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Methane release from marine sediments strongly influences the local seafloor environment and ecosystems, and may impact Earth's climate system. Recent studies revealed anomalous molybdenum (Mo) enrichment in seep sediments, which was linked to methane release events. Marine seep sediments are a potential sink of Mo for the global ocean, while the mechanisms leading to local Mo enrichment are not fully understood. The sediments from a gas hydrate-bearing area of the South China Sea analyzed herein reveal authigenic Mo (Mo auth) contents ranging from 0 and 31.4 µg/g and δ98Mo auth values ranging from 0.18 ‰ to 3.31 ‰. The range of δ98Mo auth values of seep sediments is therefore similar to values of modern iron-rich sediments with low concentrations of dissolved porewater hydrogen sulfide and sediments deposited under weakly euxinic sediments. Among the obtained South China Sea data, the more positive δ98Mo auth values (>ca. 1.5 ‰) are interpreted to reflect diffusion of seawater Mo into the sediment at moderate seepage rates and Mo isotope fractionation during the formation of thiomolybdates in the sulfidic seep environment. The lower δ98Mo auth values (

Abstract Molybdenum isotopes are considered to be a promising paleoredox proxy in reducing facies for evaluating either global-ocean redox conditions (given quantitative Mo uptake) or local redox conditions (given non-quantitative Mo uptake). Here, we examine the Mo-isotopic composition of black to gray shales from the Permian-Triassic boundary (PTB) section at Chaohu, eastern China, in order to assess the utility of this proxy under a range of paleoredox conditions. We evaluated local redox conditions using a combination of redox-sensitive trace-element concentrations (Mo, U) and Fe T /Al ratios, recognizing six intervals of more reducing conditions at the PTB and Induan-Olenekian boundary (IOB), in the mid-Smithian, at the Smithian-Spathian boundary (SSB), and in the mid- and upper Spathian. These six intervals are associated with variably weak to moderate enrichments of Mo, and they show variable shifts away from background δ98Mo values of ca. −0.4 to +0.1‰ (recording detrital Mo-isotopic compositions) toward δ98Mo values as high as +2.3 ± 0.2‰. Based on an analysis of local redox proxies versus δ98Mo as well as of sample-to-sample variance in δ98Mo values, we conclude that only one of the six reducing intervals in the study section is likely to record contemporaneous seawater δ98Mo (as a result of quasi-quantitative Mo uptake), i.e., the interval from the Hindeodus typicalis conodont Zone close to the PTB, for which δ98Mo values of +2.3 ± 0.2‰ suggest average global-ocean redox conditions similar to those of the modern ocean. For the remaining five reducing intervals, non-quantitative Mo uptake led to Mo-isotope signatures reflecting local, mostly suboxic redox conditions. The highest Mo concentrations (17–23 ppm) were yielded by the mid- and upper Spathian intervals, and the lack of concurrent U enrichment in the mid-Spathian interval suggests the local operation of a Mn-Fe particulate shuttle, an inference that is consistent with its relatively low δ98Mo values (0 ± 0.3‰). The lack of evidence for strong oceanic anoxia in the South China Craton following the PTB crisis may have been a factor in the relatively rapid recovery of marine ecosystems in this region during the Early Triassic. The present contribution serves as a case study of the application of Mo-isotopes as a paleoredox proxy in low-[Mo] sediments, which requires a careful assessment of the relative importance of global versus local redox influences based on additional proxies and evaluation criteria. Highlights • Case study of use of Mo-isotopes in low-[Mo] sediments for analysis of paleoredox conditions • Six intervals of more reducing conditions at the PTB, IOB, mid-Smithian, SSB, middle Spathian, and upper Spathian • Peak δ98Mo of +2.3±0.2 ‰, observed at the PTB, may be a global-ocean signal preserved by quantitative Mo uptake. • Peak Mo concentrations, in the middle and upper Spathian, indicate local operation of a Mn-Fe particulate shuttle. • The Mo-enriched Spathian beds associated with a Mn-Fe particulate shuttle yielded low δ98Mo values (0±0.3 ‰) [ABSTRACT FROM AUTHOR]

Two redox-sensitive metalloids, arsenic (As) and antimony (Sb), are examined here to determine what can be their help in the deciphering of past depositional conditions. The enrichment factors of the two elements are compared for a set of geological formations and marine deposits covering a relatively wide range of paleoenvironmental settings, from oxic to euxinic conditions. This work confirms that As and Sb are not robust paleoredox proxies but examining their relative enrichment may be useful. These preliminary results indicate that a co-enrichment of both elements with Sb being more enriched than As could be the mark of the so-called particulate shuttle effect. Notably, Sb would be more sensitive to Mn-shuttling than As. If confirmed, this trend could be used to further identify the cause of As-enrichment in marine sediments impacted by cold seepage fluids.

Abstract: Cold seeps, located on sea floors and frequently fueled with methane, have been much studied for the formation of carbonate nodules, crusts or masses resulting from the activity og communities of anaerobic bacteria. In this paper, we examine the impact of cold seepage on their caging sediments, as far as their geochemical composition and microfaunal content are concerned. To this end, two fossil cold seeps of the Jurassic Beauvoisin site (Vocontian Trough, SE-France) are studied, focusing on the redox-sensitive and/or sulfide-forming trace metals. We observe that: 1) the carbonate edifices of the cold seeps are enriched in specific elements, namely, Sr, Mo As and Sb, compared to their caging marlstones, and 2) the benthic foraminifer populations were present close to the edifices. The Sr enrichment is ascribed to a contamination of the seeping fluids at the contact with underlying evaporites; the Mo–As–Sb enrichment may have several, non-exclusive, origins. We retain two scenarios; in the first one, the enrichment is due to the ascending fluids that could collect these elements when passing through thick shaly formations; in the second scenario, Mo, As and Sb were supplied to the sites of carbonate precipitation together with Fe- and Mn-oxy-hydroxides (the so-called particulate shuttle process). We favor the second scenario (possibly coupled to the first one) because the rather uncommon Mo–As–Sb association is also observed for reducing sediments deposited under the influence of Fe–Mn-shuttling. [Copyright &y& Elsevier]

Hypersaline seafloor environments formed by brine seepage represent role model examples of extreme marine ecosystems; they may for example help to identify potential habitats for life on other planets. Trace and rare earth elements (REEs) are crucial components enabling microbial life at brine and hydrocarbon seeps. However, the behavior of trace elements at brine seeps is currently poorly understood. Because authigenic carbonates forming at brine seeps archive information on the composition of seepage fluids, we have analyzed carbonates collected from a brine seep site (Mississippi Canyon 709, MC709 for short) of the northern Gulf of Mexico (GoM). The dominance of low-magnesium (Mg) calcite among the carbonate minerals, moderately low δ13C carb values (average: −26.1‰, VPDB), as well as the presence of oil in carbonates suggest concomitant seepage of brine fluids and crude oil at this site. High δ18O carb values (up to +4.5‰, VPDB) suggest the involvement of 18O-rich fluids during carbonate precipitation, likely resulting from mineral-water reactions in deeper sediments. Significant enrichments of barium (Ba), strontium (Sr), and lithium (Li) in the samples suggest that deep ascending brine fluids contributed to carbonate formation. Enrichments in molybdenum (Mo), arsenic (As), and antimony (Sb) and a correlation between Mo, As, and Sb enrichment factors (EF) and the authigenic iron fraction (Fe/Al ratios) suggest that iron particulate shuttle processes were involved in scavenging of dissolved Mo, As, and Sb from seawater. This notion is strengthened by the co-variation of Mo EF and U EF and the shale-normalized REE patterns of the brine-seep carbonates. This study allows to constrain the enrichment mechanisms of trace and REE elements at brine seeps, providing the foundation for an improved understanding of the distribution and behavior of trace metals, as well as their role in microbial metabolisms in the local hypersaline environment. • Authigenic carbonates from a brine seep are dominated by low-magnesium calcite. • Enrichments of Sr, Ba, and Li in carbonates derive from parent fluids ascending from greater depths. • Enrichments of Mo, As, Sb are ascribed to the particulate shuttle process. • Trace elements and REEs are important for microbial metabolism at brine seeps. [ABSTRACT FROM AUTHOR]