Predominance of allochthonous and refractory carbon in sediments from two contrasting Mexican mangrove ecosystems

Mangroves are one of the most Blue Carbon-rich ecosystems worldwide, as they are highly efficient at storing and sequestering a large amount of organic carbon (C_org). The degradation of C_org inventories in mangrove sediments could cause carbon dioxide (CO₂) emissions, contributing to atmospheric warming. In this study, we used Rock-Eval pyrolysis and palynofacies identification to explore the composition and sources of organic matter (OM) and the quantity and liability of C_org in four ²¹⁰Pb-dated sediment cores from contrasting Mexican mangrove areas. The composition of terrestrial and refractory OM was similar in all cores, with variations attributed to the influence of the local river discharges on OM inputs and preservation. A progressive decrease in C_org quantity and lability from 2021 to 1990 in some cores was attributed to early diagenesis. Past precipitation and river discharge events appeared to have influenced carbon accumulation and preservation: increased influx and preservation of labile C_org in the sediments occurred during low river discharge and precipitation, whereas larger inputs and oxidation of refractory C_org occurred during high river discharge and precipitation. Sedimentary C_org stocks, assessed for 1921–2021, were primarily composed of refractory organic components, with degradation of allochthonous and autochthonous C_org mainly occurring before sediment burial. Sediments acted as efficient and long-term sinks for the C_org supplied to these mangroves, particularly in the context of increasing C_org inputs caused by an acceleration since the 1950s in continental erosion.