Millennial-aged organic matter sequestration and preservation in anoxic and sulfidic mangrove soils: insights from isotopic and molecular analyses

A buried layer rich in organic matter (OM) has been identified in the mangrove soils along the West coast of New Caledonia (South Pacific), resulting from a long period of stable sea levels during the Holocene. This study aims to characterize this OM-enriched layer isotopically, molecularly (lignin and neutral carbohydrates), and using Rock-Eval pyrolysis, while identifying the decomposition and preservation processes in these anoxic and sulfidic conditions. The study site is a mangrove forest of the West coast of New Caledonia, with a vegetation typical of this semi-arid area. The species Rhizophora stylosa develops in monospecific stand seaward and the species Avicennia marina landward. Multiple parameters such as the high total organic carbon content (10%) indicate the presence of this enriched layer below 30 cm depth beneath the A. marina stand. Stable isotopic ratios indicate that the roots of R. stylosa are the primary source of OM in this layer. Degradation mechanisms include dehydrogenation and the loss of major neutral sugars (glucose, xylose, galactose), while preservation processes involving arabinose, vanillin, and p-hydroxyacetophenone contribute to the stabilization of OM. The presence of well-preserved root material associated with pyrite observed using a scanning electron microscope, along with high S_org/C ratio provide tangible evidence of interactions between OM and minerals and OM sulfurization, reinforcing preservation in anoxic and sulfidic environments. Through radiocarbon dating, the OM-enriched layer is placed in the historical context of a period characterized by stable sea levels approximately 4,000 years ago, emphasizing the profound impact of prolonged stability on OM accumulation and preservation in mangrove soils.