Geochemistry of shallow tropical marine sediments from the Arafura Sea, Australia

Tropical shelves have not been the focus of many biogeochemical studies. This study presents new data from sediments collected in the Arafura Sea, including headspace gas and lipid contents. This allows assessment of modern depositional environments and reconstruction of environmental changes that have affected this tropical carbonate shelf during the last glacial transgression. Biomarker compositions from surface sediment and core samples from the upper 2.5 m revealed that marine algal input dominated the recently deposited organic matter over the survey area. In a region closest to the shore, high taraxerol abundance indicated a strong input of mangrove material to samples below 2.5 m, indicating strong mangrove development associated with rising sea level during the transgression following the Last Glacial Maximum. When sea level stabilized around 6000 BP, biomarker data indicate that the terrigenous contribution decreased and that palaeo-channels formed during erosion of the continental shelf were infilled with marine dominated material. Sediments within these palaeo-channels contained elevated concentration of dissolved CH4 at the base of the cores. The source of the gas was determined to be microbial in origin, based on depleted δ13C isotopic composition (−96‰) and very low concentration of wet gas components.