Carbon isotopes (delta-carbon-13 and delta-carbon-14) and trace elements (barium, manganese, yttrium) in small mountainous rivers and coastal coral skeletons in Puerto Rico

Tropical small mountainous rivers (SMRs) may transport up to 33% of the total carbon (C) delivered to the oceans. However, these fluxes are poorly quantified and historical records of land-ocean carbon delivery are rare. Corals have the potential to provide such records in the tropics because they are long-lived, draw on dissolved inorganic carbon (DIC) for calcification, and isotopic variations within their skeletons are useful proxies of palaeoceanographic variability. The ability to quantify riverine C inputs to the coastal ocean and understand how they have changed through time is critical to understanding global carbon budgets in the context of modern climate change. A seasonal dual isotope (13C & 14C) characterization of the three major C pools in two SMRs and their adjacent coastal waters within Puerto Rico was conducted in order to understand the isotope signature of DIC being delivered to the coastal oceans. Additionally a 56-year record of paired coral skeletal C isotopes (delta13C & Delta 14C) and trace elements (Ba/Ca, Mn/Ca, Y/Ca) is presented from a coral growing ∼1 km from the mouth of an SMR. Four major findings were observed: (1) Riverine DIC was more depleted in delta13C and Delta 14C than seawater DIC, (2) the correlation of delta13C and Delta14C was the same in both coral skeleton and the DIC of the river and coastal waters, (3) Coral delta13C and Ba/Ca were annually coherent with river discharge, and (4) increases in coral Ba/Ca were synchronous with the timing of depletions of both delta 13C and Delta14C in the coral skeleton and increases in river discharge. This study represents a first-order comprehensive C isotope analysis of major C pools being transported to the coastal ocean via tropical SMRs. The strong coherence between river discharge and coral delta 13C and Ba/Ca, and the concurrent timing of increases in Ba/Ca with decreases in delta13C and Delta14C suggest that river discharge is simultaneously recorded by multiple geochemical records. Based on these findings, the development of coral-based proxies for the history of land-ocean carbon flux would be invaluable to understanding the role of tropical land-ocean carbon fluxes in the context of global climate change.