| Summary |
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. |
