Xinrong Cheng

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.

Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression

Abstract A large estuary was formed by marine inundation of the paleo-Changjiang (Yangtze) incised valley during the transgression after the Last Glacial Maximum. This paper presents the sedimentary facies and architecture of the estuary fill, based on the analysis of three sediment cores (CM97, JS98, and HQ98) obtained from the present Changjiang delta plain. Estuary fill deposits showing an upward-fining succession were grouped into five depositional facies: tidal river, distributary channel, muddy intertidal to subtidal flats, transgressive lag, and estuary front, based on sedimentary textures, lithology, and physical sedimentary structures. Sand–mud couplets are common in these deposits, indicating that tides played a significant role in producing these sedimentary structures and that the estuary type was tide-dominated. Some of the successive sand–mud couplets probably recorded neap-spring cycles as well as semidiurnal tidal cycles. The nature of the estuary was very different from other representative tide-dominated estuaries in sediment facies, its distribution, and sediment source for estuarine fill. Unlike the other estuaries that receive sediments mainly from the sea, the paleo-Changjiang estuarine fill deposits were supplied largely from the river. This difference would also have a great influence on the sedimentological and morphological component in the estuary. The sediment distribution of the estuary showed fining-seaward and estuary-mouth sand bodies fed by marine-source sand were absent. The architecture model of tide-dominated estuaries should be divided into two types by the degree of fluvial sediment supply. The paleo-Changjiang estuary shows a good example for an estuary of large rivers.

Sedimentary facies and Holocene progradation rates of the Changjiang (Yangtze) delta, China

Abstract The Changjiang (Yangtze) River, one of the largest rivers in the world, has formed a broad tide-dominated delta at its mouth during the Holocene sea-level highstand. Three boreholes (CM97, JS98, and HQ98) were obtained from the Changjiang delta plain in 1997–1998 to clarify the characteristics of tide-dominated delta sediments and architecture. Based on sediment composition and texture, and faunal content, core sediments were divided into six depositional units. In ascending order, they were interpreted as tidal sand ridge, prodelta, delta-front, subtidal to lower intertidal flat, upper intertidal flat, and surface soil deposits. The deltaic sequence from the prodelta deposits to the delta front deposits showed an upward-coarsening succession, overlain by an upward-fining succession from the uppermost part of the delta front deposits to the surface soil. Thinly interlaminated to thinly interbedded sand and mud (sand–mud couplets), and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of Changjiang deltaic facies. Sediment facies and their succession combined with AMS 14C dating revealed that isochron lines cross unit boundaries clearly, and delta progradation has occurred since about 6000 to 7000 years BP, when the rising sea level neared or reached its present position. The average progradation rate of the delta front was approximately 50 km/kyear over the last 5000 years. The progradation rate, however, increased abruptly ca. 2000 years BP, going from 38 to 80 km/kyear. The possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and/or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid-Holocene.

Astronomically tuned Plio–Pleistocene benthic δ 18 O record from South China Sea and Atlantic–Pacific comparison

Based on benthic foraminiferal N 18 O from ODP Site 1143, a 5-Myr astronomical timescale for the West Pacific Plio^Pleistocene was established using an automatic orbital tuning method. The tuned Brunhes/Matuyama paleomagnetic polarity reversal age agrees well with the previously published age of 0.78 Ma. The tuned ages for several planktonic foraminifer bio-events also agree well with published dates, and new ages for some other bio-events in the South China Sea were also estimated. The benthic N 18 O from Site 1143 is highly coherent with the Earth’s orbit (ETP) both at the obliquity and precession bands for the last 5 Myr, and at the eccentricity band for the last 2 Myr. In general, the 41-kyr cycle was dominant through the Plio^Pleistocene although the 23-kyr cycle was also very strong. The 100-kyr cycle became dominant only during the last 1 Myr. A comparison of the benthic N 18 O between the Atlantic (ODP 659) and the East and West Pacific (846 and 1143) reveals that the Atlantic^Pacific benthic oxygen isotope difference ratio (vN 18 OAtlPac) displays an increasing trend in three time intervals: 3.6^2.7 Ma, 2.7^2.1 Ma and 1.5^0.25 Ma. Each of the intervals begins with a rapid negative shift in vN 18 OAtlPac, followed by a long period with an increasing trend, corresponding to the growth of the Northern Hemisphere ice sheet. This means that all three intervals of ice sheet growth in the Northern Hemisphere were accompanied at the beginning by a rapid relative warming of deep water in the Atlantic as compared to that of the Pacific, followed by its gradual relative cooling. This general trend, superimposed on the frequent fluctuations with glacial cycles, should yield insights into the processes leading to the boreal glaciation. Cross-spectral analyses of the vN 18 OAtlPac with the Earth’s orbit suggests that after the initiation of Northern Hemisphere glaciation at about 2.5 Ma, obliquity rather than precession had become the dominant force controlling the vertical structure or thermohaline circulation in the paleo-ocean. A 2002 Elsevier Science B.V. All rights reserved.

Carbon reservoir changes preceded major ice-sheet expansion at the mid-Brunhes event

The beginning of the mid-Brunhes event ca. 430 ka coincided with the largest-amplitude change in d 18 O in the global ocean over the past 6 m.y. This large d 18 O change recorded a major ice-sheet expansion that cannot be explained by small changes in orbital forcing. Our recent studies at Ocean Drilling Program Site 1143 from the South China Sea show that this large d 18 O change was preceded by a significant negative d 13 C shift. A global survey of long deep-sea records has revealed periodic d 13 Cmax episodes (i.e., maximum positive values of d 13 C), and both major ice-sheet expansion events in the Pleistocene (the mid-Brunhes event and the middle Pleistocene revolution) were preceded by d 13 Cmax ep- isodes followed by negative d 13 C shifts. This new finding suggests that disturbance in carbon reservoirs leads to major growth of ice-sheet size and challenges the prevalent concept of Arctic control of glacial cycles. Because Earth is now passing again through a d 13 Cmax episode, it is crucial to understand the causal relationship between the successive d 13 C changes and ice-sheet growth events.

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.

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.

Controlling factors of coccolith distribution in surface sediments of the China seas: marginal sea nannofossil assemblages revisited

Abstract Three nannofossil assemblages have been distinguished in the East and South China seas: the neritic assemblage at the inner shelf dominated by Gephyrocapsa oceanica ; the oceanic assemblage from the outer shelf to continental slope, with a higher diversity and with Emiliania huxleyi outnumbering G. oceanica ; and the dissolution assemblage below lysocline again with G. oceanica as dominant species. These assemblages include a total of 35 species. The ‘marginal sea nannofossil assemblage’ in the China seas is not clearly distinguishable. There is a ‘sharp biological boundary’ of coccolith assemblages between neritic and oceanic waters, but not between marginal seas and open ocean. The oceanic coccolith assemblage in marginal seas can hardly be distinguished from that of the ocean. G. oceanica dominance is specific to neritic, warm, moderate-to-high fertile water, but can not be used as an indicator of marginal sea or monsoon influence. Therefore, the use of ‘marginal sea nannofossil assemblage’ or about ‘monsoon control on coccolith assemblages’ is inappropriate.