T249 The Co-Evolution of Life and Its Environment during the Precambrian: The Rise of Oxygenic Photosynthesis and the Great Oxidation Event

Tuesday, 24 October 2017
1:30 PM - 5:30 PM
Skagit 5

QUANTIFYING ARCHEAN OXYGENATION - INSIGHTS FROM SULFIDE OXIDATION EXPERIMENTS AT LOW PO2

Paper number: T249-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

Prior to the Great Oxidation Event (GOE) at ~2.4-2.3 Ga, sedimentary abundances of the trace element molybdenum (Mo) indicate transient changes in redox conditions at Earth's surface. The best-known example is an authigenic Mo enrichment of up to ~ 40 ppm in the 2.5 Ga Mt. McRae Shale of Western Australia. The same section also contains sulfur isotopes indicating an atmosphere with pO2 <10-5 present atmospheric level (PAL). To determine if the Mo enrichments - which suggest oxygenation - and the sulfur isotopes are consistent with each other requires translating the Mo enrichments into an estimate of pO2. However, this is difficult because the delivery of Mo to marine environments hinges in part on the rate at which Mo-bearing sulfide minerals oxidize and release their Mo to solution at low pO2. While sulfide oxidation kinetics are well-constrained at present atmospheric level, informed extrapolation of these kinetics to low pO2 (<10-5 PAL) is impossible without additional experimental data.

To address this problem, we conducted novel batch experiments of pyrite and molybdenite oxidation across a range of pO2 (10-5 - 10-3 PAL) and pH (1.8 - 8.5) relevant to Archean weathering environments. Kinetics were determined by monitoring consumption of dissolved O2 with luminescence measuring oxygen sensors (LUMOS). These sensors can measure dissolved O2 at nanomolar concentrations. Oxidation rates were calculated, and a linear regression of the rates as a function of pO2 and pH allowed us to determine, for the first time, sulfide oxidation rate laws directly applicable to early Earth environments.

With these new rate laws for sulfide oxidation at low pO2, we revisit weathering models to explore the pO2 and other conditions required to produce the observed sedimentary enrichments of Mo. By exploring a range of erosion rates, weathering environment pH, mass accumulation rates, and depositional area consistent with constraints from the geologic record, we determine the quantitative sensitivity of sedimentary Mo enrichments to changes in pO2. In particular, our modeling indicates that the Mt. McRae Shale Mo enrichment is attainable at a range of conditions under 10-5 PAL pO2, consistent with sulfur isotope signatures.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306329

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