Organic carbon (OC) content, nitrogen content, C/N ratio, carbonate content, δ13C of OC, mean grain size, surface area, calculated fraction, content and loading of terrestrial and marine OC in surface sediments of Helgoland Mud Area in the North Sea

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., in preparation).