Oceanic molybdenum drawdown by epeiric sea expansion in the Mesoproterozoic

Abstract

Molybdenum is a bioessential micronutrient whose abundance in the global oceans may have played a primary role in evolution of the Earth's nitrogen cycle and, ultimately, in the timing of the ecologic expansion of early eukaryotes. Because molybdenum (Mo) is delivered to the ocean under conditions of oxic weathering and is removed in suboxic and euxinic environments, the concentration of Mo in the oceans reflects a complex function of global redox and the hydrologic conditions that influence the areal extent of euxinic sedimentation. Recent compilations of Mo within euxinic shales (Scott et al., 2008; Och and Shields-Zhou, 2012; Sahoo et al., 2012) indicate that the oceanic reservoir of Mo did not rise substantially above crustal values until the Great Oxidation Event (~2.3Ga), and a modern Mo cycle did not develop until the contraction of ocean euxinia in the latest Proterozoic.At present, a paucity of data from euxinic shales of the late Mesoproterozoic (1.3 to 1.0Ga) limits our ability to relate marine redox evolution to biological innovation during a critical interval of eukaryotic development. Here we present data from marine shales of the 1.1Ga Atar and El Mreiti groups, Mauritania, which were deposited during sea level highstand when shallow, epeiric seas covered much of the West African craton. Epicratonic strata of the El Mreiti Group were deposited under fluctuating redox conditions near a shallow chemocline (as evidenced by iron speciation), and record generally low Mo concentrations (<15ppm) and Mo/TOC ratios (<2ppm/wt.%), along with a weak covariance between Mo and TOC. By contrast, craton margin strata of the coeval Atar Group were deposited under relatively persistent euxinic conditions, yet record Mo concentrations largely <1ppm and show no covariance between Mo and TOC. Combined, data suggest that Mo sourced from terrestrial weathering was preferentially sequestered in proximal regions of the epeiric sea, and that onshore sequestration led directly to critically low Mo concentrations in offshore waters. We suggest that a Mesoproterozoic expansion of nearshore euxinia (at least in pore waters) within epeiric seas led directly to a critical depletion of the global oceanic Mo reservoir. This hypothesis is supported by a series of first-order sensitivity tests that estimate the extent to which expansion of epeiric seas would have drawn down oceanic Mo concentrations. Ultimately, this study suggests that substantial lateral redox heterogeneity may have limited the availability of bioessential trace metals in the Mesoproterozoic oceans, despite evidence for increased biospheric oxygenation. Extreme Mo limitation in open oceans environments would have had profound effects on the global nitrogen cycle and may help explain observed onshore–offshore patterns in early eukaryotes.