Geochemical Dynamics and Depositional History from Mangrove Sediments Within the Pearl River Estuary

This study investigated the geochemical dynamics of sediment organic matter (OM) and examined the usefulness of sedimentary OM delta C-13, TOC and C/N proxies for reconstructions of the depositional history from mangrove sediments in a coastal site of the Pearl River mouth region. The analyses of the delta C-13, TOC and C/N from plant leaves and sediment OM of three mangrove vegetation zones (mudflat with macro-algae, mangrove forest and supratidal mixed forest) across a mangrove wetland indicate significant exchanges of mangrove OM with marine aquatic algal OM and mixed forest plant OM caused by tidal currents and freshwater discharge in both landward and seaward directions. Despite the OM exchanges, the delta C-13, TOC and C/N of modern surface sediments show values distinctive between mudflat, mangrove and mixed forest zones. Specifically, these three signatures appear as -24.3 +/- 0.5 parts per thousand, 1.6 +/- 0.2% and 4.4 +/- 1.3 for mudflat sediment, -26.8 +/- 0.4 parts per thousand, 3.1 +/- 0.7% and 8.6 +/- 1.6 for mangrove sediment, and -28.5 +/- 0.5 parts per thousand, 8.4 +/- 2.2% and 11.9 +/- 2.0 for mixed forest sediment. Such clear divisions of sediment OM signatures between vegetational zones confirm their usefulness as coastal environmental proxies. Together with the chronologies based on radiocarbon, 210 Pb and 137 Cs analyses, these proxies were successfully applied to sediment cores to reconstruct the mangrove wetland's depositional history in the past century. This study has enhanced our understanding on organic geochemical dynamics in mangrove environments, and the improved reconstruction provides new insights into environmental responses to natural and anthropogenic drivers.