Arsenic in marine sediments: how robust a redox proxy?

Arsenic (As) is reputed to be a redox-sensitive element but has not been fully explored yet as a proxy in marine sediments and sedimentary rocks, compared to extensively examined elements such as molybdenum (Mo) and uranium (U). Here the behavior of As is compared to that of Mo and U in a number of paleo-environmental situations that have been studied before and are, therefore, rather well constrained. The results indicate that attention must be paid to the reference values used to normalize sedimentary As content in geochemical studies. The most-commonly used references (namely the Post-Archean Australian Shale and average upper crust of Earth, to be opposed to the Average Shale) tend to overestimate slightly As enrichments as shown by As enrichment factors being quasi systematically larger than 1, whatever the geological setting. The results also show that As enrichments are less straightforward compared to those of Mo and U, when using these elements as redox proxies. In addition, the depositional milieus that endured the most hydrologically-restricted and/or reducing (euxinic) conditions are not those with the highest As enrichments. Thus, arsenic shows a redox-sensitive behavior but is not adequate as a redox proxy. However As enrichments are correlated with Mo enrichments in two distinct types of situations: settings impacted 1) by particulate iron shuttling processes and, 2) by diagenetic fluid circulation (cold seeps). In both cases, the Mo-As correlation is induced by the strong relationship between these trace metals and reactive iron: Fe conveys both As and Mo to the sediment where they are trapped. Thus arsenic has a promising potential to allow recognition of past depositional environments associated with iron shuttling and (cold) seep circulations. Lastly, high-TOC, high-Mo, low-As situations point out possible occurrences of sulfurized organic matter in sediments. High Mo concentrations indicate reducing, sulfidic conditions prone to organic-matter accumulation (high TOC values), but low As concentrations indicate that reactive iron was a limiting factor, preventing As capture. Under such conditions, free H₂S was not associating with iron and could react with organic matter, causing its sulfurization.