The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland

Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth's oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H2S similar to 200 mu M in the bottom waters (or about 300 mu M total sulfide: Sigma S2- = H2S + HS- + S2-). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a delta Mo-98(oxic) = 0.9 +/- 0.1 parts per thousand, which matches that of the riverine input, delta Mo-98(river) = 0.9 +/- 0.1 parts per thousand. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 +/- 0.1 parts per thousand, but the dissolved Mo is even heavier at delta Mo-98(sulfidic) = 1.8 parts per thousand. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with delta Mo-98 values significantly fractionated at -0.8 parts per thousand to -1.2 parts per thousand both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by similar to 0.3-1.5 parts per thousand. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive delta Mo-98 trend with depth, from delta Mo-98 = 1.2 parts per thousand to 1.4 parts per thousand while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of delta Mo-98 = 0.9-1.0 parts per thousand. These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H2S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with