Role of the Deglacial Buildup of the Great Barrier Reef for the Global Carbon Cycle

The carbon isotope C-13 is commonly used to attribute the last deglacial atmospheric CO2 rise to various processes. Here we show that the growth of the world's largest reef system, the Great Barrier Reef (GBR), is marked by a pronounced decrease in delta C-13 in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a prominent minimum in atmospheric delta(CO2)-C-13 and the Younger Dryas. The event follows the flooding of a large shelf platform and initiation of an extensive barrier reef system at 13 ka. Carbon cycle simulations show the coral delta C-13 decrease was mainly caused by the combination of isotopic fractionation during reef carbonate production and the decomposition of organic land carbon on the newly flooded shallow-water platform. The impacts of these processes on atmospheric CO2 and delta(CO2)-C-13, however, are marginal. Thus, the GBR was not contributing to the last deglacial delta(CO2)-C-13 minimum at similar to 12.4 ka. Plain Language Summary An outstanding problem in our understanding of the global carbon cycle is unraveling the processes that were responsible for the rise of atmospheric CO2 during the last deglaciation (19,000-11,000 years ago). The carbon isotope C-13 is commonly used to attribute the last deglacial atmospheric CO2 rise to various processes. The growth of tropical coral reefs has been controversially discussed in this context. To test this, well constrained reef carbonate records that span the last deglaciation are necessary, but such records are generally not available. Here we make use of a multi-proxy coral reef record obtained at the Great Barrier Reef (GRB). The delta C-13 signal in the carbonate skeletons of fossil corals indicates a pronounced minimum that precisely coincides with a prominent minimum in atmospheric delta(CO2)-C-13 as indicated by ice core records for the Younger Dryas cold period. We show, by carbon cycle simulations, that the GBR coral delta C-13 signal can be explained by changes in reef carbonate production and decomposition of organic land carbon on a newly flooded wide area. However, the simulations indicate that that the world's largest reef system in existence appears to have little effect on the last deglacial atmospheric CO2 and delta(CO2)-C-13 changes.

Automated summary

This study reveals a decrease in the C-13 isotope in fossil coral skeletons of the Great Barrier Reef between 12.8 and 11.7 thousand years ago, which coincides with a decrease in atmospheric delta(CO2)-C-13 and the Younger Dryas cold period. Carbon cycle simulations suggest that this decrease was mainly caused by isotopic fractionation during reef carbonate production and the decomposition of organic land carbon on the newly flooded platform, but the impacts on atmospheric CO2 and delta(CO2)-C-13 were marginal.