New model for molybdenum behavior in euxinic waters

Rogoznica Lake, on the Adriatic coast of Croatia, undergoes vertical mixing in autumn, but quickly restratifies and becomes intensely sulfidic during the succeeding 11 months. Autumn's vertically homogeneous dissolved Mo concentrations quickly revert to a summer profile in which Mo concentrations are lower by an order of magnitude in the sulfidic layer than in the oxic layer. Other redox sensitive elements (As, U and V) are much less affected on this time scale. Below the depth where sulfidic waters become saturated with iron monosulfide, Mo concentrations become nearly constant. A similar pattern can be recognized in other euxinic basins and seems inconsistent with Mo removal by sorption on sinking particles. Precipitation of a previously unrecognized, probably nanoscale Fe(II)–Mo(VI) sulfide mineral is postulated to explain this phenomenon. By fitting the Rogoznica data, a provisional composition and solubility product constant for this material are derived. The mineral's formula approximates Fe5Mo3S14. Its Mo content is similar to that of a synthetic precipitate previously shown by X-ray spectroscopy to resemble molybdenum's host phase in black shales. Assuming saturation with this mineral, dissolved Mo in the deep water column of a number of other euxinic basins can be predicted in most cases to within a factor of three. The model has important implications for Mo behavior in euxinic basins. For example, the model attributes the near-total Mo removal from deep Black Sea waters to a serendipitous combination of pH and H2S concentration, implying that such removal would not be a general property of euxinic basins. This is a key point with regard to Mo isotope systematics. Contrary to previous views, the model assigns a critical role to pH in controlling the efficiency of Mo removal from euxinic basins.

Research highlights
► Measurements at Rogoznica Lake (Croatia) suggest that Mo concentrations and FeS ion activity products are linked; a Mo limiting concentration is attained at depths where saturation with FeS is reached.
► Existence of a new Fe–Mo–S mineral is postulated; its composition is inferred to approximate FeMo0.6S2.8 and its solubility is determined.
► Assuming that other major euxinic basins become saturated with this mineral at depth, their limiting Mo concentrations are computed reasonably well based on a Rogoznica-calibrated thermodynamic model.
► For euxinic basins having modern values of pH and Mo input concentration, the model predicts that nearly complete removal of Mo can occur only in a limited sulfide concentration window and that Mo isotope fractionation is to be expected in many euxinic basins because their sulfide concentrations fall outside this window.