Reconstruction of palaeoenvironmental changes and the response of the Great Barrier Reef (GBR) ecosystem since 30,000 BP: implications from REY (rare earth elements and yttrium) and element ratios of reef-building carbonates

Abstract

Our understanding of the development of the Great Barrier Reef (GBR) in response to sediment inputs over the past 30 ka is limited. Geochemical proxies including rare earth elements and yttrium (REY) along with element ratios (Pr/Yb, Y/Ho, Mn/Ca, Nd/Yb and Ce anomalies) are known to be closely related to sediment and nutrient input. This research conducts novel geochemical analysis on almost 200 fossil reef building samples that formed since the Late Pleistocene, the last 30 ka. The first part of this research focuses on improving the methodology for ICP-MS analysis. Then a series of experiments are conducted to evaluate factors that may bias REY results, including the inter-taxa differences of genera within a specific family, sampling position and potential influence meteoric diagenesis effects. The results indicate the REY are not affected by these factors, which greatly expands the range of samples available for the reconstruction of palaeo-water quality. Both coral and microbialite samples from the GBR since 30 ka show typical geochemical features of well-oxygenated, shallow ambient seawater. However, both spatial and temporal differences exist in these geochemical signals. The northern-central GBR samples record higher values for ΣREY and other geochemical proxies than southern-central GBR samples. The reasons include differences in shelf geomorphology, timing and rates of shelf flooding, and palaeo-mangrove development near the northern-central GBR as an additional source for the sediments. Temporally, five distinct geochemical peaks in ΣREY and other geochemical proxies are observed, at 22-20 ka, 18-17 ka, 16-15 ka, 12-10 ka and 8-6 ka. These peaks are associated with a degradation in reef development, characterised by major changes in coral-algal assemblages, decreases in accretion rates and even death events, that are consistent with previous studies, including those on downhole gamma ray and palaeoecology work.