Distribution and sources of bulk organic matter (OM) on a tropical intertidal mud bank in French Guiana from elemental and isotopic proxies

The mobile French Guiana coast is a shoreface region downdrift of the Amazon River, where enormous quantities of inorganic and organic materials are exchanged with the Atlantic Ocean. The rapid accumulation of these materials forms highly unstable shore-attached mud banks, which can be temporally emerged and then rapidly colonized and stabilized by microphytobenthos and opportunistic mangroves (i.e. Avicennia germinans). Mud banks are preferential sites for the accumulation and significant remineralization of organic matter (OM) due to intense erosion/deposition cycles and potential biological colonization. The distribution and sources of bulk sedimentary OM were characterized by elemental and isotopic analyses of four sediment cores, together with samples from three potential OM sources (mangrove plants, suspended particulate matter (SPM) and microphytobenthos), all collected from the landward face of the Macouria mud bank (French Guiana). Total organic carbon (TOC) and nitrogen (TN) concentrations in the sediment cores showed that OM sources were characterized by spatio-temporal variations in this mud bank. The relative contributions of mangrove plants, SPM and microphytobenthos were estimated using a three end-member mixing model based on [TN/TOC]atomic ratios and δ13C values. Sedimentary OM is mostly controlled by SPM associated with variable amounts of OM derived from mangrove plants and microphytobenthos. These variations could be explained by topography and bed elevation, which decrease submersion time and increase desiccation. Higher contributions of microphytobenthos are associated with black OM-rich laminae, identified in sedimentary cores and linked to temporal emersion phases of the mud bank, which favor the growth of benthic microalgae. This result is confirmed by the calculation of the average sediment accumulation rate (around 36.7 ± 14.8 cm yr− 1), taking into account the emersion of the study site every spring tide (a fortnightly cycle). This value is within the range of the previous results from other mud banks in French Guiana but is more than ten times greater than values generally obtained in other coastal contexts.