Coral skeletal carbon isotopes (δ13C and Δ14C) record the delivery of terrestrial carbon to the coastal waters of Puerto Rico

Tropical small mountainous rivers deliver a poorly quantified, but potentially significant, amount of carbon to the world's oceans. However, few historical records of land-ocean carbon transfer exist for any region on Earth. Corals have the potential to provide such records, because they draw on dissolved inorganic carbon (DIC) for calcification. In temperate systems, the stable- (delta C-13) and radiocarbon (Delta C-14) isotopes of coastal DIC are influenced by the delta C-13 and Delta C-14 of the DIC transported from adjacent rivers. A similar pattern should exist in tropical coastal DIC and hence coral skeletons. Here, delta C-13 and Delta C-14 measurements were made in a 56-year-old Montastraea faveolata coral growing similar to 1 km from the mouth of the Rio Fajardo in eastern Puerto Rico. Additionally, the delta C-13 and Delta C-14 values of the DIC of the Rio Fajardo and its adjacent coastal waters were measured during two wet and dry seasons. Three major findings were observed: (1) synchronous depletions of both delta C-13 and Delta C-14 in the coral skeleton are annually coherent with the timing of peak river discharge, (2) riverine DIC was always more depleted in delta C-13 and Delta C-14 than seawater DIC, and (3) the correlation of delta C-13 and Delta C-14 was the same in both coral skeleton and the DIC of the river and coastal waters. These results indicate that coral skeletal delta C-13 and Delta C-14 are recording the delivery of riverine DIC to the coastal ocean. Thus, coral records could be used to develop proxies of historical land-ocean carbon flux for many tropical regions. Such information could be invaluable for understanding the role of tropical land-ocean carbon flux in the context of land-use change and global climate change.