Significance of Halimeda bioherms to the global carbonate budget based on a geological sediment budget for the Northern Great Barrier Reef, Australia

Since the correlation between carbon dioxide 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 link between the marine carbonate budget and atmospheric CO2 levels draws attention to the fact that this budget is still not adequately quantified.

It is widely accepted that the calcareous green alga Halimeda is a prominent contributor of carbonate to reef sediment facies in the tropics, however, Halimeda bioherms remain one of the main reservoirs of unaccounted for carbonate. In fact, it has been suggested that Halimeda bioherms on the shelf of the Northern Great Barrier Reef may contain an equal or greater volume of carbonate than the reefs themselves.

Here we attempt to determine the quantitative relationship between Halimeda bioherm and coral reef derived carbonate accumulated during the Holocene on the outer shelf of the Northern Great Barrier Reef Province using data from seismic profiles, reef drill cores and Halimeda bioherm piston and vibrocore cores from the published literature.

The methodology used is detailed in the document availble for download through this record.

Calculations reveal that the ribbon reefs of the Lizard Island and Cooktown regions cover areas of approximately 18.5 and 20.5 km2 and comprise approximately 0.52 and 0.92 Gt CaCO3 respectively. The Halimeda bioherms of the Lizard Island and Cooktown regions cover areas of approximately 184 and 118 km2 and comprise approximately 2.06 and 0.86 Gt CaCO3 respectively.

Thus, that ratio of ribbon reef to Halimeda bioherm CaCO3 is approximately 1:4 and 1:1 for the Lizard Island and Cooktown regions respectively. If we assume that the carbonate content of the Lizard Island region bioherms ranges from 20 to 90 %, we estimate that the mass balance of Halimeda bioherm CaCO3 is between 0.55 and 2.5 Gt. The ratio of ribbon reef to Halimeda bioherms CaCO3 may vary between 1:1 to 1:5 depending on the carbonate content of the Lizard Island bioherms.