Holocene Oxygen and Carbon Isotopic Records of Core OR102-3 off Southeastern Taiwan: Paleoceanographic Implications

The Holocene carbon and oxygen isotopic records of the planktonic foraminifera Globigerinoides sacculifer from core OR102-3 off southeastern Taiwan are presented. The isotopic values for this taxon from the Holocene period show ranges from -2.45°10 0 to -1.70°/oo for- 8180 and from 2.41°100 to 1.72°/00 for 813C, respectively. Paleo-temperatures revealed from trans­ fer function estimates of foraminifera assemblages in this core and in nearby core OR216-17 show that the surface· waters in this region remained rel­ atively warm and constant during the Holocene. The 8180 values of the surface seaw�ters overlying the two core locations during the Holocene were then calculated using the paleotemperature equation. Results show that oxy­ gen isotopic variations of the local surface waters during the Holocene were within 1°fo0 for core OR102-3 and 0.5°/00 for core OR216-17, respectively. Two freshwater spikes are detected for core OR102-3 at about 2400 yrs B.P. and 5700 yrs B.P., but these spikes are not found in core OR216-17. These two freshwater spikes may be related to precipitation increases over eastern Taiwan during those two periods. Carbon isotopic records indicate that the c5"13C of the total dissolved C02 in surface waters were almost the same for both cores during the Holocene.

fer function estimates of foraminifera assemblages in this core and in nearby core  show that the surface· waters in this region remained rel atively warm and constant during the Holocene. The 8180 values of the surface seaw�ters overlying the two core locations during the Holocene were then calculated using the paleotemperature equation. Results show that oxy gen isotopic variations of the local surface waters during the Holocene were within 1°fo0 for core OR102-3 and 0.5°/00 for core OR216-17, respectively.
Two freshwater spikes are detected for core OR102-3 at about 2400 yrs B.P. and 5700 yrs B.P., but these spikes are not found in core OR216-17. These two freshwater spikes may be related to precipitation increases over eastern Taiwan during those two periods. Carbon isotopic records indicate that the c5"13C of the total dissolved C02 in surface waters were almost the same for both cores during the Holocene.

INTRODUCTI ON
For Monsoon Asia, the Kuroshio �urrent is an important oceanographic current of the western Pacific. It fl ows from the southeast towards the northeast off eastern Taiwan at a speed of 1.5-2 knots (Chu, 1974). Surface water in this current maintains a temperature of 28-29°C in summer and 25-26°C in winter. The salinity of the surface water is in the 1 Institute of Earth Sciences, Academia Sinica, P.O. Box 1·55, Nankan g , Taipei, Taiwan, R.O.C.
2 Institute of Oceano g raphy, National Taiwan University, Taipei, Taiwan 10764, R.O.C. range of 34.2-34.7°100 (Fan, 1985(Fan, , 1987. Records of the surface temperature and water characteristics of the Kuroshio Current and nearby region during the geologic past are of interest and essential to the PAGES study.
Studying the marine sediments close to Taiwan has both advantages and disadvantages.
On the advantage side, one can recover very high-sedimentation-rate cores and consequently get high resolution records, both of marine and terrestrial inputs, from them. On the other hand, due to the high-sedimentation-rates and terrestrial interferences, the use of oxygen isotopes as a stratigraphic tool, normally valuable for open ocean sediments, is often not applicable. Thus, we have turned to other dating methods to obtain a reliable chronology.
In this study, we present detailed Holocene carbon and oxygen isotopic records from core ORl 02-3 off southeastern Taiwan. Our objective is to examine the isotopic records with a view to obtaining a better understanding of the paleoceanography of the Kuroshio current during the Holocene.

MATERIALS AND METHODS
Piston core OR102-3 was raised from the slope of the Huatung Ridge (22°20.l'N, Samples weighing about 2 mg ("'20 tests) were treated with NaClO (5%) at room tem peratures for 24 hours to remove organic matter. Carbon dioxide was released by reaction with iOO% orthophosphoric acid at 35°C and analyzed in a triple-collector mass spectrometer.

14 C Ages and Sedimentation Rates
The AMS 14 C dates obtained from mixed planktonic foraminifera for core OR 102-3P are listed in Table 1. Using the time scale proposed by Martinson et al. (1987) it is obvious that the core covers the periods of oxygen isotope stages 1 to 3. Although the age of the core bottom is beyond the limit of 14 C dating, it is estimated from the oxygen isotope record of this core that the core-base age is close to the oxygen isotope stage 314 boundary. Thus, an age of about 55ka is assumed for the core base. In the following discussion, ages for individual samples are estimated by interpolation between adjacent dates. Figure 2 shows the age vs. depth .relationship for the core OR102-3P. Sedimentation rates indicated in this figure clearly show that the interval of isotope stage 1 had the highest average rate (21 cm/kyr), * Reported age is the conventional radiocarbon age before present (BP). Age and · n 14 c are as defined in Stuiver and Polach (1977). The last column represents the reservoir-corrected age obtained by subtracting 400 years (Bard, 1988). (4 cmlkyr). Because this report discusses only the Holocene part, the cause for the low sedimentation rates during the last glacial time will be presented elsewhere.

The Isotope Records
The Holocene carbon and oxygen isotopic results from G. sacculifer for core OR102-3P (< 10,000 yr B.P.) are listed in Ta ble 2. To obtain a better picture for the studied area of Holocene conditions, we compared isotopic records from core OR102-3 with those of a nearby Holocene core OR216-17 previously studied (Shieh et al., 1991). The Holocene chronology in core OR216-17 was also based on AMS 1 4 C dates of mixed planktonic foraminifera. Figure  3 is the plot of isotopic data vs. age for both cores. Over the interval of time represented, 0180 values show ranges from -2.45°100 to -1.70°/00 for OR102-3 and from -2.42°100 to -2.07°/00 for OR216-17. The 6 1 3C values show ranges from 2.47° /00 to 1.72°/00 for OR102-3 and from 2.19°100 to 1.84°/00 for0R216-17. These isotopic values are comparable to those observed in other western Pacific cores (Berger & Killingley, 1977;Berger et al., 1978aBerger et al., , 1978bWang et al., 1985;Wang and Chen, 1988). The amplitude of the oxygen isotopic variations in both cores is also relatively small ("-'0.7°/00), indicating that temperature and oxygen isotopic variations of surface seawater were relatively minor during the Holocene.
The 613C trends of G. sacculifer are almost identical for both OR102-3 and OR216-17 cores, presumably as a result of having the same 13C/ 1 2C ratio of the bicarbonate in the surface water during the Holocene. The sea-surface temperatures were calculated by the transfer function technique (lmbrie & Kipp, 1971) for both OR102-3 and OR216-17 cores using the score of Thompson (1981) for the western Northern Pacific. Table 2 also lists sea surface temperatures derived from transfer functions of foran:iinifera assemblages. Figure 4 shows the plot of estimated sea surface temperature vs. age for cores OR102-3 and OR216-17. These estimates indicate that the surface waters over the two cores remained relatively warm and constant during the Holocene. The range of temperature variations was about l.5°C for core OR102-3 and 1.3°C for core OR216-17, respectively. Today, the average yearly sea surface temperature in this region is about 28°C and is in good agreement with the Holocene average of core 0R102-3. It is also interesting to note from the transfer function estimates that temperatures between 4000 and 7000 yrs B.P. of both cores were relatively and consistently warmer than those of other Holocene times. This high temperature stage might be related to the hypsithennal period of the Holocene. With temperature and foraminifera 8180 values at hand, the.6180 values of the surface seawaters at the two core locations during the Holocene can be calculated by using the paleotemperature equation of Craig (1965). Results suggest that the range of oxygen isotopic variations of surface waters (i.e. 8180 se a water ) during the Holocene were about 0.9°100 for core 0Rl02-3 and 0.4°/ 00 for core OR216-17 (Table.2; Figure 5). Core OR102-3, which is closer to the island of Taiwan may exhibit larger amplitudes in oxygen isotopic values of the surface seawaters than core OR2 l 6-17 because of increased influence of freshwater input.
Two 8180 minima, inferred to represent freshwater spikes, are detected in core 0R l 02-3 at about 2400 yrs B.P. and 5700 yrs B.P., but these spikes are not found in core OR216-17. These two minima were carefully duplicated as other samples did to verify that they are the real signals. Similar freshwater spikes are also found in other Holocene cores raised off eastern Taiwan in previous studies (Wang et al., 1985). Normally, a freshwater source contributes both depleted carbon and oxygen isotopes for the sea surface waters. But corre sponding carbon isotope anomalies are not found in OR102-3 as freshwater spikes showed in other cores. The main reason for this discrepancy may be due to heavier carbon isotope compositions in rivers of southeastern Taiwan. Heavy 813C values of-0.5°100 to -2°100 were reported for rivers of eastern Taiwan because they flow through thick and exposed marine marble formations which contain relatively heavy carbon isotope compositions (Lee, 1991).
If the two minima in OR 102-3 were true freshwater spikes, then the source river water should have similar heavier carbon isotopic compositions. This unusual case illustrates that, under special situations as we presented here, freshwater spikes may not show carbon anomalies for marine isotope records.
These freshwater spikes may be related to precipitation increases over eastern Taiwan during the Holocene. However, the exact "timing of freshwater spikes in OR102-3 is first reported here from this well-dated core. The absence of freshwater spikes in core OR216-17 implies that the Kuroshio current, flowed over the location of core OR216-17, but did not reach to that of OR102-3 during the Holocene. Apparently, the Kuroshio current can serve as an effective banier for freshwater dispersion to cores within its main flow (Figure 1).
Using the 8180-salinity relationship proposed by Craig and Gordon (1965), the paleo salinities can also be calculated from the deduced 8 1 80 values of the surface seawater ( Table   2). Results show that the range of salinity variations of the local surface waters during the Holocene were about 1.7°/00 for core OR102-3 and 0.8°/00 for core OR216-17 ( Figure 5) and are well within observed habitat ranges for G . sacculifer (Pastouret et al., 1978). In summary, temperature records generated by transfer functions from two cores raised off eastern Taiwan show that the temperatures of surface seawater remained relatively high (27 -29°C) for the past 9000 yrs. Estimated temperatures between 4000 and 7000 yrs B.P. are relatively warmer than those of other Holocene times, and might be related to the hypsithermal period of the Holocene. The amplitudes of the oxygen isotopic records of surface dwelling foraminifera from these two cores are primarily a reflection of oxygen isotopic variations of local seawater. Fresh water spikes detected in OR102-3 may be related to precipitation increases over eastern Taiwan during the middle and late Holocene period.