Rong Xiang

Last deglaciation in the Okinawa Trough: Subtropical northwest Pacific link to Northern Hemisphere and tropical climate

[1] Detailed deglacial and Holocene records of planktonic delta O-18 and Mg/Ca - based sea surface temperature (SST) from the Okinawa Trough suggest that at similar to 18 to 17 thousand years before present (kyr B. P.), late spring/ early summer SSTs were approximately 3 degrees C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.

Variations of the Last Glacial Warm Pool: Sea surface temperature contrasts between the open western Pacific and South China Sea

[1] High-resolution paleoceanographic records from the southern South China Sea (SCS) and the tropical open western Pacific reveal multicentennial- to millennial-scale variations of the Western Pacific Warm Pool (WPWP) over the last 40 kyr. During the Last Glacial Maximum (LGM), in marine isotope stage (MIS) 2, planktonic Globigerinoides sacculifer dO-18 in core NS93-5 from the southern SCS and in core WP92-5 from the central WPWP registered a similar average value of about - 1.5P. However, the average dO-18 value was about 0.5P more negative in the SCS than in the central WPWP, as was their difference before and after the LGM in the later part of MIS 3 and early portion of MIS 1. These results indicate weaker monsoonal and fluvial effects on the southern SCS during the last glacial than during pleniglacial and postglacial periods owing to different oceanic circulation modes and regional topography. The calculated seasonal temperature differences of summer and winter sea surface temperatures (SSTs) between the open WPWP and the southern SCS were up to 3 - 4.5 D C during the LGM but only slightly over 3 D C in the southern SCS and less than 2 D C in the open WPWP during other periods. From about 29 to 25 ka the MIS 2/3 transition was marked by a sudden SST increase in the southern SCS but a decrease in the open WPWP. Over this 4 kyr period the average annual SST appeared to be only 28 D C in the central WPWP but more than 28 D C in the southern SCS, indicating a shift of the central WPWP toward the far west during that time. Furthermore, our results imply a period of nearly 6 kyr from 22.5 to 16.5 ka without an apparent warm pool of 28 D C in the western Pacific.

Phase evolution of Holocene paleoenvironmental changes in the southern Yellow Sea: Benthic foraminiferal evidence from core C02

Assemblages of benthic foraminifera in a sediment core (C02) near the western margin of the southern Yellow Sea Mud were studied to decipher the phase evolution of Holocene paleoenvironmental changes associated with the Holocene marine transgression. It appears that during the early Holocene (11.2–10.1 kyr BP), the faunal was dominated by low salinity and shallow water species Cribrononion subincertum, Buccella frigida and Ammonia beccarii, reflecting a near coast depositional environment. A rapid increase of the relative abundance of Ammonia compressiuscula between 10.1–9.3 kyr BP indicates that the sea level rose rapidly during that time period. From 9.3–7.7 kyr BP, the benthic foraminiferal assemblage was dominated by high percentage of A. compressiscula, suggesting that the sea level was relatively stable. An obvious transition of benthic foraminifera, from the A. compressiuscula-dominated assemblage to an Ammonia ketienziensis-dominated assemblage, occurred between 7.7–6.2 kyr BP, possibly corresponding to a second sea level rapid rise period in the Yellow Sea during the Holocene. This transition may correspond to the gradually strengthened Yellow Sea warm current (YSWC) and finally is established the modern-type circulation in the Yellow Sea. It may also mark the formation of the Yellow Sea cold bottom water (YSCBW) during that period. Since then, the benthic foraminiferal assemblage based on core C02 was dominated by typical YSCBW species, A. ketienziensis, Astrononion italicum and Hanzawaia nipponica, at 6.2–4 kyr BP. A non-deposition period occurred since ∼4 kyr BP, which possibly related to the hydrology changes caused by the East Asia monsoon. The two obvious benthic foraminiferal transitions recorded in core C02 during the early and middle Holocene provide evidence that the Yellow Sea has undergone a two-phase rapid sea level rise during the Holocene marine transgression.

Influence of the Kuroshio Current intrusion on Holocene environmental transformation in the South China Sea

Numerous studies have been conducted on the evolutionary history of the Kuroshio Current (KC) and other extensions. However, the history of the KC intrusion into the South China Sea (SCS) through the Luzon Strait has been little studied. Here we trace the Holocene developmental history of the KC intrusion and its impact on environmental transformation in the northern SCS. This study involved a comprehensive analysis of planktonic foraminifera, clay minerals and geochemical elements of core 05E306 sediments from the northeastern SCS. AMS 14C dates indicated that the sedimentation rate at the core site was most rapid (>1000 cm/ka) before ~6.0 ka, relatively rapid (~65 cm/ka) between 6.0 and 4.6 ka and slow (~24 cm/ka) since 4.6 ka. The Pulleniatina minimum event (PME) occurred between 4.6 and 2.7 ka, together with high percentages of Neogloboquadrina dutertrei and Globorotalia menardii. Planktonic foraminifera indicated that the KC intrusion experienced three significant changes at 6.0, 4.6 and 2.7 ka in turn. Depositional environments, including input of terrigenous materials and development of biogenic materials, similarly underwent three important transformations at 6.0, 4.1 and 3.0 ka, respectively. Abrupt changes in sedimentary records at the study core are contemporary with records from the tropical Pacific during the Holocene. This indicates that development of the KC intrusion and its impact on depositional environments in the SCS can be correlated with tropical factors from the low-latitude oceans.

Biomarker records of phytoplankton productivity and community structure changes during the last 14000 years in the mud area southwest off Cheju Island, East China Sea

The ecological environment in the East China Sea (ECS) and the Yellow Sea (YS) has changed significantly due to sea-level rising and the Kuroshio incursion since the last deglaciation. In this study, biomarker records of core F10B from the mud area southwest off Cheju Island (MSWCI) were generated to evaluate phytoplankton productivity and community structure changes in response to environmental evolution during the last 14 kyr. The contents of diatom, dinoflagellate and haptophyte biomarkers (brassicasterol, dinosterol and C37 alkenones) display similar trends, with increasing phytoplankton productivity during the last 14 kyr due to the increased influences of the Kuroshio, and especially due to the eddy-induced upwelling during the late Holocene. On the other hand, the contents of terrestrial biomarkers (C28 +C30 +C32n-alkanols) and terrestrial organic matter (TOM) proxies (TMBR’ and BIT) all reveal decreasing TOM input into the area around the sampling site for the 14 kyr, mostly due to sea-level rising. Phytoplankton biomarker ratios reveal a shift from a haptophyte-dominated community at 6.2–2.5 kyr BP to a diatom-dominated community at 2.5–1.45 kyr BP, likely caused by a stronger cold eddy circulation system at 2.5–1.45 kyr BP in the MSWCI.

Distribution of living radiolarians in spring in the South China Sea and its responses to environmental factors

Using a planktonic net (62-μm mesh) and a Rose-Bengal staining method, we studied the spatial distribution of living radiolarians in spring along two sections of the South China Sea (SCS) in spring and discussed the responses of living radiolarian distribution to tropical environmental factors. Generally, the highest abundance of living radiolarians occurred at the depth range of 25–75 m, where the chlorophyll-a maximum and the highest primary productivity were. In contrast, the maximum living abundance occurred in the top 25 m in cold eddies in the open seas and the abundance decreased with depth. We found that the inhibition effect of changing salinity (due to runoffs) on living radiolarians was much stronger than the promotion effect of mesoscale cold eddies. We observed that large variation of temperature was unfavorable for living radiolarians. The dominant species composition consisted of tropical-subtropical warm species. We identified some indicator species for tropical environments. Living Didymocyrtis tetrathalamus tetrathalamus could be an indicator for tropical surface water or mixed-layer water, and even for tropical oligotrophic water. Living Tetrapyle octacantha could be used to indicate tropical thermocline and eutrophic environment. Living Acanthodesmia vinculata could indicate tropical surface and subsurface waters. T. octacantha and A. vinculata should only be used as indicators for upwelling in the open seas, i.e., far away from river mouths. Living Siphonosphaera polysiphonia preferred to form colonies, which might be related to the effect of warm eddies. Living Cyrtopera laguncula and living Cornutella profunda occurred in the tropical upper layer, even in the surface layer, which suggests that they should not be used as indicators for intermediate and deep waters.

Comparison of modern pollen distribution between the northern and southern parts of the South China Sea

The authors conducted a palynological analysis based on different number of air pollen samples for the northern and southern parts of the South China Sea, respectively, in order to give a reference to reconstruct the paleoclimate of the area. (1) Fifteen air pollen samples were collected from the northern part of the South China Sea from August to September 2011, and 13 air pollen samples were collected from the southern part of the South China Sea in December 2011. The pollen types were more abundant in the north than in the south. The total pollen number and concentration in the north was 10 times more than that in the south, which may be because of the sampling season. Airborne pollen types and concentrations have a close relationship with wind direction and distance from the sampling point to the continent. (2) Seventy-four samples were collected from surface sediments in the northern part of the South China Sea in the autumn. Thirty-three samples were collected from surface sediments in the southern part of the South China Sea in the winter. Pollen concentrations in the north were nearly 10 times higher than that in the south. This is because trilete spores are transported by rivers from Hainan Island to the sea and also by the summer monsoon-forced marine current. (3) Ten air pollen samples and 10 surface sediments samples were selected for comparison. The pollen and spores in the air were mainly herbaceous and woody pollen, excluding fern spores, having seasonal pollen characteristics. Pollen in the surface sediments were mainly trilete, Pinus, and herbaceous, and may also show a combination of annual pollen characteristics.

Original component of grain size index in core sediment from southwestern slope of the South China Sea and its paleoenvironmental implication

A new approach to the basic composition character of deep sea sediment is discussed in this paper for indicating its paleoenvironment meaning. For this purpose high precision grain size character of sediments from core NS93-5 was analysed by using the Laser Diffraction Particle Size Analyzer, combining with results of micropalaeontologic counting and trace element and calcium carbonate testing, to illustrate some main grain size indexes and their relationship with micropalaeontologic makeup and terrigenous input. It was showed that the grain sizes of deep sea sediment may reflect materials properties and their buildup framework. There are obvious relations between indexes of grain size ranges and abundances of different micropalaeontologic groups and terrigenous matters, in which grain size of <2 μm represents terrigenous composition, that of >5 μm is mainly made up of various microfossil shells, except for containing volcanic dust layers, and median diameter (MD) of grain size may be taken as proxy of paleoproductivity or paleoecological dynamic. These index and proxy were consistent with micropaleontologic records for well indicating important features of paleoceanographic ecology, showing that the micropalaeontologic growth in the southwestern slope area of the South China Sea (SCS) was negatively correlated to terrigenous input since late Pleistocene and corresponded to glacial-interglacial global climatic changes incompletely, but had a regional particularity in this sea area.

Radiolarian biogeography in surface sediments of the Northwest Pacific marginal seas

Overall abundance and species composition of radiolarian faunas were analyzed in surface sediment samples from representative areas of South China Sea, East China Sea, Sea of Japan, Sea of Okhotsk, Bering Sea, Philippine Sea, and the western boundary current regions of the North Pacific, in order to understand the biogeographic distribution of radiolarians in the Northwest Pacific and explore its relationship with the main environmental factors and the North Pacific circulation. The results showed that radiolarians in the Northwest Pacific surface sediments can be divided into two large biogeographic provinces—cluster A and cluster B. Cluster A is characterized by the dominance of warm-water species and distributed primarily in tropical and subtropical seas with high radiolarian abundance and diversity; whereas cluster B is predominated by cold water species and distributed mainly in the Arctic and subarctic seas with comparably low abundance and diversity. Cluster A is further divided into five subclusters, A1 to A5, which correspond to East China Sea, Philippine Sea, South China Sea, Sea of Japan, and Kuroshio Current, respectively; cluster B is divided into three subclusters, B1 to B3, which correspond to Sea of Okhotsk, Bering Sea, and subarctic gyre area, respectively. Based on the relationships between radiolarian faunas and major environment parameters in different biogeographic provinces, we suggest that the sea surface temperature (SST) and sea surface salinity (SSS) are primary factors that influence productivity, composition, and distribution pattern of the radiolarian fauna in the Northwest Pacific regions, while water depth is likely responsible for regional differences in the radiolarian fauna in each marginal sea. In addition, according to the distribution and abundance patterns of common radiolarian species in different areas, we identified five special radiolarian assemblages, which may be used as indicators for main Kuroshio Current, Kuroshio-East China Sea Branch, Kuroshio-South China Sea Branch, Tsushima Current, and Oyashio Current water masses.

Clay mineral assemblages at IODP Site U1340 in the Bering Sea and their paleoclimatic significance

Clay mineral assemblages and crystallinities in sediments from IODP Site 1340 in the Bering Sea were analyzed in order to trace sediment sources and reconstruct the paleoclimatic history of the Bering Sea since Pliocene (the last ∼4.3 Myr). The results show that clay minerals at Site U1340 are dominated by illite, with a moderate amount of smectite and chlorite, and minor kaolinite. Sediment source studies suggest that the clay mineral assemblages and their sources in the studied core are controlled primarily by the climate conditions. During the warm periods, clay minerals originated mainly from the adjacent Aleutian Islands, and smectite/(illite+chlorite) ratios increased. During the cold periods, clay minerals were derived primarily from the Alaskan region, and smectite/(illite+chlorite) ratios decreased. Based on smectite/(illite+chlorite) ratios and clay mineral crystallinities, the evolutionary history of the paleoclimate was revealed in the Bering Sea. In general, the Bering Sea was characterized by warm and wet climate condition from 4.3 to 3.94 Myr, and then cold and dry condition associated with the enhanced volcanism from 3.94 to 3.6 Myr. Thereafter, the climate gradually became cold and wet, and then was dominated by a cold and dry condition since 2.74 Myr, probably induced by the intensification of the Northern Hemisphere Glaciation. The interval from 1.95 to 1.07 Myr was a transitional period of the climate gradually becoming cold and wet. After the middle Pleistocene transition (1.07 to 0.8 Myr), the Bering Sea was governed mainly by cold and wet climate with several intervals of warm climate at ∼0.42 Ma (MIS 11), ∼0.33 Ma (MIS 9) and ∼0.12 Ma (MIS 5), respectively. During the last 9.21 kyr (the Holocene), the Bering Sea was characterized primarily by relatively warm and wet climatic conditions.