Evolution of the inter-reef Halimeda carbonate factory in response to Holocene sea-level and environmental change in the Great Barrier Reef

The inter-reef Halimeda bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief and up to four times greater volume of calcium carbonate sediment than the nearby coral reefs during 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. However, the geochronological record of initiation and development of the Halimeda bioherm carbonate factory was poorly constrained and based on very few datapoints. A comprehensive age dataset is presented, comprising sixty-three new 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. Facies transitions and environmental changes are recorded from lithological and palynological analyses spanning Last Interglacial Halimeda deposits, Last Glacial Maximum terrestrial palaeosols, transgressive mangrove vegetated shorelines, and the turn-on of the Halimeda bioherm carbonate factory. Mangrove pollen and coral records from the study area provide additional spatial, temporal and environmental context.Halimeda had established by 11.1 cal kyr BP, just 450 years after transgressive shelf inundation 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 at 8.9 cal kyr BP from Boulder Reef (15.4°S, 145.4°E). Inter-reef Halimeda bioherms play a major role in carbon and nutrient cycling and potentially preserve a near-continuous geochemical record of northeast Australian Holocene oceanographic and climatic changes, filling spatial and temporal gaps not covered by coral and other marine sediment proxies.