Sedimentary pyrite proxy for atmospheric oxygen: evaluation of strengths and limitations

Abstract

Predicting the concentration and trends in atmospheric oxygen over Earth history has become a major challenge for scientists. Some consensus has been reached on the general pattern, but there is considerable debate on the detail. Here we discuss a relatively new geochemical proxy based on the trace element content of sedimentary pyrite in black shales. Although the proxy shows particular promise as an estimator of atmospheric oxygen through time, it has several potential weaknesses and requires further rigorous testing before general acceptance is likely.The proxy utilises the concentration of Se and Co in pyrite, two redox sensitive trace elements that exhibit opposite mobility behaviour on the continent and in the oceans that depends on the concentration of available oxygen. The advantages of the pyrite proxy over other geochemical proxies that depend on redox sensitive elements and their isotopes are: 1) application to one mineral, pyrite, rather than a bulk rock shale composite, 2) routine investigation of textures and alteration in samples prior to analysis to check for metamorphic recrystallisation, surface oxidation and hydrothermal effects, 3) Laser ablation ICP-MS avoids the time consuming process of crushing, grinding, and chemically dissolving the rock with strong acids as well as allowing for the separate analysis of both pyrite and organic-clay sediment matrix, 4) Development of a pyrite proxy time series checked by data from the same time period but from different geographic locations. Limitations of the proxy that require further attention include: 1) variability between pyrite analyses on a single sedimentary pyrite grain, 2) effect of sediment pore fluids on the original pyrite composition, 3) lack of studies on modern sedimentary pyrite. Not withstanding these limitations, new insights gained from application of the proxy include: a sustained peak in oxygenation from 2200 to 1800 Ma that may eclipse the GOE; a decline in atmospheric oxygen from 1800 to 1450 Ma; first recognition of the 1400 Ma oxygenation event; and a cyclic behaviour of atmospheric oxygen that matches the supercontinent cycles.