Evaluating mangrove proxies for quantitative relative sea-level reconstructions

Abstract

Deciphering the timing, magnitude, and geographic variability of relative sea-level changes from geological records is critical for projecting future sea-level changes. Mangrove sediments provide an archive of past sea-level changes because they occupy intertidal zones along coastlines in low latitude locations. Such locations are important because they are far from present and former polar ice sheets that influence relative sea levels via glacio-isostatic adjustment. However using mangrove sediments for quantitative relative sea-level reconstructions is challenging due to variable fossil preservation and issues with radiocarbon dating. To evaluate the utility of mangrove sediments for high-resolution relative sea-level reconstructions, I studied two mangroves on Mahé, Seychelles.
To determine what materials are best suited for radiocarbon analysis, I compared radiocarbon ages from bulk sediments, organic concentrates, and macrofossils. I find that radiocarbon ages from bulk sediment and organic concentrate are indistinguishable and therefore both methods likely sample younger carbon. These results demonstrate that macrofossils sourced from above-ground (e.g. leaves) are the most reliable materials for constructing radiocarbon chronologies in mangrove sediments.
I examined the utility of mangrove pollen as a sea-level indicator through vegetation mapping and pollen trapping in two Seychelles mangroves. Mangrove species are broadly zoned according to elevation, but the results from annual pollen traps demonstrate pollen rain is poorly associated with vegetation. Additionally, low pollen concentrations in surface sediments suggest that it is unlikely pollen-based proxies will improve existing uncertainties on relative sea-level reconstructions in Seychelles.
To test the utility of geochemical properties for relative sea-level reconstructions in Seychelles mangroves I characterised the TOC, TIC and lipid biomarker compositions in surface sediments at two mangroves sites. Sedimentation is controlled by hydrogeomorphological factors, and therefore TOC and TIC do not provide sufficient information alone to reconstruct past elevation changes. However, biomarker distributions may be indicative of different vegetation and elevation zones.