Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.
Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093.
Proc Natl Acad Sci U S A. 2022 Oct 11;119(41):e2209152119. doi: 10.1073/pnas.2209152119. Epub 2022 Oct 6.
Marine dissolved organic matter (DOM) is a major reservoir that links global carbon, nitrogen, and phosphorus. DOM is also important for marine sulfur biogeochemistry as the largest water column reservoir of organic sulfur. Dissolved organic sulfur (DOS) can originate from phytoplankton-derived biomolecules in the surface ocean or from abiotically "sulfurized" organic matter diffusing from sulfidic sediments. These sources differ in S/S isotope ratios (δS values), with phytoplankton-produced DOS tracking marine sulfate (21‰) and sulfurized DOS mirroring sedimentary porewater sulfide (∼0 to -10‰). We measured the δS values of solid-phase extracted (SPE) DOM from marine water columns and porewater from sulfidic sediments. Marine DOM δS values ranged from 14.9‰ to 19.9‰ and C:S ratios from 153 to 303, with lower δS values corresponding to higher C:S ratios. Marine DOM samples showed consistent trends with depth: δS values decreased, C:S ratios increased, and δC values were constant. Porewater DOM was S-depleted (∼-0.6‰) and sulfur-rich (C:S ∼37) compared with water column samples. We interpret these trends as reflecting at most 20% (and on average ∼8%) contribution of abiotic sulfurized sources to marine DOS and conclude that sulfurized porewater is not a main component of oceanic DOS and DOM. We hypothesize that heterogeneity in δS values and C:S ratios reflects the combination of sulfurized porewater inputs and preferential microbial scavenging of sulfur relative to carbon without isotope fractionation. Our findings strengthen links between oceanic sulfur and carbon cycling, supporting a realization that organic sulfur, not just sulfate, is important to marine biogeochemistry.
海洋溶解有机质 (DOM) 是连接全球碳、氮和磷的主要储库。DOM 对海洋硫生物地球化学也很重要，因为它是水柱中有机硫的最大储库。溶解有机硫 (DOS) 可以源自海洋表层的浮游植物衍生生物分子，也可以源自从硫化沉积物中扩散出来的非生物“硫化”有机质。这些来源的 S/S 同位素比值 (δS 值) 不同，浮游植物产生的 DOS 追踪海洋硫酸盐 (21‰)，而硫化的 DOS 则反映了沉积孔隙水硫化物 (-10‰)。我们测量了来自硫化沉积物的海洋水柱和孔隙水的固相萃取 (SPE) DOM 的 δS 值。海洋 DOM 的 δS 值范围为 14.9‰至 19.9‰，C:S 比为 153 至 303，较低的 δS 值对应于较高的 C:S 比。海洋 DOM 样品随着深度呈现出一致的趋势：δS 值降低，C:S 比增加，δC 值保持不变。与水柱样品相比，孔隙水 DOM 中硫含量较低 (-0.6‰)，硫含量较高 (C:S 为 37)。我们将这些趋势解释为反映了最多 20%（平均约 8%）的非生物硫化源对海洋 DOS 的贡献，并且得出结论，硫化的孔隙水不是海洋 DOS 和 DOM 的主要组成部分。我们假设 δS 值和 C:S 比的异质性反映了硫化孔隙水的输入以及微生物对硫的优先摄取（相对于碳），而没有同位素分馏。我们的研究结果加强了海洋硫与碳循环之间的联系，支持了这样一种认识，即有机硫而不仅仅是硫酸盐对海洋生物地球化学很重要。
