Employing lipid biomarkers to constrain environmental controls on the export of plant, soil and -derived organic matter.

River systems transport and transform organic carbon (OC) from the terrestrial realm, before delivering this organic matter to deposition centers. Organic carbon with different ages, such as i) modern organic matter, ii) pre-aged organic matter from surface soils or riparian zones or iii) petrogenic or rock-derived organic matter, is transported under different environmental conditions. Using a 30-month high-resolution time series study of the organic matter content of the suspended load in the subalpine Sihl River watershed (Switzerland), the impact of hydrology and seasonality on the amount and source of organic matter was determined.

Previous work on the distribution and amount of suspended bulk OM and vegetation derived lipid biomarkers (long chain fatty acids and n-alkanes) revealed that hydrology and seasonality determine their fluxes (i.e. Schwab et al., 2025). Specifically, storms are interpreted to promote the mobilization of both contemporary plant detritus and surface soils. Because plant waxes are sourced from both modern vegetation and pre-aged soils, the unique contribution of pre-aged soil material was not targeted. Now, the analysis of branched GDGTs, bacterial membrane-spanning lipids produced in high abundance in soils, allows to track this specific C pool. Furthermore, these three lipid classes are expected to show a different recalcitrance to degradations (fatty acids>GDGTs>n-alkanes), which allows to determine the effect of age and degradation on the composition of suspended organic matter.

Across the sampling period, the export of branched GDGTs closely follows the hydrograph. High discharge conditions (>12.7 m³ s⁻¹), typified by a high suspended sediment load, result in a high brGDGT export flux. The distribution of brGDGTs in these conditions points towards a higher altitude source of brGDGTs during winter, compared with summer. This is distinct from the lower altitude source derived from plant wax distributions (Schwab et al., 2023). Changes in relative contribution of the three biomarker classes indicate the presence of three end-members, i) an end-member of recently produced fresh organic matter, dominated by long-chain fatty acids, ii) an end-member with strongly degraded organic matter (n-alkane Carbon Preference Index < 2), dominated by n-alkanes and iii) a poorly defined end-member with increased n-alkane and GDGT concentrations, interpreted as an input of soils. Remarkably, the content of the radio-active isotope 14C (F¹⁴C), is not uniform for given end-member mixtures, indicating that age alone does not determine the relative abundance of the lipid classes.

In low discharge conditions, the low contribution of soil-derived GDGTs is overwritten by GDGTs produced in the aquatic system. As GDGT distributions reflect their production environment (soil versus aquatic), the use of GDGT ratios to quantify soil-derived versus aquatic bacterial biomass is evaluated. The direct effect of temperature on GDGTs produced in low discharge conditions, however, results in large ranges of their ratio values, complicating their proposed interpretation as a tracer for the provenance of aquatic biomass in river systems.

References:
Schwab, M. S., Haghipour, N. & Eglinton, T. I. Geochimica et Cosmochimica Acta 391, 31–48 (2025).