Anomalous molybdenum isotope trends in Upper Pennsylvanian euxinic facies: Significance for use of δ ⁹⁸Mo as a global marine redox proxy

The use of molybdenum isotope data (δ ⁹⁸Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H ₂S] _aq>11μM) water column to the sediment, thus preserving the seawater δ ⁹⁸Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ ⁹⁸Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ ⁹⁸Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ ⁹⁸Mo values (+1.1 to +1.8‰). These results contrast with the pattern of redox - δ ⁹⁸Mo covariation in modern marine environments, in which the heaviest δ ⁹⁸Mo values are found in the most intensely euxinic facies.We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater-freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ ⁹⁸Mo by water-column redox cycling of Mn, with particulate Mn-oxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ ⁹⁸Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ ⁹⁸Mo trends within the Hushpuckney Shale, seawater δ ⁹⁸Mo must have varied by ~1.2‰ at a ~100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was <100kyr. Although both the second and third hypotheses are viable based on the present limited δ ⁹⁸Mo dataset, we discuss how each model might be tested through additional Mo isotope data.