Speciation Of Organic Sulfur In Marine Sediments Of The East China Sea And Jiaozhou Bay: Evidence From Sulfur XANES Spectroscopy

Sulfur cycling is one of important processes of global biogeochemical cycling inmarine sediments. Early diagenesis of sulfur in marine sediments exerts a stronginfluence on global cycle of carbon, phosphorus, iron, and other trace elements. EastChina Sea (ECS) and Jiaozhou Bay with unique regional features are important studyareas of oceanographic community. Sulfate reduction is the main pathway coupled tomineralization of organic matter. Organic sulfur (OS) is an important sulfur pool inmarine sediments and exerts a great influence on the preservation of organic matterand geochemical cycle of a multitude of elements. The research on the speciation andrelative fractions of OS in sediments of the ECS, Jiaozhou Bay, and the correspondingrivers is of importance on determining the sources and formation mechanisms of OSin coastal sediments and assessing the influence of human activities and terrestrialinput on OS in the marine sediments. In this study, chemical extraction techniqueswere applied to quantify humic acid sulfur (HA-S), fulvic acid sulfur (FA-S) and nonchromium reducible organic sulfur (non-CROS) in two sediment cores and fifteensurface sediments collected from the ECS and Jiaozhou Bay. Sulfur K-edge X-rayabsorption near edge structure (XANES) spectroscopy was used to characterize, onthe molecular level, the speciation and relative fractions of the three OS pools.Several issues can be addressed on basis of data obtained. First, the data can provideinformation on the spatial and vertical distribution of the relative fractions of OSfunctionalities; second, it is likely to assess the difference on OS sources andformation mechanisms between humic sulfur (HA-S+FA-S) and non-CROS, and theinfluence of environmental changes on OS compositions in marine sediments. In addition, sulfur K-edge XANES spectroscopy was also used to characterize andquantify the speciation and relative fractions of humic sulfur in surface sedimentsfrom the Yangtze River and four rivers around Jiaozhou Bay. The influences of humanactivities on speciation and relative fractions of humic sulfur in river sediments wereassessed. By comparing the sources and functionalities of OS in river sediments andthe marine sediments, the influences of terrestrial inputs on the OS in marinesediments were revealed. In this study, the main results and conclusions are asfollows:1. In ECS surface sediments, oxidized OS is generally the predominant species inhumic sulfur (HA-S+FA-S), accounting for58.6-75.1%and48.9-90.8%. whereasreduced OS is the predominant species in the non-CROS, accounting for82%.Diagenesis is likely the predominant factor leading to the high relative fractions ofreduced OS in non-CROS in the ECS surface sediments. There is an obviousdifference in the speciation of humic sulfur and non-CROS in marine sediments, thushumic sulfur cannot comprehensively reflect the compositions and the sources of theOS in the bulk sediments.2. In sediment core of ECS (DH7-1), oxidized OS is generally the predominantspecies in humic sulfur, accounting for38.4-73.9%and52.3-82.8%, and is ascribableto biogenesis. At the bottom of the core, the enhancement of highly reduced OSfractions with depth is the result of enhanced sulfurization; however, the contributionof biogenesis cannot be excluded. In the FA-S, the relative fractions of the highlyreduced OS, which is ascribable to sulfurization, account for less than30%. Thussulfurization is not the key geochemical process responsible for the burial of FA-S inthe sediments. Although the predominant species of non-CROS (that is, reduced OS)is different from that of the humic sulfur, which is predominated by oxidized OS, boththe two OS pools is ascribable to biogenesis.3. In marine sediments, the relative importance of reduced OS in HA-S isinsensitive to redox conditions, which is different from the case in the terrestrialsystem. Terrigenous inputs and sulfurization may be the important factors determiningthe relative fractions of the reduced and oxidized OS in HA-S of the marine sediments.4. For core sediment of Jiaozhou Bay (M1), the reduced and oxidized OS werecomparable, each accounting for about50%of humic sulfur. The formation of humicsulfur be ascribable to sulfurization, which enhanced from the uppermost layer to thebottom of the core. The relative fractions of humic sulfur in sediments of JiaozhouBay are significantly different from those in the ECS. It can be concluded thateutrophication is conducive to the sulfurization of humic sulfur, and environmentalchange has a significant influence on the formation mechanisms of humic sulfur inJiaozhou Bay.5. Speciation of the HA-S in the Yangtze River sediments is not different formthat in the rivers around Jiaozhou Bay. For the FA-S, however, the average relativefraction of the reduced OS in the river sediments of Jiaozhou Bay is47.3%andapparently higher than those in the Yangtze River sediments (27.6%). Enhancedhuman activities, sulfurization, and coarse-sized grains are likely responsible for thehigher relative fraction of the reduced OS in river sediments of Jiaozhou Bay than thatin Yangtze River sediments.6. In HA-S, the speciation in the Yangtze River sediments is apparently differentfrom that in the ECS. In FA-S, the speciation in the Yangtze River sediments is similarto that in the ECS. The speciation in river sediments of Jiaozhou Bay is similar to thatin Jiaozhou Bay. Humic sulfur in marine sediments of ECS and Jiaozhou Bay ismainly of terrigenous origin, which controls the relative fractions of humic sulfur. It islikely that, in Jiaozhou Bay, simultaneously enhanced terrigenous inputs andweakened coastal currents have enhanced the similarity of the speciation and relativefractions in humic sulfur of the river and marine sediments.