The last glacial maximum, ca. 21,000 years ago, caused a fall in eustatic sea level of ca. 120 m below present. The low-gradient, shallow Sunda Shelf, Southeast Asia was subaerially exposed during this sea-level lowstand and experienced rising sea level thereafter. Sea level rose to a +1.3––5 m highstand ca. 6,500 cal yr BP, and then fell to modern sea level. The objective of this research is to characterize environmental change on the Sunda Shelf in response to the post-glacial rising eustatic sea level. To address this objective, six gravity cores were collected along a transect crossing the paleo-Chao Phraya incised river valley complex between peninsular Malaysia and southern Vietnam. Thirteen AMS radiocarbon samples, 130 bulk sediment magnetic susceptibility samples (BMS), 66 X-ray fluorescence spectrometry (XRF) samples, and 54 samples for the analysis of foraminiferal assemblages were used to characterize change in the depositional environments of the cored sediments. BMS, XRF, and foraminiferal analysis distinguish two main units. Unit 1 is found in the lower part of the cores and typically contains more terrestrial material than sediments further up-core as shown by higher BMS values, higher % Ti, % Al, % Fe, and lower indicators of marine influence, for example, lower % Ca, % planktonic foraminifera, and percentages of deeper water benthic foraminifera such as Heterolepa dutemplei. Unit 2 is characterized by a significant increase in % Ca, % Heterolepa dutemplei and % planktonic foraminifera ca. 6,500 cal yr BP. Unit 1 is consistent with shallower water depths and is part of a transgressive systems tract (TST, ca. 1 m thick) that terminates ca. 6,500 cal yr BP. Unit 2 represents the overlying highstand systems tract (HST, ca. 1 m thick) and is characterized by an increase in % planktonics, % Ca, and shifts in benthic foraminiferal assemblages, indicating deeper water conditions than the sediments below. This shift from a TST to a HST ca. 6500 cal yr BP is consistent with the Sunda Shelf sea-level record.

Two gravity cores were collected from 43 km and 90 km offshore of Kuala Terengganu (western Sunda Shelf, southern South China Sea) at ~60 m water depth to characterize late Quaternary paleoenvironments by using bulk sediment magnetic susceptibility (BMS), elemental analysis via X-ray fluorescence spectrometry (XRF), and foraminiferal analyses. Radiocarbon age estimates (using benthic foraminiferal carbonate material) indicated the nearshore core was Holocene (ca. 10,000-6,000 cal years BP) in age while the offshore core was deposited prior to the last glacial maximum (ca. 45,000 cal years BP and possibly older) during marine isotope stage (MIS) 3 (ca. 60,000 - 30,000 yrs BP) within the late Pleistocene. Elemental and BMS data agree well within both cores, except Pleistocene sediments exhibit higher BMS (ca. 1x10-4 si) and higher calcium content (ca. 2%) than Holocene sediments. These data can be interpreted as representing stronger terrestrial influence during the deposition of Pleistocene sediments or a higher dissolution of calcium carbonate. BMS data, elemental data, and micropaleontological evidence, relative abundances of 64 benthic foraminiferal species, indicate that both cores were deposited in an inner shelf environment similar to the modern shelf environment at ca. 50-60 m water depth. However, a higher ratio of planktonic foraminifera (Holocene: 0-1.3%, Pleistocene: 0-8.5%) suggests a greater influence of open marine waters in the Pleistocene sediments. The position of sea level during deposition of the late Pleistocene cored sediments is higher (ca. 60-0 m depending on the data set) than eustatic published sea-level data for MIS 3. This incongruity could be caused by age inaccuracies from limitations of the radiocarbon dating technique, although this seems unlikely as the lower range of age estimates fall within the technique and the uppermost range falls on the border of the limitations of the technique. Other options for the incongruity include large margins of error for late Pleistocene sea-level reconstruction and vertical crustal movement post-deposition from isostatic adjustment or tectonic activity.

In order to understand Holocene paleoenvironmental evolution of the western Sunda Shelf (southern South China Sea), two gravity cores TER15-GC10A and TER15-GC9A were collected (2015) 33 and 37 km offshore of Kuala Terengganu (KT), Peninsular Malaysia at ca. 60 m water depth. The cores were sampled every 1 cm and analyzed for three paleoenvironmental proxies; 1) bulk sediment magnetic susceptibility (BMS); 2) elemental analysis by x-ray fluorescence (XRF) spectrometry, and 3) foraminiferal content. Radiocarbon age estimates indicate that cores have a maximum age of ca. 8,400 cal. yr BP. BMS values decrease up-core (1.95 x 10-4-1.04 x 10-4 SI). XRF data also indicate a decrease up-core for Al (12.82%-17.05%), Fe (5.45%-3.65%), and Ti (0.78%-0.61%) with an inverse relationship to the concentration of Ca (3.29%-7.77%). This is interpreted to reflect a decrease in amount of terrestrial material supplied to the shelf over the last ca. 8,400 years, a dilution of detrital material by the local productivity of marine carbonates, and/or a stabilization of sea level. Cluster analysis indicates four groups of benthic foraminifera within the two cores. Within all four clusters Textularia sp. A, Heterolepa dutemplei, Asterorotalia milletti, and Hanzawaia nipponica were the most abundant species. Within three of the four groups, Textularia sp. A was the most abundant. Benthic foraminiferal assemblages, along with the presence of planktonic foraminifera suggest an open inner-shelf marine environment. Within the two cores, three paleoenvironmental episodes were recognized. The first episode occurred from ca. 8,400 cal. yr BP to ca. 6,500 cal. yr BP and was characterized by relatively high terrigenous input. At the beginning of this episode, sea level was at about -5 m rising to a maximum height of around +5 m by the mid-Holocene highstand (6,500 BP). The second episode (6,500 cal. yr BP-4,000 cal. yr BP) was transitional, from the relatively high influence of terrigenous material to a third episode, which ranged from 4,000 cal. yr BP-1,000 cal. yr BP, and was characterized by lower terrigenous influence and higher marine influence. During this episode, sea-level fell to its current position.

Variations in the East Asian Monsoon (EAM) control weather and regional climate conditions in the heavily populated, agriculturally dependent regions of eastern and southeast Asia. The South China Sea (SCS), especially the deep, northern SCS, has yielded many high-resolution Quaternary records of the EAM due to its high sedimentation rates and central location within the EAM system. The Sunda Shelf, a low-gradient, shallow region in the southwestern semi-enclosed SCS, is characterized by thin (~1 m) Holocene deposits and thus has not been a target for paleoclimate reconstructions. However, flooded fluvial paleochannels on the Sunda Shelf contain thicker, muddy Holocene sediments. Herein we test the hypothesis that the sediment fill of fluvial paleochannels contains a record of Holocene paleoclimate. Two piston cores were collected in fluvial paleochannels ~80 km offshore of Bintulu, Sarawak, Malaysia. Core D45, collected on the edge of a v-shaped paleochannel, exhibits ~7200 years of sediment accumulation, whereas core D42, retrieved in a shallower paleochannel to the west, yielded a shorter (~3,000 year) but higher resolution record. Mg/Ca ratios of Globigerinoides ruber and Globigerinoides sacculifer, in combination with the stable oxygen isotopic composition of G. ruber, allow for the estimation of sea surface temperature (SST), the stable oxygen isotopic composition of seawater, and salinity. Furthermore, the stable carbon isotopic composition of G. ruber illuminates trends in nutrient input and continental runoff. Based on decreased salinity estimates, a high sedimentation rate (~0.064 cm/yr), and low stable carbon isotopic values, core D45 records an interval of increased rainfall and continental runoff from 6-4 ka, in keeping with nearby speleothem and continental pollen records. Additionally, this core records coeval reduced SST values, a trend not recognized in regional deep-sea records, suggesting a strong terrestrial runoff effect on the sedimentary record. The Medieval Climate Anomaly and Little Ice Age are also tentatively recognized in SST data. Core D42 shows similar trends in data for the late Holocene, confirming that flooded fluvial paleovalleys can provide Holocene paleoclimatic records on continental shelves characterized by generally low sediment accumulation rates.

Post-Last Glacial Maximum (since ca. 20,000 cal yr BP) sediments of the Sunda Shelf (southern South China Sea) are generally thin, with the exception of incised valleys where thicker accumulations tend to occur. These valleys provide a sedimentary record that preserves Holocene environmental changes. In the summer of 2014, two ca. 2 m gravity cores were collected on the Sunda Shelf in ca. 60 m of water ca. 52 km from the mouth of the Terengganu River off northeast peninsular Malaysia. Both cores were sampled in 1 cm contiguous intervals and analyzed for magnetic susceptibility of bulk sediment (BMS), elemental composition (X-ray fluorescence, XRF), and oxygen and carbon stable isotopes on the planktonic foraminifer Globigerinoides ruber. The chronology of the cores, based upon eight AMS radiocarbon age estimates on the benthic foraminifer Cavarotalia annectens, covers most of the Holocene. According to previous work, the Holocene evolution of the Sunda Shelf was largely controlled by the transgression of the South China Sea (SCS) shoreline following the Last Glacial Maximum. Several sea-level curves suggest sea level in the area had reached ca. -60 m by the start of the Holocene (ca. 11,700 cal yr BP). According to this study, at ca. 10,000 cal yr BP, the Sunda Shelf off northeast Peninsular Malaysia was already covered by ca. 30 m of water, evident by the presence of benthic foraminifera and marine mud throughout both cores. The study area transitioned from a shallow, muddy, nearshore environment to an open shelf environment (ca. 60 m water depth and at least 50 km from shore) during the 10,000 - 6,400 cal yr BP time interval. This is indicated by steadily decreasing Al, Ti, and Fe concentrations up-core, steadily increasing Ca concentrations up-core, and the first occurrence of planktonic foraminifera at ca. 7,000 cal yr BP. Between 6,000 - 4,000 cal yr BP, previous studies indicate the mid-Holocene sea-level highstand was reached. This study suggests the presence of a possible diastem at ca. 6,400 cal yr BP in TER14-GC5 based on an abrupt change in BMS and XRF data, which could be related to the maximum flooding surface. Core TER14-GC7 reveals the same shift in elemental and BMS trends at ca. 6,400 cal yr BP, but the transition is more gradual. The up-core profile of XRF and BMS trends reveal a shift from higher BMS values to lower BMS values, steady Ca concentrations, and higher but fluctuating presence of planktonic foraminifera after ca. 6,400 cal yr BP. The changes recorded by the two cores during the ca. 6,400 - present time interval are interpreted as the late transgressive systems tract transitioning to the highstand systems tract (HST). The HST is typically characterized by fine-grained sediments, with abundant foraminifera, that were deposited as the rate of sea-level rise slowed. However, the changes recorded during this time interval could also be explained by a decrease in precipitation in the area, possibly due to a decrease in monsoon intensity. Further investigation is needed to properly understand the time interval from ca. 6,400 cal yr BP - present in order to determine whether trends in the data sets are due to climate or other environmental change.

The Yangtze River (Changjiang) and the Red River (Song Hong) that originate in highlands of the Himalayas and Tibetan Plateau, two of the largest rivers in East and Southeast Asia in terms of water and sediment discharge into oceans, play a pivotal role in the global biogeochemical cycle. The Yangtze River Delta is located at the west coast of the East China Sea and the Red River delta at the west coast of the Gulf of Tonkin in the South China Sea. Both were initiated during the early to middle Holocene.