ALBERT

Description: Additionally, we employed a local resident to collect surface water suspended sediment samples at river km 865 throughout the 2017-2018 water year. These samples were collected in a bucket, the sediment was allowed to settle, and then the water was decanted off the top. Recovered sediment was stored in sterile Whirlpak bags, and then dried in an oven at 40°C. We homogenized and disaggregated the dry sediment using a mortar and pestle, and removed coarse plant material 〉1 mm. For each sample, we weighed an aliquot of sediment and loaded the material into aluminum cells for lipid extraction. Total lipid extracts (TLE) were recovered using an accelerated solvent extraction system (Dionex ASE) with 9:1 v/v dichloromethane: methanol. We added exactly 10 µg of internal standard (5-a-Androstane) to the TLE for unknown compound quantification. We then separated the TLE into three fractions using silica gel column chromatography with hexane (alkanes), 1:1 v/v hexane: dichloromethane (ketones), and 1:1 v/v dichloromethane: methanol (alcohols + acids) (Rach et al., 2020; doi:10.1016/j.orggeochem.2020.103995). Unsaturated compounds were removed from the alkane fraction using AgNO3-silica gel column chromatography with n-hexane (saturated n-alkanes) and DCM (unsaturated n-alkanes). n-alkanes were identified and quantified using an Agilent gas chromatograph (GC 7890-A) with flame ionization detection (FID) coupled to a single quadrupole mass spectrometer (MS 5975-C). We quantified n-alkane concentrations relative to the peak response of the internal standard, and then normalized the abundance to the sediment mass. We measured n-alkane d13C via GC-C-IRMS (gas chromatography/combustion/isotope-ratio mass spectrometry) with helium as a carrier gas (Agilent 7890N, ThermoFisher Delta V Plus). All compounds were measured in triplicate with a standard deviation of =0.5‰. Measurement quality was checked regularly by measuring n-alkane standards (nC15, nC20, nC25) with known isotopic composition (provided by Campro Scientific, Germany). d13C values were normalized to the Vienna Pee Dee Belemnite (VPDB) standard. We measured n-alkane d2H via GC-IRMS using a ThermoFisher Scientific Trace GC 1310 coupled to a Delta-V isotope ratio mass spectrometer. All d2H measurements were made in duplicate, and measurement quality was checked with d2H values were normalized to the Vienna Standard Mean Ocean Water (VSMOW) standard using an n-alkane standard mix with known d2H values (nC16 - nC30, from A. Schimmelman/Indiana University).