Whether a particular black shale represents local, regional or global-scale conditions has major implications for palaeoceanographic and paleoatmospheric models, particularly in the Precambrian when the areal extent of black shale deposition is lost from the geologic record. Therefore, a means of quantifying the global balance between anoxic and oxic bottom water conditions at a given time would be a valuable paleoenvironmental tool. We propose that the molybdenum (Mo) stable isotope system may provide such a tool.

We are conducting a survey of mass-dependent variations in the isotopic composition of Mo in sediments, using MC-ICP-MS. Variations in the isotopic composition of Mo are reported as d⁹⁷Mo=((⁹⁷Mo/⁹⁵Mo)_sample/(⁹⁷Mo/⁹⁵Mo)_standard - 1) x 1000 per mil. External precision is < +/- 0.25 per mil (2s). Our data demonstrate an offset > 1 per mil between sediments deposited under anoxic conditions (d⁹⁷Mo=+1.02 - +1.52 per mil relative to our in-house standard) and ferromanganese nodules (d⁹⁷Mo=-0.63 - -0.42 per mil). d⁹⁷Mo of Pacific Ocean seawater (d⁹⁷Mo=+1.48 per mil) lies within the range of values for anoxic sediments, closest to modern Black Sea anoxic sediments. Molybdenites from continental ore deposits have intermediate d⁹⁷Mo (-0.26 - +0.09 per mil).

We hypothesize that the isotopic offset between anoxic sediments and ferromanganese nodules results from fractionation during inefficient scavenging from seawater by Mn oxides under oxic conditions. The similarity in d⁹⁷Mo of anoxic sediments and seawater is consistent with the very efficient removal of Mo from seawater under anoxic conditions in the presence of H₂S. The data can be interpreted in terms of a steady-state mass balance between the Mo flux into the oceans from the continents and the Mo flux out of the oceans into oxic and anoxic sediments. Such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of Mo to anoxic and oxic sediments. These findings suggest that d⁹⁷Mo in seawater may co-vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans, and that this variation may be recorded in d⁹⁷Mo of anoxic sediments. Hence the Mo isotope system may be useful in exploring the link between localized observations and regional or global paleoredox conditions.