Subsurface dissection of Holocene inter-reef Halimeda bioherms: morphology, facies and latitudinal variations in the northern Great Barrier Reef

Halimeda, calcareous green algae, bioherms are among the largest inter-reef biogenic structures in the Great Barrier Reef, spanning over >6000 km² of the continental shelf – an area exceeding the adjacent coral reefs at equivalent latitudes¹. Previous studies have shown the peculiar circular to reticulate shapes, the internal structure and volume of these accumulations, underscoring their significant contribution to the global neritic carbonate factory throughout the Holocene^2,3. However, a comprehensive understanding of the formation and development of these uniquely shaped bioherms has been hindered by the absence of densely spaced core samples that target bioherm morphologies.

This study presents new data from the 2022 RV Investigator voyage IN2022_V07 “Halimeda bioherms: Origins, function and fate in the northern Great Barrier Reef (HALO)”. Forty-two densely spaced vibrocores were collected (up to 6 m length) over 3 inter-reef sites between lat 15⁰ 48’ 45” S and lat 13⁰ 21’ 11” S. Core locations to target bioherm morphotypes were collected with the aid of 50 cm resolution multibeam bathymetry data, and closely spaced sub-bottom profiles collected during the voyage. A total of almost 200 m of cores have been scanned with high-resolution CT, 50 m of core have been split, logged, scanned with multi-sensor core logger (magnetic susceptibility, spectrophotometer, X-ray fluorescence) and subsampled for grain size, composition and microfossil analysis to show a variety of facies ranging from estuarine to coral-rich deposits. Selected cores have been sub-sampled for radiocarbon dating of Halimeda grains, benthic foraminifers, and organic rich mud (23 samples), spanning from 12 ka to present, aligning with previous findings³. Initial observations revealed further facies complexity than previously thought in morphotypes, including Halimeda floatstone-rudstone and Foraminiferal wackestone-packstone facies in the south, while having facies minor mud matrix and richer in coral, mollusc, rhodolith and lithified clumps abundance towards north. This new dataset significantly advances our understanding of Halimeda bioherm morphology, development, and regional influences, providing new insights into their formation processes and ecological significance.

 

References:

• McNeil, M. A., Webster, J. M., Beaman, R. J., and Graham, T. L., 2016, New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef, Australia: Coral Reefs, v. 35, no. 4, p. 1343-1355. doi: 10.1007/s00338-016-1492-2
• McNeil, M., Nothdurft, L. D., Dyriw, N. J., Webster, J. M., and Beaman, R. J., 2021, Morphotype differentiation in the Great Barrier Reef Halimeda bioherm carbonate factory: Internal architecture and surface geomorphometrics: The Depositional Record, v. 7, p. 176– 199. doi: https://doi.org/10.1002/dep2.122
• McNeil, M., Nothdurft, L. D., Hua, Q., Webster, J. M., and Moss, P., 2022, Evolution of the inter-reef Halimeda carbonate factory in response to Holocene sea-level and environmental change in the Great Barrier Reef: Quaternary Science Reviews, v. 277. doi: 10.1016/j.quascirev.2021.107347