TRACE METALS IN PRECAMBRIAN BLACK SHALES AND THE EVOLVING REDOX STATE OF THE EARLY OCEAN

The initial accumulation of oxygen in Earth's atmosphere introduced a suite of transition elements to the ocean whose geochemical cycles are sensitive to changes in redox potential. Elements like Mo, Re and U require oxygen to liberate them from continental rocks and are removed from the ocean most efficiently where oxygen is absent. Today these elements are relatively abundant in the ocean as a result of pervasive oxygenation of bottom waters and the associated restriction of highly efficient anoxic sinks. During much of the Precambrian, however, oxygen deficiency was likely more pervasive, and the dynamic balance between trace-metal sources and sinks may have resulted in their depletion from seawater on regional to global scales.

Here we compare trace metal enrichments in various modern reducing environments with analyses of Precambrian black shales. Black shales older than ca. 2.4 Ga are depleted in redox-sensitive elements, which is consistent with the lack of oxidative weathering prior to the rise of atmospheric oxygen. In younger Proterozoic black shales where C-S-Fe systematics independently indicate deposition under a euxinic water column, we find that trace metals are depleted relative to analogous modern environments. Similar depletions are observed in the Black Sea where euxinia is widespread and, due to restricted circulation in the basin, dissolved trace metal concentrations are a fraction of open ocean values. This observation is consistent the hypothesis that expansion of sulfidic environments during the Proterozoic, both in the sediments and in the water column, resulted in depletion of bioessential redox-sensitive trace metals. In the late Neoproterozoic, the appearance of extreme trace metal enrichments, similar in magnitude to those of Phanerozoic black shales, indicates a transition to fully oxygenated oceans and restriction of euxinic environments to specific regions or basins. As with any proxy, the challenge is to distinguish between local and global signals. Nevertheless, trace metal analysis of black shales may help to constrain the pattern and timing of changes in the ocean's redox state.