TEMPORAL TRENDS IN TRACE METAL ENRICHMENT IN PRECAMBRIAN BLACK SHALES AND OXIDATION OF SURFACE ENVIRONMENTS

The history of Proterozoic ocean and atmosphere oxygenation remains a topic of considerable interest and debate. Geochemical analyses of marine sedimentary rocks, including C and S isotope relationships as well as Fe systematics and Mo isotope studies, suggest that the oxidation state of Earth’s surface increased in a step-wise fashion. Steps included a broadly oxygen-deficient Archean, a long-lived intermediate state during much of the Proterozoic characterized by an oxidizing atmosphere and surface ocean, and finally oxygenation of the deep ocean during the Neoproterozoic.

Redox sensitive trace elements like Mo and U are ideally suited for testing the time scales and patterns of oxygenation intrinsic to this model. Their utility is grounded in two fundamental aspects of their general geochemistry. First, delivery to the ocean is dominated by oxidative weathering of continental crust. Second, removal from the water column is favored by reducing environments whose sediments can become significantly enriched in these elements relative to average crust. The primary requirements for enrichment are the presence of hydrogen sulfide and dissolved Mo and U in the water column and/or sediments. Thus, enrichment is more generally linked to trace metal delivery and the presence of reactive organic matter, sulfate, and bacterial sulfate reduction in the absence of oxygen.

We have generated and compiled trace metal data for a suite of Precambrian black shales. We find trace metal enrichments were absent prior to the first oxidation event, ca. 2.3 Ga, when delivery of Mo, U, and sulfate was limited. After this event, as oxygen began to accumulate in the atmosphere, significant enrichments appeared for the first time. Maximum enrichments then declined during an extended Proterozoic period of intermediate redox state, suggesting that trace metal limitation occurred as oxygen deficient deep waters allowed for the common establishment of sulfidic environments. Not until the second oxidation event, ca. 0.75 Ga, do maximum enrichments rebound to approach Phanerozoic levels.