Geochemical dynamics and depositional history from mangrove sediments within the Pearl River estuary

Abstract

This study investigated the geochemical dynamics of sediment organic matter (OM) and examined the usefulness of sedimentary OM δ 13 C, 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 δ 13 C, 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 δ 13 C, 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‰, 1.6 ± 0.2% and 4.4 ± 1.3 for mudflat sediment, −26.8 ± 0.4‰, 3.1 ± 0.7% and 8.6 ± 1.6 for mangrove sediment, and −28.5 ± 0.5‰, 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. • Sediment OM δ 13 C, TOC and C/N are distinctive between mangrove vegetation zones. • There are multiple levels of OM exchange between mangrove forest and mudflat. • δ 13 C, TOC and C/N proxies can be used to reconstruct mangrove sedimentary history. • The reconstruction indicates the expansion of mangroves over the past century.