Mid-Holocene sea surface conditions and riverine influence on the inshore Great Barrier Reef

We present measurements of Sr/Ca, δ18O, and spectral luminescence ratios (G/B) from a mid-Holocene Porites sp. microatoll recovered from the nearshore Great Barrier Reef (GBR). These records were used as proxies to reconstruct sea surface temperature (SST), the δ18O of surrounding seawater (δ18Osw), and riverine influence, respectively, and compared with records from a modern Porites sp. microatoll growing in the same environment. Strong riverine influence in the mid-Holocene record is indicated by (1) an increased annual δ18Osw range in the mid-Holocene record, (2) negative peaks in δ18O characteristic of flood events, and (3) a higher G/B luminescence ratio. Seasonal cycles in G/B suggest that humic acid inputs were elevated for a longer portion of the year during the mid-Holocene. The seasonal cycle of δ18Osw peaked earlier in the year in the mid-Holocene record relative to the modern, while mean δ18Osw values from the mid-Holocene record were similar to modern values. These records provide an insight into the oceanographic conditions the nearshore GBR experienced during mid-Holocene climatic shifts and are consistent with a strong Australian–Indonesian Summer Monsoon (AISM) system at ~ 4700 cal. yr BP.