Baoqi Huang

Clay mineral assemblages in the northern South China Sea: implications for East Asian monsoon evolution over the past 2 million years

Abstract Clay mineral assemblages at ODP Site 1146 in the northern South China Sea are used to investigate sediment source and transport processes and to evaluate the evolution of the East Asian monsoon over the past 2 Myr. Clay minerals consist mainly of illite (22–43%) and smectite (12–48%), with associated chlorite (10–30%), kaolinite (2–18%), and random mixed-layer clays (5–22%). Hydrodynamic and mineralogical studies indicate that illite and chlorite sources include Taiwan and the Yangtze River, that smectite and mixed-layer clays originate predominantly from Luzon and Indonesia, and that kaolinite is primarily derived from the Pearl River. Mineral assemblages indicate strong glacial–interglacial cyclicity, with high illite, chlorite, and kaolinite content during glacials and high smectite and mixed-layer clay content during interglacials. During interglacials, summer enhanced monsoon (southwesterly) currents transport more smectite and mixed-layer clays to Site 1146 whereas during glacials, enhanced winter monsoon (northerly) currents transport more illite and chlorite from Taiwan and the Yangtze River. The ratio (smectite+mixed layers)/(illite+chlorite) was adopted as a proxy for East Asian monsoon variability. Higher ratios indicate strengthened summer-monsoon winds and weakened winter-monsoon winds during interglacials. In contrast, lower ratios indicate a strongly intensified winter monsoon and weakened summer monsoon during glacials. Spectral analysis indicates the mineral ratio was dominantly forced by monsoon variability prior to the development of large-scale glaciation at 1.2 Myr and by both monsoon variability and the effects of changing sea level in the interval 1.2 Myr to present.

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.

Response of upper ocean structure to the initiation of the North Hemisphere glaciation in the South China Sea

Abstract Planktonic foraminifer as well as oxygen and carbon isotopic records from Ocean Drilling Project Core 1146, located at the northern continental slope of the South China Sea, were used to study the response of the upper water column structure to the formation and progressive intensification of the Northern Hemisphere glaciation during the Late Pliocene (3.2–2.0 Ma). Variations in the relative abundance of the Globigerinoides ruber , Globigerinoides sacculifer , Globorotalia menardii , and Globorotalia inflata groups, Globorotalia crassaformis and Neogloboquadrina pachyderma showed that sea surface temperatures gradually decreased, coinciding with heavier oxygen isotope values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma. After 2.7 Ma, the relative abundance of mixed-layer species decreased, while that of thermocline dwellers, dominated by high productivity species, increased. Δδ 13 C variations among G. sacculifer , Pulleniatina obliquiloculata and Cibicidoides wuellerstorfi suggest that the mixed-layer depth and vertical exchange was enhanced with the strenghtening of the East Asian winter monsoon, which in turn is associated with the progressive intensification of the Northern Hemisphere glaciation.

Carbon isotopic record of foraminifers in surface sediments from the South China Sea and its significance

The study is based on stable carbon isotopic measurements of 112 foraminiferal samples from surface sediments at 40 sites in the South China Sea (SCS). δ13C of foraminifers and Δδ13C between planktonic and benthic foraminiferal species exhibit a low value area at the north-eastern and southern ends of the SCS. It is correlated with the nutrient distributional pattern in the SCS and circumjacent area, the influence of the northeastern and southwestern monsoons on water flow and water chemistry in the SCS. The monsoons have not only brought nutrients to the upper part of the sea but also disturbed water and decreased difference between the surface and bottom water. Its influence is most obvious at both ends, which resulted in the low value areas in δ13C and Δδ13C at the ends. The distributional pattern of the stable carbon isotope in the SCS is a reflection of the East Asian monsoons.

Global deep water circulation between 2.4 and 1.7 Ma and its connection to the onset of Northern Hemisphere Glaciation

We have generated an early Pleistocene benthic isotopic record for the Ocean Drilling Program (ODP) Site 807 (2804 m) from the western equatorial Pacific. Between 2.4 and 1.7 Ma, the benthic δ13C of this site and a few other deep Pacific sites was consistently higher than the Southern Ocean Site MV0502-4JC (4286 m), pointing to a reversal relative to the current gradient and hence implying a different circulation regime. We reconstructed the deep water mass distribution of this interval using a collection of benthic isotope records from 15 Pacific and 10 Atlantic sites and a δ13C-δ18O mixing model. A two-end-member mixing regime between the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), with properties very different from today, was identified. The Southern Ocean showed strong signs of stratification and AABW with low benthic δ13C but high δ18O values reached out to other basins only below ~4000 m. In contrast, NADW ventilated most of the ocean interior, contributing ~70% to the Pacific Deep Water (PDW) volumetrically. Our model results also reveal a strong remineralization effect at the bottom sites of the Pacific and the Atlantic, suggesting significant accumulation of respired carbon in the bottom water between 2.4 and 1.7 Ma. We propose that such a circulation pattern was initiated by the reversal of salinity gradient between AABW and NADW from 3.0 to 2.4 Ma, possibly linked to Antarctic sea ice expansion and reduced southward heat transport during the onset of Northern Hemisphere Glaciation (NHG).