Geochemical Records from Porites Corals: Ocean Circulation, Pollution, and Climate

Abstract

Oceanic variables and human impacts on coastal environments are not well-understood. My PhD study has extracted the environmental information preserved in the skeletons (CaCO3) of Porites corals to understand the long-term effects of these influences. Porites corals are ideal biomonitors because they are long-lived and during the process of calcification, chemical signatures are incorporated into the skeletal aragonite providing time histories of impacts to the corals and reef systems. This thesis is aimed at (1) using coral radiocarbon (14C) to understand the seasonal to decadal scale ocean circulation and climatic variability in the South China Sea (SCS) and in the Great Barrier Reef (GBR) and (2) developing and applying multiple elemental ratios in coral skeletons to evaluate the century-long environmental changes (temperature and water quality) in the GBR.

The SCS radiocarbon study showed that Pacific water intrudes into the eastern SCS as reflected by the detection of 'early radiocarbon bomb peak' in late 1955 and that the circulation in the SCS is more affected by East Asian Monsoon than previously thought. The GBR radiocarbon study showed that GBR seawater 14C is mainly affected by the East Australian Current. The extended GBR 14C record also provides insights into the long-term overturning of the 14C reservoirs. In both study sites, climatic variabilities such as El Nino-Southern Oscillation were found to be correlated with abnormally higher or lower 14C signals tracking surface water circulation.

Multiple coral cores were collected from the nearshore GBR to assess the robustness of sea surface temperature (SST) proxies (Li/Ca, B/Ca, Mg/Ca, Sr/Ca, U/Ca). Results showed that U/Ca was more robust than other elemental ratios in these GBR corals. An improvement of Li/Ca and B/Ca as SST proxy can be made by normalizing Li/Ca and B/Ca to Mg/Ca which eliminates the impacts of vital effects. The empirical calibrations established in this study have contributed to the site-specific SST calibrations in the GBR and have been used to revisit a universal model to reconstruct the SST.

The Gladstone Harbour region of Queensland is a major port for coal and alumina export. It is also subjected to major port expansion and dredging events. Corals were collected around the Gladstone Harbour to test if they are good indicators of environmental pollution throughout the historical development of the Gladstone Harbour. Results showed that a few heavy metals (Al, Ti, V, Mn, Cu, Zn, Pb) were elevated in coral skeletons during the post-industrial era (1950-present). These contaminants were mainly delivered to the coastal seawater by river runoff, but other modes of delivery also exist.

Corals subjected to nearshore river runoff can record terrigenous input. Corals were collected from the Central and Southern GBR to cover different hydrological conditions. Results showed that coral Ba/Ca are effective tracers of river runoff over the longer timescale, but the seasonal-scale variations of Ba/Ca can be terrigenous, temperature-dependent and/or bio-regulated depending on their geographic locations. The quantification of terrestrial runoff can be made from coral Ba/Ca ratios, but this approach should be preceded by careful site selection.