Zhimin Jian

Holocene variability of the Kuroshio Current in the Okinawa Trough, northwestern Pacific Ocean

Abstract Cores 255 and B-3GC from the southern and northern Okinawa Trough, respectively, were studied for determining the variability of the Kuroshio Current on centennial scales during the Holocene based on detailed AMS 14 C dates, stable isotope, and planktonic foraminiferal distribution. The main flow of the Kuroshio Current was inferred to re-enter the Okinawa Trough at ∼7.3 calendar (cal.) kyr BP, leading to abrupt changes in sedimentation rate, remarkably increased abundance of the Kuroshio Current indicator Pulleniatina obliquiloculata , increased sea surface temperature (SST) and depth of thermocline (DOT). During ∼4.6–2.7 cal. kyr BP, the abundance of P. obliquiloculata sharply decreased, corresponding to a decreases in SST and DOT, implying that the influence of the Kuroshio Current weakened at that time, possibly as a result of the intensified winter monsoon. Significantly, the Kuroshio Current proxies (e.g. the difference in SST between the southern and northern Okinawa Trough) display periodicities of ∼1500 yr and ∼700–800 yr (ascribed to the second harmonic of the 1500 yr cycle in oceanic thermohaline circulation) during the Holocene. The Holocene events (at ∼0.6, 1.7, 3.3, 4.6, 5.9, 8.1, 9.4 cal. kyr BP) in the Okinawa Trough appear to be the most recent manifestation of the millennial scale climate cycle, recorded in the North Atlantic, Arabian Sea marine sediment cores and Greenland ice cores, suggesting global climatic tele-connections.

Geochemical and Nd isotopic variations in sediments of the South China Sea: a response to Cenozoic tectonism in SE Asia

Secular variations in geochemistry and Nd isotopic data have been documented in sediment samples at ODP Site 1148 in the South China Sea. Major and trace elements show significant changes at ca. 29.5 Ma and 26^23 Ma, whereas ONd values show a single change at ca. 26^23 Ma. Increases in Al/Ti, Al/K, Rb/Sr, and La/Lu ratios and a decrease in the Th/La ratio of the sediments beginning at 29.5 Ma are consistent with more intense chemical weathering in the source region. The abrupt change in Nd isotopes and geochemistry at ca. 26^23 Ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments, implying a drastic change in sedimentary provenance and environment at the drill site. Comparison of the Nd isotopes of sediments from major rivers flowing into the South China Sea suggests that pre-27 Ma sediments were dominantly derived from a southwestern provenance (Indochina^Sunda Shelf and possibly northwestern Borneo), whereas post-23 Ma sediments were derived from a northern provenance (South China). This change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the South China Sea, associated with a southwestward expansion of the ocean basin crust and a global rise in sea level. Thus, the geochemical and Nd isotopic changes in the sediments at ODP Site 1148 are interpreted as a response to a major plate reorganization in SE Asia at ca. 25 Ma. B 2003 Elsevier Science B.V. All rights reserved.

Modern distribution patterns of planktonic foraminifera in the South China Sea and western Pacific: a new transfer technique to estimate regional sea-surface temperatures

Abstract We present 30 new planktonic foraminiferal census data of surface sediment samples from the South China Sea, recovered between 630 and 2883 m water depth. These new data, together with the 131 earlier published data sets from the western Pacific, are used for calibrating the SIMMAX-28 transfer function to estimate past sea-surface temperatures. This regional SIMMAX method offers a slightly better understanding of the marginal sea conditions of the South China Sea than the linear transfer function FP-12E, which is based only on open-ocean data. However, both methods are biased toward the tropical temperature regime because of the very limited data from temperate to subpolar regions. The SIMMAX formula was applied to sediment core 17940 from the northeastern South China Sea, with sedimentation rates of 20–80 cm/ka. Results revealed nearly unchanged summer temperatures around 28°C for the last 30 ky, while winter temperatures varied between 19.5°C in the last glacial maximum and 26°C during the Holocene. During Termination 1A, the winter estimates show a Younger Dryas cooling by 3°C subsequent to a temperature optimum of 24°C during the Bolling/Allerod. Estimates of winter temperature differences between 0 and 100 m water depth document the seasonal variations in the thickness of the mixed layer and provide a new proxy for estimating past changes in the strength of the winter monsoon.

Benthic foraminiferal paleoceanography of the South China Sea over the last 40,000 years

Benthic foraminifera in gravity and piston cores from two sites of the northern and southern slopes of the South China Sea (SCS) were analyzed to evaluate changes in surface productivity and deep-water mass characteristics over the last 40,000 years. Our observations suggest that changes in organic carbon flux, that is food supply, and chemical and=or physical properties of the ambient water mass may be the two primary and intercorrelated factors controlling the distribution patterns of benthic foraminifera. When organic carbon flux increased above 3.5 g C m 2 yr 1 in the southern SCS during the last glacial maximum and in the northern SCS during the first part of the Holocene around 10 ka B.P., a group of detritus feeders including Bulimina aculeata and Uvigerina peregrina dominated the benthic foraminiferal assemblage as shown by relative abundance (%) and accumulation rates. This may reflect episodes of increased surface productivity, possibly induced by increased input of nutrients from nearby river runoff. Suspension feeders such as Cibicidoides wuellerstorfiand a group of ‘opportunistic’ species including Oridorsalis umbonatus, Melonis barleeanum and Chilostomella ovoidea gradually became more abundant than detritus feeders as soon as the organic carbon flux decreased to 2.5‐3.5 g C m 2 yr 1 . Similar glacial to interglacial changes in relative abundance and accumulation rates were observed in both cores for a number of species, including Eggerella bradyi, Globocassidulina subglobosa , Astrononion novozealandicum , Sphaeroidina bulloidesand Cibicidoides robertsonianus. These changes were not correlated to the distribution patterns of organic carbon in both cores and may have been related to yet unspecified changes in chemical and=or physical properties of the ambient water mass, independent of changes in organic carbon flux.

Foraminiferal responses to major Pleistocene paleoceanographic changes in the southern South China Sea

A detailed age model for core 17957-2 of the southern South China Sea was developed based on delta(18)O, coarse fraction, magnetostratigraphy, and biostratigraphy for the last 1500 kyr. The delta(18)O record has clear similar to 100-kyr cycles after the Mid-Pleistocene Revolution (MPR) at the entrance of marine isotopic stage (MIS) 22. Planktonic foraminifera responded to the MPR immediately, showing the increased sea surface temperature (SST) and dissolution after the MPR. Benthic foraminifera did not respond to it until the Brunhes/Matuyama boundary. Since the MPR, the depth of thermocline gradually became shallower until MISs 6-5. This major change within MISs 6-5 was also reflected in the decreased SSTs and increased productivity and Deep Water Mass. Thus two major Pleistocene paleoceanographic changes were found: One was around the MPR; the other occurred within MISs 6-5, which speculatively might be ascribed to the reorganization of surface and deep circulation, possibly induced by tectonic forces.

Quantitative composition of benthic foraminiferal assemblages as a proxy indicator for organic carbon flux rates in the South China Sea

Abstract The organic carbon flux signal embedded in the distribution data of benthic foraminifera from 43 box core surface samples from the South China Sea (SCS) is compared to data from two gravity cores on the northern (core 17940-2) and southern (core 17964-3) slope of the SCS. Surface sampling stations cover almost the entire area of the South China Sea, including areas with preferential summer monsoonal upwelling or winter monsoonal upwelling, as well as a wide range of water depths (300–4000 m). We use correspondence analysis (AFC) to examine the relation between the quantitative composition of benthic foraminiferal assemblages, water depth and organic carbon flux rates. We find a clear correlation between benthic foraminiferal assemblage composition (factor 1 of the AFC) and organic carbon flux rates in the surface samples and use the same model to calculate paleo-flux rates for the two cores. Significantly increased carbon flux rates are observed in the southern SCS (Sunda Shelf margin) during the last glacial maximum (LGM), whereas the organic carbon flux rates along the northern slope of the SCS seem to be highest during the early Holocene.

Pliocene–Pleistocene stable isotope and paleoceanographic changes in the northern South China Sea

Abstract Based on the stable isotopic analysis of planktonic and benthic foraminifers from Ocean Drilling Program Core 1148 of the northern South China Sea (SCS), Pliocene–Pleistocene isotope stratigraphy and events have been reconstructed. The benthic foraminiferal δ18O record shows that the Pacific intermediate water had a greater influence upon the SCS or the Pacific deep water above ∼2600 m was warmer before ∼3.2 Ma than at present. After that, the benthic δ18O conspicuously increased during the ∼3.2–2.5 Ma period, in correspondence to the formation of the Northern Hemisphere ice sheet, whereas the planktonic δ18O signal suggests a stepwise overall decrease of sea surface temperature during the ∼2.2–0.9 Ma period. Compared to the equatorial Pacific records, the decrease in planktonic (Globigerinoides ruber) δ13C during the ∼3.2–2.2 Ma period is particularly striking, suggesting that fertility of surface water increased noticeably. According to the modern δ13C distribution of G. ruber in the northern SCS, it is inferred that the East Asian winter monsoon strengthened during this interval. Afterwards, there were several conspicuous decreases of G. ruber δ13C at ∼1.7, 1.3, 0.9, 0.45 and 0.15 Ma BP, that is, about every 0.4 Ma, suggesting that the East Asian winter monsoon became episodically stronger. This is confirmed by changes in relative abundance of planktonic foraminifer species Neogloboquadrina dutertrei, a typical East Asian winter monsoon proxy. The deep-water δ13C of the SCS is close to that of the Pacific, but lighter than that of the Atlantic, implying that the pattern of deep water originating mainly from the Atlantic and through the Pacific entering the SCS existed at least since the early Pliocene. After 1.4 Ma, the benthic δ13C signal decreased conspicuously but with a periodicity of ∼100 ka, suggesting that the deep-water ventilation of the SCS was reduced, probably corresponding to a decrease of the North Atlantic Deep Water and/or further isolation of the SCS deep basin from the Pacific during glaciations.

Thirty million year deep-sea records in the South China Sea

In the spring of 1999 the Ocean Drilling Program (ODP) Leg 184 Shipboard Party cored 17 holes at 6 deep water sites in the northern and southern parts of the South China Sea (SCS). Chinese scientists actively participated in the entire process of this first deep-sea drilling leg off China, from proposal to post-cruise studies. More than 30 categories of analyses have been conducted post-cruise in various Chinese laboratories on a large number of core samples, and the total number of analyses exceeded 60 thousand. The major scientific achievements of the Leg 184 studies are briefly reported in three successive papers, with the first one presented here dealing with deep-sea stratigraphy and evolution of climate cycles. This ODP leg has established the best deep-sea stratigraphic sequences in the Western Pacific: the 23-Ma isotope sequence from the Dong-Sha area is unique worldwide because of its continuity; the last 5-Ma sequence from the Nansha area represents one of the best 4 ODP sites worldwide with the highest time-resolution for that time interval, and the sequences of physical properties enable a decadal-scale time resolution. All these together have provided for the first time high-quality marine records for paleoenvironmental studies in the Asian-Pacific region. This new set of stratigraphic records has revealed changes in climate cyclicity over the last 20 Ma with the fluctuating power of the 100 ka, 400 ka, 2000 ka eccentricity cycles, indicating the evolving response of the climate system to orbital forcing along with the growth of the Antarctic and Northern Hemisphere ice sheets.

Foraminiferal responses to upwelling variations in the South China Sea over the last 220 000 years

Abstract Late Quaternary benthic and planktonic foraminiferal and isotopic data were obtained from two sediment cores in the present upwelling areas of the South China Sea: Core 17928 off northwestern Luzon and Core 17954 off eastern Vietnam. Quantitative analyses of foraminifera in the two cores revealed the foraminiferal responses to paleoceanographic changes over the last 220 000 years. High productivity species such as Neogloboquadrina dutertrei , Bulimina acculeata , Uvigerina peregrina , Melonis barleeanus and Chilostomella oviodea displayed high abundance during interglacial periods in Core 17954 and during glacial periods in Core 17928. Correspondingly, variations in benthic foraminifera accumulation rates, the ratio of infaunal to epifaunal benthic foraminifera and the presence of the benthic foraminifers Cibicidoides , Miloilids and Oridorsalis umbonatus , all indicate that bottom water was oxygen-rich and oligotrophic during glacial periods off Vietnam (Core 17954) but such bottom conditions occurred during interglacial periods off Luzon (Core 17928). Sea-surface temperature results from planktonic foraminifera transfer function and variations in the abundance of planktonic foraminifera, typical tropical species Globigerina sacculifer , tropical–subtropical species Globorotalia menardii , Globigerinoides ruber , and Pulleniatina obliquiloculata , transition species Globorotalia inflata and Arctic species Neogloboquadrina pachyderma , indicate an increase in cool waters during marine isotope stage (MIS) 5, except for MIS 5.5, and glacial intervals in the cores 17954 and 17928, respectively. The ratios of mixed-layer/thermocline dwelling species and depth of thermocline (DOT) estimates using transfer function suggest that the DOT was shallower during the interglacial than the glacial periods off Vietnam (Core 17954), but the DOT was shallower during the glacial periods off Luzon (Core 17928). Our results show that in the glacial periods lower sea surface temperature, shoaled thermocline, and high productivity increased off northwestern Luzon (Core 17928), and that, on the contrary, similar event occurred in the interglacial off Vietnam (Core 17954). These differences in the upper water column structure and productivity in the two areas were induced by coastal upwelling, which in turn is driven by monsoons. The intensity of upwellings is also closely related to the evolution of the eastern Asian monsoon. Therefore, we conclude that differences in the upwelling intensity in the two sites indicate strengthening of winter and summer monsoon during the glacial and interglacial periods, respectively.

Deep water exchanges between the South China Sea and the Pacific since the last glacial period

Deep ocean circulation is widely considered as one of the important factors for increasing CO2 concentration and decreasing radiocarbon activity (Δ14C) of the atmosphere during the last deglaciation. The AMS 14C ages of benthic and planktonic foraminifers from 18 samples of Core MD05-2904 (water depth of 2066 m) in the northern South China Sea (SCS) and 15 samples of Core MD05-2896 (water depth of 1657 m) in the southern SCS were analyzed in this study for reconstructing the intrabasin deep oceanic processes and hence exploring the deep water exchanges between the SCS and the Pacific since the last glacial period. The results show that during the Holocene the average apparent ventilation age of deep water was younger in the southern SCS (~1350 years) than in the northern SCS (~1850 years) due to relatively strong vertical mixing and advection, consistent with modern observations. However, during the last glacial period and deglaciation the deep water was older in the southern SCS (~2050 years and ~1800 to 1200 years, respectively) than in the northern SCS (~1600 years and ~670 years, respectively), indicating reduced deep mixing and advection. Moreover, the northern SCS deep water was significantly younger during the last deglaciation than during the Holocene and the last glacial period, implying the existence of northern sourced newly formed and relatively young North Pacific deep water. Our records do not support the intrusion of anomalously 14C-depleted deep water to the middepth of the low-latitude western Pacific and the SCS during the “Mystery Interval” (17.5–14.5 kyr B.P.).