MOLYBDENUM ISOTOPES IN BLACK SHALES

The molybdenum isotope system is emerging as an indicator of regional or global redox conditions. The basis of this application is the relatively small (< 1 ^o/oo; d^97/95Mo) fractionation between Mo in seawater and sediments accumulating under euxinic conditions, probably because of mass balance constraints, and the comparatively large (~ 2 ^o/oo) fractionation between Mo in seawater and ferromanganese crusts and nodules (1, 2). The latter effect has recently been reproduced in the laboratory (3) and probably dominates Mo isotope fractionation in the oceans (Mo isotope fractionation during removal in suboxic settings [4] is probably not of quantitative importance in the global Mo isotope budget). The Mo isotope composition of seawater should therefore vary over geologic time with the extent of global ocean anoxia: shifts toward lighter values would reflect decreased removal to Mn oxides associated with expanded anoxia. Such variations may be recorded in d^97/95Mo of black shales.

A shift of up to 1.3 ^o/oo toward lighter d^97/95Mo in mid-Proterozoic black shales may record expanded deep water anoxia under a low PO₂ atmosphere (5). A smaller d^97/95Mo shift, only ~ 0.5 ^o/oo, is seen in the Huron Member of the Ohio (black) Shale of the Late Devonian (USGS SDO-1). Interpretation of smaller variations in the range observed in SDO-1 requires further study but may reflect less pervasive oceanic anoxia. In euxinic sediments of the Black Sea and the Cariaco Basin, the world's largest modern euxinic basins, we observe small (< 1 ^o/oo) but measurable variations both temporally and spatially. There is a ~ 0.5 ^o/oo difference between Unit I and Unit II sediments of the Black Sea, and an offset of similar magnitude between the Black Sea Unit I and the euxinic sediments of the Cariaco Basin.

Factors that may affect the black shale Mo isotope record at < 1 ^o/oo may include massive local or regional changes in the riverine Mo flux; inefficient removal of Mo under mildly euxinic conditions; or lack of open communication between euxinic basins and the global ocean. The importance of such factors is being investigated.

1. Barling et al., EPSL, 2001; 2. Siebert et al., EPSL, 2003; 3. Barling and Anbar, EPSL, in revision; 4. McManus et al., G^3, 2002; 5. Arnold et al., Goldschmidt 2002.