Morphotype differentiation in the Great Barrier Reef Halimeda bioherm carbonate factory: Internal architecture and surface geomorphometrics

Abstract

The calcareous Halimeda bioherms of the northern Great Barrier Reef, Australia are the largest actively accumulating Halimeda deposits worldwide. They contribute a substantial component of the Great Barrier Reef neritic carbonate factory, as well as the geomorphological development of Australia's northeast continental shelf.Halimeda bioherm geomorphology is complex, expressing three distinct variations in morphotype patterns: annulate, reticulate and undulate. Similar regular and irregular geomorphological patterning often results from scale-dependent biophysical feedback mechanisms. Therefore, a better understanding of morphotype differentiation can inform the biotic and abiotic drivers of spatial heterogeneity in the bioherm ecosystem. Here, 3D LiDAR bathymetry is integrated with 2D sub-bottom profile datasets to investigate surface topography and internal sedimentary architecture of Halimeda bioherms through space and time. Using the ESRI ArcGIS 3D Analyst and Benthic Terrain Modeller extensions, the bioherm surface and subsurface geomorphometric characteristics were quantified for the annulate, reticulate and undulate morphotypes. Significant variation was found between the three bioherm morphotypes in their surface topography, internal structure, volume, slope gradients and terrain complexity. Therefore, their geomorphology is probably influenced by differing processes and biophysical feedback mechanisms. The complex surface topography does not appear to be inherited from the antecedent substrate, and preferred aspect orientations resulting from hydrodynamic forcing appear to be limited. It is suggested here that autogenic dynamics or biotic self-organization similar to patterns and processes in other marine organo-sedimentary systems modulates Halimeda bioherm geomorphology, and some hypotheses are offered towards future studies. Morphotype differentiation has implications for the development of the Halimeda bioherm carbonate factory, rates of sediment aggradation and progradation, and variable capacity to fill accommodation space. Self-organization dynamics and morphology differentiation in Modern bioherm systems could potentially inform | 177 MCNEIL Et aL.

New constraints on the postglacial shallow-water carbonate accumulation in the Great Barrier Reef

“…Halimeda bioherms are a significant component of the region’s postglacial carbonate budget. A comparison of the most up-to-date morphometric data 27 shows that the postglacial Halimeda deposits are equivalent in mass to 5.5–10.5% of the Holocene reef mass on the GBR shelf (Table 1 ). Halimeda can form mounds in inter-reef areas of the GBR up to a thickness of 20 m 25 – 27 , 59 , 60 .…”

“…A comparison of the most up-to-date morphometric data 27 shows that the postglacial Halimeda deposits are equivalent in mass to 5.5–10.5% of the Holocene reef mass on the GBR shelf (Table 1 ). Halimeda can form mounds in inter-reef areas of the GBR up to a thickness of 20 m 25 – 27 , 59 , 60 . Recent reviews of published and new high-resolution bathymetry have revealed that at least 6000 km 2 of the northern and central GBR are covered by Halimeda bioherms 26 , 27 .…”

“…Globally, drowned reefs may constitute an important fraction of postglacial carbonate 21 , 23 , 24 and an alternative earlier source of postglacial carbon. Halimeda bioherms are another contributing component, with recent investigations from the GBR suggesting they are volumetrically relevant in postglacial carbonate budgets 25 – 27 .…”

“…325 well-dated fossil reef cores 30 , 31 , seismic lines 32 , 33 , multibeam bathymetry 34 , and surface sediment and rock dredge samples 35 , 36 . New detailed bathymetry and interpretations are also providing new constraints on the spatial distribution and volume of Halimeda bioherms in the northern GBR 26 , 27 .…”

Morphotectonic Analysis of the East Manus Basin, Papua New Guinea

“…The BTM is a pixel-based surface tool that allows the user to interrogate bathymetric data, on a pixel-topixel comparison (Walbridge et al, 2018). This tool is useful for identifying surface variation, characterizing and identifying seafloor morphology (Verfaillie et al, 2006;Rengstorf et al, 2012;Ismail et al, 2015;Watson et al, 2017;McNeil et al, 2020). This method provides a quantitative measure of seafloor variation, removing interpretation bias.…”