Influence of Physical and Mineralogical Sediment Properties on the Molecular Composition and Stability of Organic Carbon in Fluvial Sediments

In the context of climate change, the long-term storage of organic carbon (OC) in soils and sediments is increasingly important, as these systems can act as both carbon sinks and potential sources under changing environmental conditions. Rivers transport 0.9 to 1.9 Pg of carbon per year from terrestrial sources into the ocean, with a large proportion assumed to be transformed, mineralized, or permanently stored. However, due to the dynamic nature of fluvial sediments, stored OC is heterogeneous in both source and composition. Consequently, the influence of sediment properties such as grain size, elemental composition, and depositional conditions on the molecular stabilization of OC remains poorly understood, and OC-matrix heterogeneity complicates molecular characterization.

Here, we investigate the molecular composition of OC in sediments from four depth profiles of a hydrologically inactive meander of the Oder River, where sediments have been deposited over long periods and include sediment types of varying ages. OC fractions were directly analyzed from milled sediments by laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). To our knowledge, this represents the first application of LDI-FT-ICR-MS to fluvial sediments, enabling direct molecular characterization of complex organic matter without solvent extraction or chemical pre-treatment. This preserves fragile and high-molecular-weight compounds and provides unique insights into matrix-bound OC pools. In addition, total organic carbon (TOC), stable carbon isotopes (δ¹³C), and radiocarbon (¹⁴C) were used to assess sources, transformation, and residence times.

LDI-FT-ICR-MS detected between 900 and 2,000 molecular formulae across sediment types. Molecular composition differed systematically between sediment matrices: the aromaticity index (AI) was higher in fine-grained sediments such as loam, clay, and peat-like deposits (AI up to ≈0.65), whereas coarse-grained sands showed lower AI (≈0.45). Sands exhibited higher double bond equivalents (DBE ≈10) and mean m/z values, indicating less aromatic, more aliphatic structures. The O/C ratio was lowest in peat-like sediments (≈0.25) and higher in sands (≈0.40), reflecting differences in oxidation state and carbon sources. Molecular variability decreased with depth and was lowest in very fine-grained clay and loam, suggesting enhanced stabilization and long-term persistence of OC.

These results highlight the central role of sediment properties in OC stability and provide insights into molecular mechanisms of carbon sequestration in river systems, with direct relevance for long-term carbon storage and climate protection strategies.