C) BIOGEOCHEMISTRY OF SMALL MOUNTAINOUS RIVERS AND COASTAL CORAL SKELETONS IN PUERTO RICO

Tropical small mountainous rivers (SMRs) transport significant amounts of carbon (C) and other materials to the coastal ocean. However, the fluxes and fate of these materials are poorly constrained and historical records of land-ocean C delivery are rare in tropical SMRs. Corals growing near the mouths of tropical SMRs may provide such records in the tropics because they are long-lived, assimilate dissolved inorganic C (DIC) for calcification, and isotopic variations within their skeletons are useful proxies of palaeoceanographic variability. In this study, a ¹³C and ¹⁴C characterization of dissolved and particulate organic C (DOC and POC, respectively) and DIC in two SMRs and adjacent coastal waters of Puerto Rico was conducted over two years in order to understand the amounts and sources of C delivered to the coastal ocean. Additionally, a 56-year record of paired δ¹³C and Δ¹⁴C measurements is presented from a coral growing ~1 km from the mouth of an SMR. During increased river discharge, organic C in coastal waters showed relatively greater terrestrial character, indicating that tropical SMRs may transport relatively aged and unaltered C directly to the coastal ocean. Overall, riverine DIC had lower δ¹³C and Δ¹⁴C than seawater DIC, and both isotopes were correlated in both the DIC of the river and coastal waters and in the coral skeleton. In addition, increases in river discharge were synchronous with the timing of depletions of both δ¹³C and Δ¹⁴C in the coral skeleton and increases in river discharge. Thus, the δ¹³C and Δ¹⁴C signatures of these corals represent a promising proxy record for terrestrially derived DIC. Further development of such a proxy for land-ocean C flux would be invaluable to understanding how land-use and modern global climate change have altered C flux in tropical SMRs.