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Methods and materials

Samples were taken during Expedition 316 (see the “Expedition 316 methods” chapter [Expedition 316 Scientists, 2009a]). The shipping and storage temperature for all sediment cores was –20°C, which suppresses rapid oxidation. For determination of acid volatile sulfide (AVS) and chromium reducible sulfur (CRS) species, frozen wet samples (0.5–1 g) were treated with a two-step acid Cr(II) distillation method (Fossing and Jørgensen, 1989). AVS was extracted cold for ~1 h. This step was followed by Cr(II) extraction at 200°C for ~2 h. The released sulfide was trapped in a 5% w/v Zn acetate solution. After sonication, the trap samples were shaken, and an aliquot of each sample was diluted and analyzed spectrophotometrically by the methylene blue method (Cline, 1969). The data are reported in dry weight units (Table T1). During the AVS extraction, mainly iron monosulfides, such as mackinawite, and ~66% greigite were extracted. In the Cr(II) reduction step, mainly pyrite, elemental sulfur, and ~33% greigite were extracted (e.g., Cornwell and Morse, 1987). For simplification, we refer to AVS as iron monosulfide and CRS as pyrite throughout the text.

To analyze the sulfur isotope composition of CRS (δ34S-CRS), zinc sulfide was converted to silver sulfide (Ag2S) with a 5% silver nitrate (AgNO3) solution. The precipitate was rinsed with deionized water and washed with 1 M NH4OH. In a final step, all Ag2S precipitates were rinsed with deionized water and dried at ~50°C in an oven overnight. Sulfur isotope measurements were performed by continuous flow gas source stable isotope ratio mass spectrometry (GS-IRMS) (model Delta V, Finnigan; Thermo Fisher Scientific, Waltham MA, USA). The sulfur isotope measurements were calibrated with reference materials NBS 127 (δ34S = 20.3‰) and IAEA-SO-6 (δ34S = –34.1‰) relative to the Vienna Cañon Diablo Troilite standard. The standard deviation (1σ) of replicate measurements of an in-house standard was <0.2‰ for δ34S values. All samples were analyzed at the Max Planck Institute for Marine Microbiology, Bremen, Germany.