Sulfidity controls molybdenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea

Abstract

Molybdenum (Mo) isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry. It is therefore fundamental to understand the processes controlling Mo partitioning into modern marine environments. This study identifi es the availability of dissolved sulfi de as the dominant control on overall Mo removal from the water column in euxinic systems. Mo isotopic composition of surface sediments from different localities of the Black Sea demonstrates complete fi xation of Mo only below 400 m water depth, above a critical concentration of 11 µmol l -1 aqueous hydrogen sulfi de in the bottom water. The Mo isotopic composition of these sediments refl ects the homogeneous seawater isotopic composition of 2.3‰. In contrast, signifi cant Mo isotope fractionation into less euxinic sediments is evident at shallower depths in the Black Sea, as well as in temporarily euxinic deeps of the Baltic Sea, consistent with the observed lower maximum sulfi de concentrations in the respective water columns. Therefore, Mo isotope signatures in the modern Black Sea constrain the processes responsible for global Mo removal from the ocean by euxinic sediments. Furthermore, models of past ocean anoxia reconstruction have to consider that the seawater Mo isotopic composition is not per se archived in euxinic sediments.

Molybdenum evidence for expansive sulfidic water masses in ~750Ma oceans

“…a) Total sulfide: ∑S(-II) = H2S + HS -+ S Table S1: Chemical characteristics of modern euxinic basins and inferred composition for the Walcott basin [20][21][22].…”

“…In fact, tidal influence is reported throughout the section [19] suggestive of a low estuarine Richardson number. 98 Mo of global seawater [21], and encourages the use of Mo isotopes to track global ocean redox conditions in the past from restricted marine basins.…”

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

“…The overall Mo/U ratio decreases in the iron reduction zone. Molybdenum content in all but two carbonate concretions (Cal-C and Cal-N) is consistently greater than the crustal average (1e2 ppm; Taylor and McLennan, 1985) and much higher than that expected for pure carbonate rocks (0.02 ppm; Neubert et al, 2008 (Algeo and Tribovillard, 2009). Anomalously high Mo concentrations are believed to reflect the formation of iron sulfides after the production of hydrogen sulfide during AOM (Sato et al, 2012;Hu et al, 2014).…”

“…In well-oxygenated water Mo behaves conservatively, whereas it normally enriches within organic matter and iron sulfides under anoxic conditions (Helz et al, 1996;Neubert et al, 2008;Algeo and Tribovillard, 2009). In anoxic conditions Mo is completely removed from seawater and accumulates in surface sediments (Neubert et al, 2008).…”

“…In well-oxygenated water Mo behaves conservatively, whereas it normally enriches within organic matter and iron sulfides under anoxic conditions (Helz et al, 1996;Neubert et al, 2008;Algeo and Tribovillard, 2009). In anoxic conditions Mo is completely removed from seawater and accumulates in surface sediments (Neubert et al, 2008). Under oxic conditions U is present in its oxidized form U(VI) showing a conservative behavior (e.g., Hu et al, 2014), whereas in suboxic conditions (within the iron reduction zone) U(VI) is reduced to the insoluble U(IV).…”