Sedimentology of the Vietnam shelf

On the Vietnam Shelf more than 1000 miles of shallow high-resolution seismics were analyzed to unravel post-glacial evolution in a tropical, siliciclastic environment together with 25 sediment cores from water depths between 21 and 169 m to determine stratigraphy, distribution and style of sedimentation. Fourty-seven samples were dated with the AMS-14C technique. The shelf was grouped into three regions: a southern part, a central part, and a northern part. On the broad Southern Shelf, sedimentation is influenced by the Mekong River, which drains into the SCS in this area. Here, incised valley fills are abundant that were cut into the late Pleistocene land surface by the Paleo-Mekong River during times of sea level lowstand. Those valleys are filled with transgressive deposits. The Holocene sedimentation rate in this low gradient accommodation-dominated depositional system is in the range of 5-10 and 25-40 cm/ky at locations sheltered from currents. The Central Shelf is narrow and the sedimentary strata are conformable. Here, numerous small mountainous rivers reach the SCS and transport large amounts of detrital sediment onto the shelf. Therefore, the Holocene sedimentation rate is high with values of 50-100 cm/ky in this supply-dominated depositional system. The broad Northern Shelf in the vicinity of the Red River Delta shows, as on the Southern Shelf, incised valleys cut into the Pleistocene land surface by paleo river channels. In this accommodation-dominated shelf area, the sedimentation rate is low with values of 5-10 cm/ky. Where applicable, we assigned the sampled deposits to different paleo-facies. The latter are related to certain intervals of water depths at their time of deposition. Comparison with the sea-level curve of (Hanebuth et al., 2000, doi:10.1126/science.288.5468.1033) indicates subsidence on the Central Shelf, which is in agreement with the high sedimentation rates in this area. In contrast, data from the Northern Shelf suggest tectonic uplift that might be related to recent tectonic movements along the Ailao Shan-Red River Fault zone. Data from the Southern Shelf are generally in agreement with the sea-level curve mentioned above.

Sedimentology on 19 cores from the South China Sea

Based on the study of 10 sediment cores and 40 core-top samples from the South China Sea (SCS) we obtained proxy records of past changes in East Asian monsoon climate on millennial to bidecadal time scales over the last 220,000 years. Climate proxies such as global sea level, estimates of paleotemperature, salinity, and nutrients in surface water, ventilation of deep water, paleowind strength, freshwater lids, fluvial and/or eolian sediment supply, and sediment winnowing on the sea floor were derived from planktonic and benthic stable-isotope records, the distribution of siliciclastic grain sizes, planktonic foraminifera species, and the UK37 biomarker index. Four cores were AMS-14C-dated. Two different regimes of monsoon circulation dominated the SCS over the last two glacial cycles, being linked to the minima and maxima of Northern Hemisphere solar insolation. (1) Glacial stages led to a stable estuarine circulation and a strong O2-minimum layer via a closure of the Borneo sea strait. Strong northeast monsoon and cool surface water occurred during winter, in part fed by an inflow from the north tip of Luzon. In contrast, summer temperatures were as high as during interglacials, hence the seasonality was strong. Low wetness in subtropical South China was opposed to large river input from the emerged Sunda shelf, serving as glacial refuge for tropical forest. (2) Interglacials were marked by a strong inflow of warm water via the Borneo sea strait, intense upwelling southeast of Vietnam and continental wetness in China during summer, weaker northeast monsoon and high sea-surface temperatures during winter, i.e. low seasonality. On top of the long-term variations we found millennial- to centennial-scale cold and dry, warm and humid spells during the Holocene, glacial Terminations I and II, and Stage 3. The spells were coeval with published variations in the Indian monsoon and probably, with the cold Heinrich and warm Dansgaard-Oeschger events recorded in Greenland ice cores, thus suggesting global climatic teleconnections. Holocene oscillations in the runoff from South China centered around periodicities of 775 years, ascribed to subharmonics of the 1500-year cycle in oceanic thermohaline circulation. 102/84-year cycles are tentatively assigned to the Gleissberg period of solar activity. Phase relationships among various monsoon proxies near the onset of Termination IA suggest that summer-monsoon rains and fluvial runoff from South China had already intensified right after the last glacial maximum (LGM) insolation minimum, coeval with the start of Antarctic ice melt, prior to the d18O signals of global sea-level rise. Vice versa, the strength of winter-monsoon winds decreased in short centennial steps only 3000-4000 years later, along with the melt of glacial ice sheets in the Northern Hemisphere.