm downcore and as the sedimentation rates decreased.

Particle Size Distribution
Results of particle size analyses for core E25a are summarized in Figure 2, which the geometric mean of particle size distributions are shown in the upper part of Figure  (for core interval between 440-490 cm; r-.. 14900-5400 years B.P.), but it shifts toward around 12 µm in curve B (for 400-440 cm; 4600-4900 years B.P.). Curve A represents the general size distribution pattern for the core section above 400 cm, peaking in the category of fine silt (7-15.6 µm according to grade scale). The ge. ometric mean of particle size is generally around 11 µm except for the base section (before 5000 years B.P., Figure 5). A distinctive peak up to more than 20 µ.m is recorded around 4000 ye. ars B .P ..

The Oxygen Isotope Records
Even though the size fraction of planktonic foraminifera was prohibited due to the low abundance, the 8180 values generated from core 13 are very similar to the same core reported by Gorbarenko and his associates ( 1995). It seems, to some extent, that the potential influence caused by size fraction is not very discernible, especially under the conditions of a broad sampling interval (5 to 10 cm vs. traditional 1 cm). Therefore, the following discussion is based on the assumption that such restrction are vaild.

Planktonic Records
The b180 results derived from the N. dutertrei from cores E25a and J3 are plotted versus estimated age in the upper panel of Figure 3 with different vertical scales at the left (-2 to 0° I oo ) and right (0 to 2° I 00 ), respectively. When studied in detail no obvious covariation  :: : . .. ..
. between these two records is apparent. Over the last 2000 years, variations in 8180 in E25a show a progressively depleting trend compared to the genera] pattern for the same interval in the J3 record. It is clear that the 2°/00 offset in the 8180 compositions of N. dutertrei between these two cores was primarily caused by the different hydrological settings of these two sites. Currently, the difference in the sea surface. temperatures between these two sites is about 7°C (29°C at E25a relative to 21.6°C at J3 for surface; 21.5°C at E25a relative to 15.5 at 75 m) based on measurements at the time of the cruise (Figure 4; Chen and By·chkov, 1992). The temperature effects alone, therefore, account for about 1.7° I 00 of the difference in the average 8180 composition (Craig, 1965). In general, the 8180 fluctuation range for J3 (about J .6°/00) is two times greater than that for E25a (0.15°100, Figure 3). The different hydrographic settings of these two localities may have contributed to the difference in the amplitudes of the 8180 signal, since the semi-enclosed Japan Sea is expected to be more sensitive to environmental changes, which \\lould tend to amplify its signals.

Benthic Record
The oxygen isotopic record for Uvigerina spp. from core E25a is shown in Figure 5. Most of the benthic 8180 values in this record are around 2.5°100 except for a few 180 depleted spikes. Surprisingly, the total fluctuation range for the benthic 8180 signal (1.6° I 00) is larger than that of the planktonic signal (0.8° I 00; Figure 5). The larger amplitude. of the benthic 8180 record than the planktonic one is not likely to be due to species-dependent effects. Instead, it seems more likely that the larger fluctuations of the benthic c5180 values resulted from variations in the ambient water since the. 8180 composition of Uvigerina spp. is very close. to the 18 0 equilibrium values (Graham et al., 1981 ). In particular, there is an abrupt and brief positiv·e-shift in the interva1 between 4000 and 5000 years B.P., followed by· a sharp and pronounced 018-depleted spike around 4000 years B.P .. A step-wise 8180enriched trend then brings the 8180 values from f'..1 1.3°100 to t*v2.6°/00 ( Figure 5). After 3000 )'ears B.P., the oxygen isotopic compositions of Uvigerina spp. are relatively stable at around 2.5° I 00, but are slightly depleted (0.2-0.3 ° I 00) in the last 2000 years, which is similar to the trend in the .N. d1utertrei records.

Planktonic Records
The lower panel in Figure 3 shows the b13 C records from N. dutertrei for cores E25a and J3. In contrast to the 2° I 00 offset bet\11een the two sites for the 8180 records in the uppe. r panel, there is only a 1° /00 difference in the b13C between the two cores. The 6'13C values of N. duter·tr·ei from core E25a fluctuate between 1.1 and 1.6° I 00, while those from 13 are between -0.1 and 0.9° I 00 with most values heavier than 0.2° I 00. Of special interest is the fact that these two c513C curves are generally more coherent over the period of 1000 to 5000 years B.P .. Over the last 2000 years, the depletion shift in 13C in core E25a relative to the Japan Sea core 13 in the Japan Sea seems to match the general 18 O depletion trend except that it has a smaller amplitude (Figure 3).
In surface water, b13C is controlled by the relative importance of biological and thermo dynamically controlled processes. Carbon isotopes are fractionated by photosynthes is during the production of organic matter and to a lesser extent, during the formation of CaC0 3 . The preferential incorporation of 1 2 C into the organic matter leaves the surface seawater enriched in 813C. N. dutertrei is generally thought to have a 813C composition in equilibrium with the ambient 813C of �C02 (Fairbanks et al., 1982;Sautter and Thunell, 1991). As a dweller at a depth of around 100 m, N. dutertrei lacks the endosymbionts that appear to contribute to the ''vital effects" observed in many planktonic species (Helmleben et al., 1989) As previously discussed, the differences in the hydrographic setting of these two local ities probably play an important role in the observed differences in the foraminiferal isotopic compositions. Nutrient concentrations (especially P04), which reflect the balance between nutrient supply and surface productivity, are closely related to the 813C of L-C02. The inverse correlation between the 813C and P0 4 is 0.93°100 per µMlkg according the calcu lations of Broecker and Peng (1982). In this case, the phosphate concentrations in the East China Sea are generally more depleted than those of the Japan Sea by about 0.2""0.3 µM for the upper 100 mi n the water column (Figure 4; Chen and Bychkov, 1992), which in tum could result in a O. l 9rv0.28° I 00 change in 813C. This nutrient-induced shift, therefore, seems to be adequate in accounting for the missing one quarter difference between the estimated of the :EC02 813C and foraminiferal 813C discussed above.

Benthic Record
The lower part of Figure 5 shows the 813C record of Uvigerina spp. for core E25a. Unlike the 8180 records, the range of fluctuations in the benthic 813C record is more or less similar to that in the planktonic one. Most of the c513C values are around -1.2± 0.2° I 00, except for a fe'Ai' brief C13-depleted intervals. Among these 13C-depleted intervals, there is a distinct negative 813C excursion around 4000 ye. ars B.P. which corresponds to the conspicuous 8180 depletion in the benthic 8180 record ( Figure 5). The combination of both depletion in b180 and 813C, together with the sudden increase in the geometric mean of particle size as indicated at the top of Figure 5, strongly suggests an increase in the influence of freshwater. The mechanism responsible for this freshwater imprint, however, does not go undebated since the 8180 depletion is not shown in the planktonic record.  .. . .
Age (Ka) Fig. 5. Time series of 8180 and 813C of N. dutertrei (planktonic) and Uvigeri1ia spp. (benthic) from core E25a with the geometric mean of partic1e size distribution across the top. Different scales are labeled at ri �ht and left for N. dutertrei and Uvigerina spp., respectively. Both 81 O and 813c are calibrated to PDB. The * sign superimposed on the particle size distribution curve marks the abrupt increase in particle-diameter around 4000 years B.P ..

233
· stratigraphic variations in foraminiferal 8180 over the Pleistocene largely represent the waxing and waning of continental ice sheets and hence reflect both the climate and sea level from a long-term perspective. In comparison with changes in the ocean-water isoto ic recorded in foraminiferal shells. For example, a 1.2° /00 difference in the <5180 composition of inorganic precipitated carbonate between the last glacial and the Holocene is generally expected (e.g., Fairbanks, 1989) and according to Craig (1965) would account for about a 6° C temperature change. In this study, however, excursions in both the planktonic and benthic 8180 ·records are interpreted as local temperature and/or salinity effects since the size of the polar ice sheets and, hence, the 8180 composition of the global mean ocean water have been relatively constant for the last 6000 years. It is true that the ''spiky'' pattern of 8180 in the benthic record ( Figure 5) is not conceivable in an oceanographic perspective.
Turbidites could be easily invoked to interpret such abrupt changes. But with five AMS 14C dates from core E25a (Table l ), the continuity of these isotopic records seems plausible. In this paper, the 8180 records of N. dutertrei and Uvigerina spp. provide information on the upper water column and bottom water in the East China Sea through time, respectively.
The generally similar patterns of the two curves in Figure 5 suggest that the mechanisms controlling the isotopic composition of the watermass, and hence the hydrographic setting in this area, are rather uniform within the water column. Because the prevailing current in this region is the Kuroshio, the strong western boundary current, it is likely that any variability in the Kuroshio is responsible for the recorded 818 0 signals� However, it is unusual that fluctuations in the amplitude of the benthic record are significantly larger than those in the planktonic one ( Figure 5), particularly during the period between 4000 and 5000 years B.P ..
Since the property of watermasses in the deep ocean is generally more stable than that of the surface, it is conceivable that the 8180 composition in the bottom water signaled a more direct influence of the variation source than in the upper water column which is replated by the dominant Kuroshio water. The spike-like negative deviations in the benthic 81 0 record, however, probably were derived from the pulsive input of watermasses from the continental side of the Kuroshio, e.g., the Ta iwan Current War1n Water and the Yellow Sea Cold Water (Beardsley et al., 1985;Gong and Liu, 1994), if the water had been cold enough to sink. Over the last 2000 years, a progressive depletion of 8180 in both N. dutertrei and Uvigerina spp. is observed. Unlike the earlier interval, the amplitude of the planktonic signal is larger than the benthic signal, suggesting a more influential impact in the upper water column. It is probable that the 8180 depletion in the planktonic record resulted from warmer temperatures and/or less saline seawater due to increased net precipitation.
The present geographic distribution of 813C in the deep ocean is closely related to the oxygen and nutrient contents of the various water masses and is strongly dependent on thermohaline circulation patterns (Kroopnick, 1985). On the other hand, the distribution of 813C in surface waters is more complex and depends on the magnitude of the C02 exchange with the atmosphere, mixing with the thermocline waters and isotopic fractionation during photosynthesis. Therefore, the correlation between planktonic and benthic b13C records, with proper species selection, can be regarded as an indicator of the presence of some active water exchange between surface and deep water (e.g., Duplessy et al., 1988;Samthein et al., 1988). However, a number of studies have shown that the 813C composition of the genus Uvigerina does not faithfully reflect that of the bottom water �C02 because of its infaunal microhabitat (Graham et al., 198 1 � Vincent et al., 1981;Duplessy and Shackleton, 1985;Oppo et al., 1990); they also reveal that isotopic composition is related to the accumulation rates of organic carbon (Zahn et al., 1986).
The lower part of Figure 5 shows the time series of b13C for N. dutertrei and Uvige rina spp.. There is a general agreement betwe. en the 813 C trends of planktonic and benthic foraminifera except in the case of few peculiar inter\1als. The relative spiky and depleted pattern of benthic b13C might have resulted from the lateral transport from the upper conti nental slope. It is difficult to imagine the scenario of freshwater input as suggested by 818 0 records ( Figure 5). Yet the intermittent down slope transport of particles that moves con temporary depositions could be an alternati,1e mechanism for those 180and 13C-depletion intervals. The lateral transport of patticulate organic carbon across the shelf break to the deep sea off northeastern Taiwan was documented by Liu et al. ( 1995). Based on repeate. d surveys, Liu and his associate. s ( 1995) concluded that the oligotrophic \\later of the Kuroshio Current was not likely a sufficient source of the patchy particulate organic carbon observed in the underlying \\later. Instead, the relatively high particulate organic carbon concentration in shelf water they found could have been the major source of the organic carbon-enriched sediments on the slope. Furthermore, based on a suite of 58 surface sediment samples from the continental margin of the East China Sea (Sheu et al., 1995), relatively light v·alues of 813C in carbonates are mostly found in water depths of le. ss than 90 m. The difference in the 513C compositions in surface sediments between shallow and dee. p sites could be over 2°100• Therefore, the scattering but relatively 180-and 13C-depleted signals of benthic foraminifera might have been resulted from the concurrent rework of particles (sediments) from shallower depths .
In summary, a comparison of the planktonic isotopic records from cores E25a and J3 reveals offsets that refl ect the hydrographic contrast between the East China Sea and the Japan Sea. The general difference in the 8180 in these areas can be fully explained by the temperature effect. On the other hand, differences in nutrient concentrations are invoked to explain that part of the 813C offset between cores E25a and 13 that can't be explained by temperature alone. The apparent covariant trend of the 813 C curv·es in Figure 3 seems to indicate that the 813C pulses observed in the Japan Sea are linked to those of the East China Sea by changes in the Kuroshio Current. The watermass in the upper water co1umn is largel)' modulated by the Kuroshio Current as reflected by the reduced amplitude of the planktonic record relative to the benthic one. The significant freshwate. r flux, or rather an increase in the lateral transport in this area before 5000 years B.P. and around 4000 y·ears B.P are shown by the synchronous shifts of 8180 and c5.13C and the relatively coarser particles ( Figure 5). The indication of warmer and/or more humid climate over the last 2000 years is indicated by the progressive 818 0 depletion in N. duter·trei.