Age determination of Halimeda bioherm sediment deposits from the Great Barrier Reef, Australia, using 14C AMS from cores collected in 1983

The inter-reef Halimeda bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief throughout the Holocene. Covering > 6000 km2, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian continental shelf geomorphology, neritic carbonate factory, and sedimentary archive of post-glacial environmental changes. Previously, the chronological record of initiation and development of the Halimeda bioherm carbonate factory was poorly constrained and based on very few age data points. A comprehensive new age dataset is presented, comprising sixty-three AMS radiocarbon measurements of Halimeda and foraminifera grains, mollusc shells and bulk soil from twelve inter-reef sediment cores, and ten previously published Halimeda ages that are newly calibrated. Radiocarbon measurements were undertaken at the ANSTO Centre for Accelerator Science in 2018, 2019 and 2020 from cores collected by the Bureau of Mineral Resources in 1983. Halimeda growth had established by 11,143 +237/-277 cal. yr BP, just ~450 years after the marine transgression commenced and approximately 1000 years earlier than previous inferred estimates. The outer-shelf carbonate factory was initially dominated by benthic foraminifera, then Halimeda was productive for at least 2100 years prior to the turn-on of Holocene coral reefs in the study area. Inter-reef Halimeda bioherm sediments including foraminiferal communities, might record a >10,000-year near-continuous geochemical record of northeast Australian Holocene oceanographic and climatic changes, potentially filling spatial and temporal gaps not covered by coral and other marine sediment proxies.