Temporal variability in the Holocene marine radiocarbon reservoir effect for the tropical and South Pacific

Understanding the marine radiocarbon reservoir effect (i.e., marine radiocarbon reservoir age (R) and/or correction (ΔR)) is important for the construction of robust radiocarbon chronologies for marine archives for various research areas including archaeology, palaeoecology, paleoceanography, Quaternary research and climate change studies. In this study, we examined temporal ΔR variability for the South China Sea (SCS) and the Great Barrier Reef (GBR) during the past ~8.1 ka based on ¹⁴C analysis of ²³⁰Th-dated corals. Results show large ΔR variations of ~410 yr and ~490 yr for the SCS and the northern GBR for ~5.5-8.1 ka and ~5.5-7 ka, respectively, and a smaller ΔR variability of ~200 yr for the SCS for ~2-3.5 ka. Our data, together with those previously published for the Tropical and South Pacific, indicate that variability in ocean upwelling bringing old subsurface waters to the surface and/or changes in the sources (or ¹⁴C level) of the upwelled waters are responsible for seeing large ΔR variations in coastal areas along the eastern Pacific and the Tropical East Pacific (TEP) of several hundred to a thousand years mostly during the Early to Middle Holocene. ΔR variations in the central and western Pacific of several and a couple of hundred years during the Middle and Late Holocene, respectively, might be due to variability in Pacific-wide ocean circulation associated with climatic changes controlling the spread of upwelled waters from the TEP to the west. This mechanism together with local/regional effects, such as changes in ocean upwelling in the SCS resulting from East Asian monsoon variability and changes in upwelling and/or horizontal advection in the northern GBR associated with variability in the El Nino Southern Oscillation, might be responsible for the observed ΔR variations in these areas. The results of our study also indicate the need for regional marine radiocarbon calibration curves for improved radiocarbon dating of marine samples as the observed Holocene ΔR values for the Tropical and South Pacific are not fully reproduced by a recent modelling work using a 3D ocean model, which takes into account climate change effects. Ocean circulation changes were included in the model for the period of 11.5-50 cal kyr BP but possibly not considered or not well represented for the Holocene, which might explain the differences between the observed and modelled ΔR values