Coral reefs of the Indo-Pacific and changes in global Holocene
University of Southampton, Faculty of Engineering Science and Mathematics, School of Ocean and Earth Science,
The key to understanding the future impact of the anthropogenic combustion of fossil fuels on the climate system, is to fully understand the complex feedback loops within the natural Earth system. One natural climate feedback that has been proposed is the Coral Reef Hypothesis whereby significant increases in coral reef growth may have contributed to the deglacial increase in atmospheric CO2 observed in the ice core records. This thesis examines the role of coral reefs in the oceanic carbonate budget and global carbon cycle both spatially and temporally during the Holocene. Using the most comprehensive reef area estimate to date, a conservative estimate of cumulative CaCO3 accumulation within coral reefs globally from 10 kyr BP to present is 7970 Gt. This estimate includes a temporal and spatial view of reef CaCO3 accumulation during the Holocene and represents coral reefs alone, whereas previous budgets have included wider neritic carbonate facies. This mass of reefal CaCO3 accumulation would have made approximately 2100 Gt CO2 available for release to the atmosphere over the Holocene.
Radiocarbon dating of coral obtained from new drill cores from Rodrigues (Southwest Indian Ocean), Lizard Island and MacGillivray Reef (Northern Great Barrier Reef (NGBR)), helps to reveal the spatial and temporal pattern of Holocene CaCO3 accumulation within these reefs and contributes to the dataset compiled to calculate the global mass balance of coral reef carbonate. The new data presented here demonstrates that the reefs at Rodrigues, like those at Reunion and Mauritius only reached a mature state (reached sea level) by 2 to 3 ka – thousands of years later than most of the reefs in the Australasian region. The windward margins at Lizard Island and MacGillivray Reef started growing ca 6.7 and 7.6 cal kyr BP respectively directly on an assumed granite basement and reached sea level approximately 4 and 5.6 cal kyr BP respectively. The leeward margin at MacGillivray Reef was initiated by 8.2 cal kyr BP directly on a granite basement, only reaching sea level relatively recently between 260 and 80 cal yr BP. The absence of Pleistocene reefal deposits indicates the possibility that the shelf in this region may have subsided relative to modern day sea level by at least 15 m since the last interglacial (125 ka).
The role of the calcareous green alga Halimeda in the marine carbonate budget is still unknown both spatially and temporally for the Holocene. Here a quantification of the carbonate mass within the ribbon reefs and Halimeda bioherms on the outer shelf of the NGBR is presented. It is estimated that Halimeda bioherms contain at least as much (possibly 400 % more) CaCO3 sediment than the adjacent ribbon reefs within the NGBR province.
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