ALBERT

Description: The South China Sea (SCS) is well connected with the western Pacific and influenced by the East Asian monsoon. We have examined temporal variations in radiocarbon marine reservoir ages (R) and regional marine reservoir corrections (DeltaR) of the SCS during the Holocene using paired measurements of AMS 14C and TIMS 230Th on 20 pristine corals. The results show large fluctuations in both R and DeltaR values over the past 7500 years (yrs) with two distinct plateaus during 7.5-5.6 and 3.5-2.5 thousand calendar years before present (cal ka BP). The respective weighted mean DeltaR values of these plateaus are 151 ± 85 and 89 ± 59 yrs, which are significantly higher than its modern value of -23 ± 52 yrs. This suggests that using a constant modern DeltaR value to calibrate 14C dates of the SCS marine samples will introduce additional errors to the calibrated ages. Our results provide the first database for the Holocene R and DeltaR values of the SCS for improved radiocarbon calibration of marine samples. We interpret the two DeltaR plateaus as being related to two intervals with weakened El Niño - Southern Oscillation (ENSO) and intensified East Asian summer monsoon (EASM). This is because the 14C content of the SCS surface water is controlled by both the 14C concentration of the Pacific North Equatorial Current (NEC) which is in turn influenced by ENSO-induced upwelling along the Pacific equator and vertical upwelling within the SCS as a result of moisture transportation to midlatitude region to supply the EASM rainfall.

Description: A palynological record spanning the last glacial–interglacial period was derived from high-resolution, deep-sea core MD03-2607, located near Kangaroo Island in South Australia. The core site lies opposite the mouth of the River Murray that, together with the Darling River, drains the extensive (1.6 x 10⁶ km²) Murray Darling Basin (MDB). The record comprises 120 samples and is compared with detailed records of sea-surface temperature (SST), the C3/C4 plant ratio obtained from the δ¹³C of n-alkanes from leaf waxes, the fluvial clay fraction and its neodymium isotopic composition, airborne dust, as well as the biomass burning component levuglosan. The chronology of the core is robust; it is built on 24 radiocarbon dates derived from planktic foraminifera, 16 OSL dates, plus 12 tie points linked to the astronomically tuned marine isotopic record. Algal remains are found in nearly all samples supporting our postulation that the palynoflora is predominantly waterborne. Major findings are that the gymnosperm Callitris, together with high percentages of herb pollen (mostly C~3~ plants), is predominant during cold, arid phases, whereas Eucalyptus, is predominant during warmer and wetter periods. High charcoal concentration coincides with high percentages of Eucalyptus, mostly during wet and warm periods. Using the geochemistry of the core's fluvial sediments, it has been possible to identify when water-transported palynoflora and charcoal originated from the Murray sub-basin (consisting of the River Murray and its main tributaries but not from central or western South Australia). During those periods, rainfall principally originated from the southeastern Indian Ocean. When the Darling sub-basin was the main source of the palynoflora, rainfall must have instead originated from northern Australia. The eolian dust record from the core shows that the dust signal generally coincides with the increased values in herb pollen, in particular during the Last Glacial Maximum (LGM) when, in addition to high herb percentages, Callitris representation also increased. This dry landscape taxon likely colonised the then-exposed Lacepede Shelf during this period of extreme low sea level. There is a good correspondence between SST and mean annual precipitation reconstructed from the pollen counts. During warm phases in the ocean, Eucalyptus was the dominant tree taxon, especially for the entirety of MIS 5, plus MIS 3 and MIS 1. Charcoal levels were particularly low during the dry phases MIS 4 and 2, and even more so during the LGM.

Description: The inter-reef Halimeda bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief throughout the Holocene. Covering 〉 6000 km2, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian continental shelf geomorphology, neritic carbonate factory, and sedimentary archive of post-glacial environmental changes. Previously, the chronological record of initiation and development of the Halimeda bioherm carbonate factory was poorly constrained and based on very few age data points. A comprehensive new age dataset is presented, comprising sixty-three AMS radiocarbon measurements of Halimeda and foraminifera grains, mollusc shells and bulk soil from twelve inter-reef sediment cores, and ten previously published Halimeda ages that are newly calibrated. Radiocarbon measurements were undertaken at the ANSTO Centre for Accelerator Science in 2018, 2019 and 2020 from cores collected by the Bureau of Mineral Resources in 1983. Halimeda growth had established by 11,143 +237/-277 cal. yr BP, just ~450 years after the marine transgression commenced and approximately 1000 years earlier than previous inferred estimates. The outer-shelf carbonate factory was initially dominated by benthic foraminifera, then Halimeda was productive for at least 2100 years prior to the turn-on of Holocene coral reefs in the study area. Inter-reef Halimeda bioherm sediments including foraminiferal communities, might record a 〉10,000-year near-continuous geochemical record of northeast Australian Holocene oceanographic and climatic changes, potentially filling spatial and temporal gaps not covered by coral and other marine sediment proxies.