Rapid, concurrent formation of organic sulfur and iron sulfides during experimental sulfurization of sinking marine particles

The redox speciation and bonding environment of sulfur and iron in filter-bound, sinking marine particles from the eastern tropical North Pacific O2-Deficient Zone were analyzed at the Stanford Synchrotron Radiation Lightsourse (SSRL). Glass fiber filter pieces were adhered onto Saint Gobain M60 S-free polyester tape and covered in 5-μm-thick SPEX 3520 polypropylene XRF film. ‘Bulk’ sulfur K-edge spectra (500 mm2 spot size) were collected on beam line 14–3 on whole particles, solvent-extracted particles, HCl-extracted particles, and lipid extracts, before and after copper exposure. Additionally, a micro-focused X-ray beam was used to map S and Fe species by rastering over selected mapping areas at specific energies (for sulfur: 2472.0, 2472.9, 2473.9, 2474.25, 2476.15, 2477.8, 2481.4, 2482.6, and 2486.0 eV; for iron: 7116.0, 7128.0, 7133.0, 7139.0, and 7147.0 eV) to create elemental and chemical distribution maps. Full XAS spectra were collected from 2460 to 2540 eV (sulfur) or 6900 to 7500 eV (iron) at selected spots.Sulfur data were collected at SSRL beam line 14–3, which is equipped with a Si(111) (Φ = 90) double crystal monochromator and calibrated to the thiol pre-edge peak of thiosulfate at 2472.02 eV. The S Ka fluorescence line was measured with a Si Vortex Si drift detector (Hitachi) using Xspress3 pulse processing electronics (Quantum Detectors). The X-ray beam was focused using an axially symmetric focusing mirror (SIGRAY) to a size of 5 x 5 µm at a flux of ~8 x 1010 photons per second; maps were collected at a resolution of 5 mm2. Sulfur XAS spectra were processed in the SIXPACK (Webb, 2005; Webb, 2020) software package using a K-edge E0 of 2473 and pre-edge and post-edge linear normalization ranges of –20 to –7 and 35 to 70 eV, respectively. Uncertainties reported in Table S3 refer to the confidence in the linear combination fit calculated in SIXPACK. Iron data were collected at SSRL beam line 2–3, a bending magnet workstation equipped with a Si(111) (Φ = 0) double crystal monochromator calibrated such that the first derivative of an Fe metal foil was set to 7112 eV. The beam line uses an axially symmetric focusing mirror (SIGRAY) to achieve a spot size of 5 x 5 µm at a flux of ~5 x 108 photons per second at 7100 eV, and uses a similar fluorescence data collection system as above with 14-3 to collect k-edge Fe spectra from 6900 to 7500 eV and elemental maps of Ca, P, Mn, Ti, S, and other metals at 5-mm resolution.