306-13 Hydrodynamic and Geochemical Controls on Organic Matter Preservation in Sediments on a Tropical Mangrove

Abstract:

Mangrove sediments are important reservoirs for organic carbon storage, but the amount of carbon preserved depends on the depositional conditions at each site. This study investigates organic matter (OM) content and sediment geochemistry from sediment cores collected at three mangrove sites in the Wouri Estuary, Cameroon: Mungo, located to the west in hydrodynamically restricted waters; KOO, situated in the central estuary within highly constricted water channels; and Dibamba, located to the east in relatively open-water conditions. The objective was to identify the factors controlling spatial differences in carbon storage across the estuary. Organic matter content was measured using loss-on-ignition (550°C), while major and trace element concentrations were analyzed using X-ray fluorescence (XRF). Mean OM content increases from Dibamba (22.1%; range 15.7–30.5%) to Mungo (27.4%; range 11.9–42.6%) and reaches its highest values at KOO (38.4%; range 24.5–49.5%). This increase is accompanied by a decrease in terrigenous element concentrations (Al, Si, Ti, and Zr) and an increase in sulfur content from Dibamba and Mungo to KOO. Lower Al, Si, Ti, and Zr values indicate reduced terrigenous sediment input and less siliciclastic dilution, while higher sulfur concentrations suggest more reducing, anoxic depositional conditions that may be associated with sulfate reduction and limited oxygen availability. The combination of restricted hydrodynamic conditions and highly constricted channels at KOO promotes low-energy sedimentation, minimizes clastic sediment input, and favors the development of anoxic, sulfur-rich sediments, resulting in the greatest preservation of organic matter. In contrast, the relatively open-water setting at Dibamba enhances water exchange and sediment transport, leading to greater siliciclastic input, more oxidizing conditions, and lower organic matter preservation. Mungo, located in restricted waters to the west of the estuary, exhibits intermediate characteristics between these two end members. Overall, the results demonstrate that organic matter accumulation and preservation in the Wouri Estuary are governed by the combined influence of hydrodynamic setting, sediment provenance, sediment chemistry, and redox conditions rather than by hydrodynamics alone. Sites with restricted circulation, low terrigenous sediment influx, and strongly reducing conditions provide the most favorable environments for long-term organic carbon preservation, highlighting the importance of these interacting processes in regulating blue carbon storage in tropical mangrove ecosystems.

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