Surface sediment were extracted 4 times by ultrasonication with dichloromethane: methanol (9:1, v/v) for 15 min for FAs and alkanes. For quantification of FAs and alkanes, known amounts of 19-methylarachidic acid and squalane were added as internal standards prior to extraction. Supernatants from each extraction were obtained by centrifugation and combined. The total lipid extracts were concentrated and evaporated under a nitrogen stream. The total lipid extracts were saponified for 2 h at 80 °C with 1 mL of KOH (0.1 M) in methanol: H2O (9:1, v/v). After saponification, the neutral fractions were liquid-liquid extracted with n-hexane and alkanes were eluted from the neutral fractions by silica gel column chromatography with n-hexane. The remaining KOH solution was acidified to pH 1, from which FA were liquid-liquid extracted into dichloromethane. The extracted and dried FAs were converted to methyl ester derivatives (FAMEs) in methanol: HCl (95:5, v/v) at 60 °C for 12 h. After methylation, the FAME fraction was further purified by silica gel column chromatography using dichloromethane: hexane (2:1, v/v) to remove residual polar compounds.
FAMEs and alkanes were analyzed on a 7890A gas chromatograph (GC) equipped with a DB-5MS fused silica capillary column (60 m, 250 µm, 0.25 µm) and a flame ionization detector (FID). Peak areas were determined by integrating the respective peaks and concentrations were calculated against the internal standards. FAME contents were subsequently corrected for the derivative methyl carbon to determine FA contents. FAs and alkanes were normalized to OC content.
Surface sediment were extracted 4 times by ultrasonication with dichloromethane: methanol (9:1, v/v) for 15 min for FAs and alkanes. For quantification of FAs and alkanes, known amounts of 19-methylarachidic acid and squalane were added as internal standards prior to extraction. Supernatants from each extraction were obtained by centrifugation and combined. The total lipid extracts were concentrated and evaporated under a nitrogen stream. The total lipid extracts were saponified for 2 h at 80 °C with 1 mL of KOH (0.1 M) in methanol: H2O (9:1, v/v). After saponification, the neutral fractions were liquid-liquid extracted with n-hexane and alkanes were eluted from the neutral fractions by silica gel column chromatography with n-hexane. The remaining KOH solution was acidified to pH 1, from which FA were liquid-liquid extracted into dichloromethane. The extracted and dried FAs were converted to methyl ester derivatives (FAMEs) in methanol: HCl (95:5, v/v) at 60 °C for 12 h. After methylation, the FAME fraction was further purified by silica gel column chromatography using dichloromethane: hexane (2:1, v/v) to remove residual polar compounds.
FAMEs and alkanes were analyzed on a 7890A gas chromatograph (GC) equipped with a DB-5MS fused silica capillary column (60 m, 250 µm, 0.25 µm) and a flame ionization detector (FID). Peak areas were determined by integrating the respective peaks and concentrations were calculated against the internal standards. FAME contents were subsequently corrected for the derivative methyl carbon to determine FA contents. FAs and alkanes were normalized to OC content.
The grain size of sediments was determined with a Cilas 1180 laser-diffraction particle analyzer (range 0.04–2500 μm) and the mean grain size was calculated. The surface area (SA) was calculated from the grain size distribution of the sediments. The OC content of surface sediments was determined using a carbon-sulfur analyzer (CS-125, Leco) after decarbonization with HCl. The total carbon (TC) and nitrogen (TN) contents of the samples were analyzed using a carbon-nitrogen-sulfur analyzer (Elemental III, Vario) and used to calculate carbonate contents (CaCO3= (TC − OC) × 8.333) and the mass ratio of OC to TON content ((C/N)OC), which was corrected for mineral-associated inorganic N. The stable carbon isotope composition of OC in surface sediments (δ13COC) was measured using a Thermo Delta isotope ratio mass spectrometer coupled to a Carlo Erba elemental analyzer. δ13COC values are given in per mil notation relative to the Vienna Pee Dee Belemnite standard. The standard deviation of duplicate analyses ranged from 0.01‰ to 0.18‰, with an average of 0.10‰. See details in related publication (Wei et al., 2025).
In March 2022, 56 surface sediments were collected from the Helgoland Mud Area and surrounding sandy areas in the North Sea. These surface sediments were analyzed for grain size, organic carbon (OC) content, total nitrogen content (TN), stable carbon isotope of OC, and abundances of source-specific alkanes and fatty acids, in aim to determine and quantify composition and sources of OC, to understand the degradation and sequestration of marine and terrestrial OC in sediments, and to estimate the burial fluxes and burial efficiencies of marine and terrestrial OC in the Helgoland Mud Area. Detailed dataset interpretation can be found in Wei et al. (2024, in preparation).