Lightning-driven pyrite oxidation under Archean atmosphere conditions

Oxidative weathering is a major source of bio-essential micronutrients on Earth today; however, this flux would have been muted on the early Earth or on Mars, where atmospheric O2(g) levels were very low. Here, we explore the hypothesis that nitrogen oxides generated by lightning in an anoxic atmosphere could have elevated pyrite oxidation levels under otherwise anoxic conditions. We performed spark discharge experiments in the presence of pyrite powder and three different gas mixtures, including 80% N2(g) with 20% CO2(g), 95% N2(g) with 5% CO2(g), and modern air. Experiments were run for 30 min, and we tracked the production of NO(g), dissolved nitrate and nitrite, pH, dissolved sulfate, and total dissolved iron. Our results reveal increasing production of nitrogen oxides with increasing CO2(g) and O2(g) levels, which is consistent with previous studies. Dissolved iron and sulfate also increase, indicating that the nitrogen oxides are able to oxidize pyrite abiotically. Extrapolating these data to global conditions suggests that this mechanism was probably insignificant on a global scale on the early Earth; however, in thunderstorm-prone areas, such as in the modern tropics where lightning rates may locally be over 10 times above the global average, lightning could have rivalled abiotic pyrite oxidation by Archean O2 levels. The lightning contribution would have been highest during time periods with elevated CO2(g), which makes it a potentially important contributor to local release of sulphur, iron, and bio-essential micronutrients on prebiotic land surfaces or on other planets with anoxic CO2-rich atmospheres.