Evolution of the Great Barrier Reef Halimeda carbonate factory in response to Holocene environmental changes, determined from radiocarbon and nitrogen isotope measurements
Dr Mardi McNeil1, Dr Quan Hua2, Dr Dirk Erler3, Dr Luke Nothdurft1, Dr Jody Webster4, Dr Patrick Moss5
1Queensland University Of Technology, Brisbane, Australia, 2Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia, 3Southern Cross University, Lismore, Australia, 4The University of Sydney, Sydney, Australia, 5The University of Queensland, Brisbane, Australia
The inter-reef Halimeda algal 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 than the nearby coral reefs during the Holocene. Covering > 6000 km² on the continental shelf, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian 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 throughout the Holocene was poorly constrained and based on very few radiocarbon dates, considering their vast area. Additionally, the main source of nitrogen (N) fuelling the productivity of these biogenic structures had not been traced geochemically, and there was no understanding of any temporal variation.
A total of 63 samples of calcium carbonate Halimeda and foraminifera grains, mollusc shells, and bulk soil were dated by ¹⁴C AMS using the VEGA facility at the ANSTO Centre for Accelerator Science. Changes in the terrestrial vegetation community were determined from palynological analysis at the University of Queensland. Nitrogen isotopes were measured on a subset of 27 Halimeda samples using isotope ratio mass spectrometry (IRMS) at the Centre for Coastal Biogeochemistry at Southern Cross University. Using the persulfate-denitrifier method previously developed for foraminifera and coral skeletal material, nitrogen bound within the Halimeda skeletal organic material (SOM) was targeted. This allowed for fossil samples down-core to be analysed, thus providing a 5000-year record of Halimeda δ¹⁵N-SOM.
A temporal sequence of facies transitions that record terrestrial and marine environmental changes over a full sea level cycle from the Last Interglacial, to Last Glacial Maximum (LGM) and deglacial, to transgressive estuarine sediments and shallow marine carbonates was reconstructed. Halimeda growth and carbonate deposition had established by 11,143 (+237/-277) cal. yr BP, just ~450 years after the marine transgression. The Halimeda carbonate factory was productive at least 2100 years earlier than Holocene coral reefs in the study area. Our results provide the first direct in-situ measurements of Halimeda bioherm ages from at or near the Holocene/Pleistocene boundary, recording the timing of bioherm initiation in response to post-glacial sea level rise.
The average Halimeda skeletal δ¹⁵N-SOM was 6.28 ± 0.26 ‰, consistent with δ¹⁵N-NO₃- from western tropical South Pacific (WTSP) thermocline waters, geochemically validating shelf-break upwelling of an oceanic N source. From 5000 to 2000 cal. yr BP, Halimeda δ¹⁵N-SOM decreased by 1-2 ‰, reaching a minima of 5.5 ‰ that persisted for almost 1000 years. The Halimeda δ¹⁵N-SOM variation reflects changes in regional climate and intensified El Niño activity and is consistent with other regional climate proxy records at this time. Thus, the inter-reef Halimeda algal bioherms of the GBR are an important carbonate ecosystem that record a >10,000-year near-continuous record of northeast Australian Holocene oceanographic and environmental changes in response to sea level and climatic drivers, potentially filling spatial and temporal gaps not covered by coral and other proxies.
Mardi McNeil is a marine geoscientist and postdoctoral researcher at the School of Earth and Atmospheric Science, Queensland University of Technology. Mardi is particularly interested in carbonate sedimentology in shallow marine and reef environments, and the biological organisms and communities that construct geological structures at various scales from micro-invertebrates to bioherms and reefs.
Mardi’s recent work has focused on the development of vast Halimeda algal bioherms in the northern Great Barrier Reef, as a sedimentological archive of environmental change in response to Holocene sea level rise and regional climatic changes.