Halimeda bioherms occur as extensive geological structures on the northern Great Barrier Reef (GBR), Australia. We present the most complete, high-resolution spatial mapping of the northern GBR Halimeda bioherms, based on new airborne lidar and multibeam echosounder bathymetry data. Our analysis reveals that bioherm morphology does not conform to the previous model of parallel ridges and troughs, but is far more complex than previously thought. We define and describe three morphological sub-types: reticulate, annulate, and undulate, which are distributed in a cross-shelf pattern of reduced complexity from east to west. The northern GBR bioherms cover an area of 6095 km 2 , three times larger than the original estimate, exceeding the area and volume of calcium carbonate in the adjacent modern shelf-edge barrier reefs. We have mapped a 1740 km 2 bioherm complex north of Raine Island in the Cape York region not previously recorded, extending the northern limit by more than 1° of latitude. Bioherm formation and distribution are controlled by a complex interaction of outer-shelf geometry, regional and local currents, coupled with the morphology and depth of continental slope submarine canyons determining the delivery of cool, nutrientrich water upwelling through inter-reef passages. Distribution and mapping of Halimeda bioherms in relation to Great Barrier Reef Marine Park Authority bioregion classifications and management zones are inconsistent and currently poorly defined due to a lack of high-resolution data not available until now. These new estimates of bioherm spatial distribution and morphology have implications for understanding the role these geological features play as structurally complex and productive inter-reef habitats, and as calcium carbonate sinks which record a complete history of the Holocene post-glacial marine transgression in the northern GBR.

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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.

The ram’s horn squid Spirula spirula (Linnaeus, 1758) is the only extant cephalopod with an internal calcareous, chambered shell that is coiled, making it the sole living representative of the once speciose order Spirulida [...]