ALBERT

Description: Changes in the Southeast Asia monsoon winds and surface circulation patterns since the last glaciation are inferred using multiple paleoceanographic indicators including planktic foraminifer faunal abundances, fauna and alkenones sea-surface temperature (SST) estimates, oxygen and carbon isotopes of planktic and benthic foraminifers, and sedimentary fluxes of carbonates and organic carbon obtained from deep-sea core SCS90-36 from the South China Sea (SCS) (17°59.70'N, 111°29.64'E at water depth 2050 m). All these paleoceanographic evidences indicate marked changes in the SCS ocean system over the last glacial toward the Holocene. Planktic foraminiferal faunal SST estimates show stable warm-season SST of 28.6°C, close to the modern value, throughout the glacial-interglacial cycle. In contrast, cold-season SST increases gradually from 23.6°C in the last glacial to a mean value of 26.4°C in the Holocene with a fluctuation of about 3°C during 13-16 ka. SST estimates by UK'37 method reveal less variability and are in average 1-3°C lower than the fauna-derived winter-season SST. These patterns reveal that the seasonality of the SST is not only higher by about 3-4°C in the glacial, but also a function of the winter season SST. Sedimentation rates decrease from the last glacial-deglacial stage to the Holocene due to a reduction in supply of terrigenous components, which led to an increase of carbonate contents. Total organic carbon (TOC) contents of primarily marine sources decrease from the last glacial-deglacial to the Holocene. The last deglaciation is also characterized by high surface productivity as indicated by increased ketones abundances and high mass accumulation rates (MAR) of the TOC and carbonates. The gradient of planktic foraminifer ocygen and carbon isotopes of between surface dwellers and deep dwellers increases significantly toward Termination I and Holocene, and is indiscernibly small in the carbon isotope gradient of between 14 and 24 ka, revealing a deep-mixing condition in surface layers prior to 10 ka. The glacial-interglacial fluctuation of the carbon isotope value of a benthic foraminifer is 0.61%. which is significantly larger than a global mean value. The large carbon isotope fluctuation indicates an amplification of marginal-sea effects which is most likely resulted from an increase in surface productivity in the northern SCS during the last glacial-deglacial stage. The multiple proxies consistently indicate that the last glacial-deglacial stage winter monsoon in the Southeast Asia was probably strengthened in the northern SCS, leading to a development of deep-mixing surface layer conditions and a more efficient nutrient cycling which supports more marine organic carbon production.

Description: A high resolution record (~1100 yr) of planktonic foraminifers from ODP Site 1144 in the northern South China Sea reveals rapid and strongly variable climatic changes during the mid-Pleistocene transition period. The abundance of warm water species decreased from an average of 60% in marine isotope stage (MIS) 29 to 〈40% at MIS 22, followed by a steady increase in cool water species toward younger intervals. Many deep dwelling, warm water species decreased to a minimum during MIS 22 and remained extremely rare or even became absent in younger glacial intervals, indicating stepwise sea surface cooling in the region. Estimated SSTs show large fluctuations mostly at glacial-interglacial transitions. A maximum winter temperature difference of 11 °C (17-28 °C) across MIS 23/22 boundary likely corresponded to a major growth of boreal ice sheets across the MPT center at 0.9 myr, coupled with a strengthened winter monsoon over East Asia . The MPT event not only led to a better correlation between changes in species abundances and glacial-interglacial cycles but also a more constrained thermocline that shoaled considerably during subsequent glacial periods. The oxygen isotope record and the abundance of shallow water species display power spectra closely in pace with the 41,000 and 100,000 years cyclicities. A lower coherence over these cyclicities between deep-water dwelling species and the planktonic delta18O, a shoaled thermocline, and more positive glacial delta18O together suggest disturbances of surface and subsurface waters by intensified winter monsoons over the last 1.1-0.5 myr in the South China Sea.