| Summary: | Since the correlation between carbon dioxide (CO2) levels and global temperatures was established in the ice core records, quantifying the components of the global carbon cycle has become a priority with a view to constraining models of the climate system. The marine carbonate budget is still not adequately constrained and the quantitative significance of the calcareous green alga Halimeda still remains particularly poorly understood. Previously, it has been suggested that Halimeda bioherms on the shelf of the Great Barrier Reef may contain a volume of carbonate equal to or greater than that contained within the shelf edge coral reefs. This study uses published datasets to test this hypothesis in the Northern Great Barrier Reef (NGBR) province. It is estimated that Halimeda bioherms on the outer shelf of the NGBR contain at least as much (and up to four times more) CaCO3 sediment as the adjacent ribbon reef facies. Globally, if these findings are even only partially applicable, the contribution of shallow water carbonate sediments to the global carbon budget based on coral reefs alone is currently substantially underestimated.
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