Effect of emersion/immersion cycles on the elemental and isotopic compositions of the organic matter from surface sediments of an intertidal mud bank (French Guiana): a preliminary study.
Résumé
In various marine environments, the combination of elemental ratios and stable isotopic compositions is usually used to trace the sources of organic matter (OM) in sediments. However, in intertidal areas, the sediments might be temporarily exposed to air during a more or less prolonged duration and the impact of this exposure on the latter parameter is unknown. The spatial variations of atomic Total Organic Carbon (TOC) and Total Nitrogen (TN) ratios, and δ(13) C and δ(15) N values, were measured on surface sediments of French Guiana, at the beginning and the end of five consecutive days of emersion during equinoctial tides, as well as at the beginning of a new emersion phase after returning to a normal tidal cycle. The concentrations of chlorophyll a and phaeopigments in sediments were also measured, in order to estimate the influence of microphytobenthos (MPB), growing at surface sediments, on these geochemical parameters. The results showed that the emersion/immersion cycles influenced the development of MPB at the surface sediments, which, in turn, significantly controlled the spatio-temporal changes in the atomic elemental ratios and the δ(13) C values. This variability seemed not to be significantly altered by OM degradation. On the contrary, sediments were always (15) N-enriched compared with OM sources, indicating that OM diagenetic processes mainly controlled their spatio-temporal fluctuations. The [TOC/TN]atomic ratios and the δ(13) C values, indicating the primary signal of OM sources, represented the most reliable geochemical proxies for calculating the relative contribution of OM sources to sediments in environments characterized by variable air exposure duration. The use of δ(15) N values in such environments is limited by OM degradation processes but their variation might enable the nature and the degree of these processes to be identified.