Influence of Organo-Mineral Associations on Terrestrial Particulate Organic Matter Dispersal in the Northern Gulf of Mexico

Abstract

We investigate the influence of organo-mineral associations on the dispersal of terrestrial organic matter (TerrOM) along a land-sea transect offshore the Atchafalaya river in the northern Gulf of Mexico (GoM). We analyzed bulk sediment properties, mineral surface area, and clay composition and used lipid biomarkers to distinguish plant-derived (long-chain n-alkanes and n-alcohols) freshwater aquatic (C₃₂ 1,15-diols) and soil-microbial (branched glycerol dialkyl glycerol tetraethers (brGDGTs)) OM-pools in different grain size fractions (≥250, 250–125, 125–63, 63–30, 30–10 and <10 μm) of marine surface sediments. Concentrations and mineral loadings of the targeted biomarkers were highest in the <30 μm fractions, suggesting an affinity with clay minerals. Spatially, concentrations of higher plant-derived n-alkanes remained relatively constant along the transect, whereas those of the other OM-pools rapidly decreased further offshore. This suggests that the association of plant-derived OM with mineral surfaces is better maintained than that of freshwater and soil-microbial OM. In addition, similar distributions among grain size fractions at each site for the C₃₂ 1,15-diols and brGDGTs suggest that these compounds are likely not associated with mineral surfaces in the marine realm. Furthermore, as TerrOM might be stripped from mineral surfaces upon discharge in the marine realm, the dispersal of TerrOM-pools could also represent a degradation signal, as n-alkanes are more resistant than long-chain diols and brGDGTs. Together, our results indicate that the stability of organo-mineral associations in the marine realm differs per TerrOM-pool and can lead to a differential dispersal of these pools, and thus OM sequestration patterns in the northern GoM.