THE APPLICATION OF δ13C AND C/N OF MANGROVE SEDIMENTARY ORGANIC MATTER TO RECONSTRUCT FORMER RELATIVE SEA LEVEL AND PALEOENVIRONMENT, PUERTO RICO

Reliable, quantitative proxies of former relative sea-level (RSL) and paleoenvironmental change are critical in integrating geologic and historical records into predictive models in order to better understand the response of coastal systems to marine inundation. The defining characteristic of a sea-level indicator is its systematic and quantifiable relationship to elevation with respect to the tidal frame. Microfossils (e.g. foraminifera, diatoms) are used to reconstruct Holocene sea level because of their potential for providing high-resolution archives of RSL; however, these biological proxies are somewhat limited due to spatial restrictions and poor preservation in the sedimentary record of temperate and more notably tropical environments. In this study, we aim to overcome the confines of existent indicators by adapting the use of stable carbon isotopes and carbon to nitrogen ratios of bulk sedimentary organic material to a tropical coastal setting.

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.