Photochemical Alteration of Dissolved Organic Sulfur from Sulfidic Porewater

Abstract

Sulfidic sediments are a source of dissolved organic sulfur (DOS) to the ocean but the fate of sedimentary DOS in the oxic, sunlit water column is unknown. We hypothesized that photodegradation after discharge from the dark sedimentary environment results in DOS molecular transformation and decomposition. To test this hypothesis, sulfidic porewater from a saltmarsh was exposed to potential abiotic transformations of dissolved organic matter (DOM) in the water column. We quantitatively investigated DOM transformations via elemental analysis and molecularly via ultrahigh-resolution mass spectrometry. Our study indicated that photoreactivity is dependent on DOM elemental composition as DOS molecular formulas were more photolabile than those without sulfur. Prior to solar irradiation, of the 6451 identified molecular formulas in sulfidic porewater, 39% contained sulfur. After 29 days of irradiation, the DOS concentration was depleted from 13 to 1 μM, together with a 9% decrease in the number of DOS molecular formulas. Comparing porewater and oceanic DOS molecular formulas, solar irradiation increased the similarity due to the removal of photolabile DOS formulas not present in the ocean. In conclusion, DOS from sulfidic sediments is preferentially photolabile and solar irradiation can be a potential mechanism controlling the stability and fate of porewater DOS.

Abiotic formation of dissolved organic sulfur in anoxic sediments of Santa Barbara Basin

“…It also highlights the fact that sulfurization reactions in SBB pore waters are not limited to CHO formulas and also involve a wide variety of compounds including nitrogen-containing DOM compounds. This latter point agrees with laboratory sulfurization experiments which show that abiotic sulfur incorporation reactions are non-selective and occur with a wide variety of DOM compounds including DON compounds (Pohlabeln et al, 2017;Gomez-Saez et al, 2017).…”

“…Our results and those of others (Gomez-Saez et al, 2016;Poulin et al, 2017;Gomez-Saez et al, 2017) reveal high sulfurization reactivity of many of the CHO and DON formulas that fall in the CRAM region on a van Krevelen diagram, and this group of formulas could be responsible for the formation of a major fraction of organosulfur compounds in anoxic and sulfidic sediments. If these DOS products are precursors of sulfur-containing protokerogen, they may play a key role in enhancing organic preservation in sulfidic sediments.…”

“…The same approach has been used to verify experimentally the occurrence of abiotic sulfurization under sulfidic conditions of natural DOM, and DOM directly derived from algal cultures (Pohlabeln et al, 2017). This approach has also been used to follow the photochemical alteration of DOS from sulfidic pore water (Gomez-Saez et al, 2017).…”

Photochemically Induced Changes of Dissolved Organic Matter in a Humic-Rich and Forested Stream

“…All CHNO components had a mass < 500 Da and were related to small molecules, with oxygen-poor and relatively aliphatic character ( Figure S4). The photo transformation of S-containing components was investigated by [80] and [84]. In contrast to these two studies, the abundance of CHOS was much smaller in our sample ( Figures S5 and S6).…”

Photochemical Production of Sulfate and Methanesulfonic Acid from Dissolved Organic Sulfur

“…We speculate that the higher mineralized fractions in these waters may be due to specific enriched DOM that is not found in typical lacustrine environments, 22 as confirmed by the low 262 DOC/DOS ratios of the porewaters compared to the other samples (Table S2). Water circulation 263 within the wetland brings the S-enriched DOS from the sediments to the surface, 22,23 where 264 oxidative transformations can occur 11,23 . Thus, we reason that the Prairie Pothole surface waters are more reactive than common surface waters due to a higher content of organic sulfur, but less reactive than the corresponding porewaters due to a lower fraction of reduced sulfur species.…”