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

Abstract

We present measurements of Sr/Ca, δ 18 O, 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 δ 18 O of surrounding seawater (δ 18 O sw ), 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 δ 18 O sw range in the mid-Holocene record, (2) negative peaks in δ 18 O 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 δ 18 O sw peaked earlier in the year in the mid-Holocene record relative to the modern, while mean δ 18 O sw 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.