Euro ea3028

Fish muscle (n = 5 for each site per species) and POM (n = 3 for each site) samples were dried in a freeze drier for 48 hr and the dried fish muscle was ground using a mortar and pestle. Isotope ratios were determined using an elemental analyzer (EA, Euro EA3028, EuroVector) coupled to an isotope ratio mass spectrometer (IRMS, Isoprime 100, Elementar). Approximately 0.8 mg of the dried sample was sealed in a tin cup and inserted into the EA and temperatures of 1,020°C and 650°C were used for the oxidation and reduction furnaces, respectively. The international isotopic reference, N−1 (0.4 ± 0.2; IAEA), was analyzed every 10 sample runs to normalize isotope ratios to the international scales (δ¹⁵N, ‰ vs. air). Analytical precision (1σ standard deviation) of the isotope analysis was better than 0.2‰.

Sediment samples were freeze-dried and homogenized using an agate mortar prior to bulk geochemical analyses. For TOC analysis, sediment samples (~1 g) were treated with 8 mL of 1 N HCl to remove carbonates. The TOC content and its isotopic composition were measured using an elemental analyzer (EuroEA3028, Eurovector, Milan, Italy) connected to an isotope ratio mass spectrometer (Isoprime, GV instruments, Manchester, UK) according to Lee et al. (2018). Isotopic values are expressed as δ¹³C values in per mil relative to VPDB. The δ¹³C values were calibrated through a CH₃ standard with a δ¹³C value of −24.7‰, certified by the International Atomic Energy Agency (IAEA, Austria). The precision for TOC and δ¹³C_TOC was better than ±0.2 wt.%, and ±0.1‰, respectively. The total sulfur (TS) content and its isotopic composition were measured using an elemental analyzer (EA1110, Thermo) connected to an isotope ratio mass spectrometer (Dual pumped 20–20 S, Secon). Isotopic values are expressed as δ³⁴S values in per mil relative to Vienna Canyon Diablo Troilite (VCDT). The δ³⁴S values were calibrated using S-2 and S-3 standards with δ³⁴S values of 22.7‰ and −32.3‰, respectively, certified by the IAEA. The precision for TS and δ³⁴S was better than ±0.4 wt.% and ±0.4‰, respectively.