The dataset includes solid-phase geochemistry data of sediment cores from Site C0023 (Hole A) that was recovered during International Ocean Discovery Program (IODP) Expedition 370 in the Nankai Trough offshore Japan in the Pacific Ocean (Drilling vessel Chikyu). Site C0023 was established on 17 September 2016. Coring terminated on 3 November 2016.Solid-phase geochemistry data include bulk iron (Fe), sulfur (S) and aluminum (Al) contents as well as chromium reducible sulfur (CRS). The bulk element contents were determined by performing total acid digestions (dissolution of dry sediment in a mixture of 65% HNO3 (3 ml), 30% HCl (2 ml), and 40% suprapur® HF (0.5 ml) in a CEM Mars Xpress microwave system). Sequential iron extractions were performed after Poulton and Canfield (2005) and Henkel et al. (2016) to quantify the reactive iron pools (Fe aca, sodium acetate extraction; Fe hyam, hydroxylamine-HCl extraction; Fe di-ct, sodium dithionite/sodium citrate extraction; Fe oxa, oxalate/oxalic acid extraction). We further determined the amounts of Al (Al aca, Al hyam, Al di-ct, Al oxa) and silicon (Si; Si aca, Si hyam, Si di-ct, Si oxa) released by the different extraction steps for a selection of samples. All elemental contents (except for CRS) were measured by inductively coupled plasma – optical emission spectroscopy (ICP-OES; iCAP 7400, Thermo Fisher Scientific Inc.). CRS was extracted using a CrCl2 solution after Canfield et al. (1986) and Zhabina & Volkov (1978). All values are given in wt%.

Total acid digestion: • Dissolution of dry sediment in a mixture of 65% HNO3 (3 ml), 30% HCl (2 ml), and 40% suprapur® HF (0.5 ml) in a CEM Mars Xpress microwave system• Elemental contents determined by ICP-OES (Inductively coupled plasma – optical emission spectroscopy; iCAP 7400, Thermo Fisher Scientific Inc.)Sequential Fe extraction:• after Poulton and Canfield (2005) and Henkel et al. (2016)• Fe aca (1 M Na-acetate, 24 h, pH 4.5); Fe hyam (1 M hydroxylamine-HCl in 25% v/v acetic acid, 48 h); Fe di-ct (Na-dithionite (50 g L-1)/0.02 M Na-citrate, 2 h, pH 4.8); Fe oxa (0.2 M ammonium oxalate/0.17 M oxalic acid, 6 h)• Elemental contents determined by ICP-OES (Inductively coupled plasma – optical emission spectroscopy; iCAP 7400, Thermo Fisher Scientific Inc.)Chromium reducible sulfur (CRS):• Extraction with CrCl2 solution (e.g., Zhabina and Volkov, 1978; Canfield et al., 1986)