Abstract
AbstractThe 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 palaeo‐environmental interpretations of fossil bioherms and phylloid algal mounds on geological timescales.
Highlights
Halimeda macroalgal bioherms are a well-recognized biogenic carbonate mound facies in tropical continental shelf-edge and platform-top settings, and contribute an important but poorly constrained component of the neritic carbonate factory (Milliman, 1993; Freile et al, 1995; Hillis, 1997; Rees et al, 2007)
This study presents a quantitative geomorphological analysis of three different Halimeda bioherm morphotypes by integrating the most recently available geophysical datasets to address three main objectives: (a) Combine high-resolution 3D surface bathymetry with co-located acoustic sub-bottom profiles and GIS spatial and morphometric terrain analyses; (b) Quantify the variation in physical characteristics such as bioherm area, thickness, volume and terrain patterns for the annulate, reticulate and undulate morphotypes; (c) Better constrain the size of the Halimeda bioherm carbonate factory by quantifying the mass and volume of accumulated CaCO3 by each morphotype; and (d) Use these analyses to determine the differences and similarities between the annulate, reticulate and undulate morphotypes, and resolve their sedimentological relationships
It is possible to correlate sub-bottom profiles with high-resolution 3D LiDAR bathymetry, allowing the substrate beneath the annulate, reticulate and undulate morphotypes to be explored for the first time
Summary
Halimeda macroalgal bioherms are a well-recognized biogenic carbonate mound facies in tropical continental shelf-edge and platform-top settings, and contribute an important but poorly constrained component of the neritic carbonate factory (Milliman, 1993; Freile et al, 1995; Hillis, 1997; Rees et al, 2007). Halimeda is not an invertebrate, but a calcareous green alga from the phylum Chlorophyta, order Bryopsidales that produces and sheds aragonitic skeletal grains In this sense, modern and fossil Halimeda bioherms are comparable in lithology, structure and morphology to the aragonitic phylloid algal mounds of the Late Palaeozoic (Wray, 1977; Drew, 1983; Drew and Abel, 1988; Kirkland et al, 1993; Purkis et al, 2015), taxonomically unrelated. A common interpretation of formation involves the upwelling of cool, nutrient-rich water from below the oceanic thermocline delivering the volume of nutrients required to fuel Halimeda growth, not otherwise available from the oligotrophic tropical waters (Wolanski et al, 1988) These consistent characteristics make modern Halimeda bioherms an important analogue for the depositional environment, facies interpretation and processes of formation of lithified fossil Halimeda deposits (discussed ), and potentially phylloid algal mounds, from the geological past. No mound-shaped accumulations are described, the authors stop short of describing these outcrops as Halimeda bioherms, but do make an analogous comparison to the modern Great Barrier Reef bioherms
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