THE APPLICATION OF δ13C AND C/N OF MANGROVE SEDIMENTARY ORGANIC MATTER TO RECONSTRUCT FORMER RELATIVE SEA LEVEL AND PALEOENVIRONMENT, PUERTO RICO
We sampled dominant vegetation and surface sediment along 8 transects taken through tidal flat, mangrove, and freshwater depositional environments from 4 sites in Puerto Rico. We find statistically distinct ranges in δ13C and C/N corresponding to these vertically zoned depositional environments. In addition, a 2.5 m core obtained from one of the sites in Rio Grande demonstrates changes in δ13C and C/N representative of a shift in depositional environment from tidal flat to mangrove that is in agreement with changes in the lithology and foraminiferal assemblages in the core. 210Pb accumulations and 14C analysis provide a chronology for these paleoenvironmental changes and enabled reconstruction of Late Holocene RSL. The good agreement between the geologic RSL reconstruction and nearby San Juan tide gauge record, an independent measure of former RSL, validates the δ13C and C/N-based reconstruction method.
Our analysis suggests that δ13C and C/N together serve as a suitable sea-level indicator in the tropics because they hold a systematic relationship to tidal elevation that is identifiable in the sedimentary record. This work provides alternative means for filling gaps in data associated with other proxies, as well as provides the first validated reconstruction of Late Holocene RSL change using the δ13C and C/N of mangrove sedimentary organic matter.