1,4,1,7,7,10,- 1Queensland University of Technology, Brisbane City, QLD, 4000, Australia
- 2James Cook University, Townsville, 4811, QLD, Australia
- 3The University of Sydney, Sydney, NWS, 2006, Australia
- 4Geoscience Australia, Canberra, 2601, ACT, Australia
- 5University of Granada, Granada, 18071, Spain
- 6University of Queensland, Brisbane, 4072, QLD, Australia
- 7The University of Tokyo, 113-8654, Japan
- 8James Cook University, Cairns, QLD, 4879, Australia
- 9University of Western Australia, Perth, 6009, WA, Australia
- 10Colorado School of Mines, Denver, 80401, CO, USA
- 11Commonwealth Scientific and Industrial Research Organisation (CSIRO), Hobart, 7000, TAS, Australia
Halimeda (calcareous green algae) bioherms are among the largest accumulations of biogenic sediment within the Great Barrier Reef Marine Park, covering more than 6000 km2 of the continental shelf and exceeding the area of adjacent coral reefs at equivalent latitudes1. Previous studies have documented their circular to reticulate shapes, the internal structure and thickness, underscoring their uncertain genesis and major contribution to the global Holocene neritic carbonate factory2,3. However, a detailed understanding of the formation and development of these uniquely shaped bioherms has been limited by the lack of high-resolution bathymetric maps, surface sediment samples, and densely spaced core material that target geomorphological variability.
This study presents new data from the RV Investigator voyage IN2022_V07 “Halimeda bioherms: Origins, function and fate in the northern Great Barrier Reef (HALO)”. The first sub-metre resolution (50 cm) multibeam bathymetry data and sub-bottom profiles reveal spectacular bioherm shapes and patterns not previously visible on 30 m models. Geomorphometric analysis of the 50 cm DEMs using ESRI ArcGISPro 3.3 quantified the surface characteristics and produced the first benthic terrain classification based on pattern recognition rather than differential geometry to define benthic features (Geomorphon Landforms tool).
Sixty-nine surface grab samples were collected using Boxcorer and Smith-McIntyre grab from three sites between 15° and 13° S. These samples were analysed for grain size, total carbonate and composition to characterise sedimentary variation across modern bioherms. Results highlight variability among different benthic structures, implying distinct and dynamic environmental settings.
In addition, forty-two densely spaced vibrocores (up to 6 m long) were recovered and scanned with high-resolution CT. Split cores were logged for facies and scanned with multi-sensor core logger (magnetic susceptibility, spectrophotometer, X-ray fluorescence). Subsamples at regular intervals (10cm) have been processed for grain size, total CaCO3% and microfossil analysis. Radiocarbon dates indicate the cores range from 12 ka to present. Initial observations revealed a range of morphotypes, including Halimeda floatstone-rudstone and Foraminiferal wackestone-packstone facies and layers of dense coral, mollusc, rhodolith and lithified clumps. 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:
1. 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
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
3. 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
How to cite: Szilagyi, Z., Nothdurft, L., Duce, S., Webster, J., McNeil, M., Braga, J.-C., Graham, T., Byrne, M., Behrens, B. C., Yokoyama, Y., Beaman, R., Paumard, V., Shragge, J., Nau, A., Berry, C., Kim, C., Goh, S., Reeves, J., Picton, L., and Bostock, H.: Surface morphology and internal architecture of Holocene Halimeda bioherms in the northern Great Barrier Reef, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14867, https://doi.org/10.5194/egusphere-egu26-14867, 2026.
