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

The carbon isotope 13C 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 δ13C in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a prominent minimum in atmospheric δ13CO2 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 δ13C 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 δ13CO2, however, are marginal. Thus, the GBR was not contributing to the last deglacial δ13CO2 minimum at ∼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 13C 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 δ13C signal in the carbonate skeletons of fossil corals indicates a pronounced minimum that precisely coincides with a prominent minimum in atmospheric δ13CO2 as indicated by ice core records for the Younger Dryas cold period. We show, by carbon cycle simulations, that the GBR coral δ13C 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 δ13CO2 changes. Key Points: Great Barrier Reef corals indicate pronounced decrease in skeletal stable carbon isotopes between 12.8 and 11.7 ka during Younger DryasEvent follows shelf flooding and barrier reef initiation at 13 ka and coincides with prominent atmospheric stable carbon isotope minimumCarbon cycle modeling reveals marginal impact of changes in reef carbonate production and land carbon decomposition on atmospheric carbon [ABSTRACT FROM AUTHOR]