The influence of mineral associations on terrestrial particulate organic matter transfer and dispersal in the northern Gulf of Mexico

River-dominated continental margins receive large inputs of terrestrial organic matter (TerrOM). This TerrOM can potentially form a long-term sink for atmospheric CO₂ upon burial in continental margin sediments, thereby forming a key component of the global carbon cycle. However, only part of the TerrOM that is delivered to the coastal zone is preserved on the seafloor, as its sequestration efficiency depends on its properties and composition. The composition and quality of TerrOM on the seafloor is usually not determined, hampering estimates of its contribution to carbon sequestration on the continental margin. Moreover, TerrOM is thought to form associations with mineral surfaces that protect it from degradation, but it is not fully known to which extent these associations persist in the marine realm and if different types of TerrOM preferentially bind to certain minerals.  

Here, we investigated the TerrOM composition in different grain size fractions (>250, 250–125, 125–63, 63–30, 30–10 and <10 μm) of surface sediments along a land-sea transect (15–600 m water depth) in the northern Gulf of Mexico, using bulk properties (TOC, TN, δ¹³C_org) and lipid biomarkers for plant- (long-chain n-alkanes), fluvial- (long-chain C₃₂ 1,15-diol) and soil-microbial derived OM (branched glycerol dialkyl glycerol tetraethers; brGDGTs). In addition, we used mineral surface area analysis and X-ray diffraction (XRD) to assess whether different TerrOM types have an affinity for certain minerals.

We found that along the land-sea transect, the concentrations and mineral loading of n-alkanes remained constant, while the concentrations of both brGDGTs and the C₃₂ 1,15-diol rapidly decreased. Moreover, the concentrations of lipid biomarkers, in particular the n-alkanes, were highest in the smaller (<30 μm) size fractions along the transect. This suggests that n-alkanes likely form associations with clay minerals, in particular smectite, as shown by our XRD results.

However, n-alkanes might also be more resistant against degradation than the brGDGTs and diols,  further explaining their presence offshore. While brGDGT concentrations also increase towards smaller grain sizes, their molecular signature is constant among size fractions at each site, suggesting that they are not bound to a specific grain size fraction. Furthermore, an increase in the degree of cyclisation of the brGDGTs between 50 and 150 m water depth indicates that the initial soil-derived signal is strongly overprinted by an in situ marine contribution in this zone.

Our results show that in coastal waters, plant-derived OM is more likely to remain associated with mineral (smectite) surfaces than soil-microbial and fluvial OM, thereby facilitating its transport further offshore through hydraulic sorting and preferential burial on the shelf.