Unraveling the controls of dissolved sulfate in the Himalayas: pyrite oxidation vs. bacterial reduction

In the mountains, the mineral pyrite (FeS2) consumes oxygen (O2) to produce sulfate (SO42-), which rivers transfer to oceans. Pyrite oxidation also produces acids that react with carbonate rocks to release carbon dioxide (CO2). Thus, pyrite oxidation affects oxygen and carbon dioxide levels in the atmosphere. Since different sources of sulfur and oxygen have different isotope ratios (34S/32S and 18O/16O), pyrite oxidation in modern rivers and marine sediments is identified using isotopes in sulfate. However, river sulfate may not faithfully record rock and atmospheric compositions if anaerobic bacteria overprint the pyrite oxidation signal.

Content and goal

Using state-of-the-art Isotope Ratio Mass Spectrometry at ETH Zurich, we will measure the chemical composition and isotope ratios of rock, river water, and sediments in the Himalayas. The presence and composition of bacterially recycled sulfur in sediments will be related to changes in the isotope ratios of river sulfate to ascertain if biology, and not geology, is the chief control on river sulfate. Significant bacterial reduction of sulfate would mean that sulfur and oxygen isotopes in modern rivers and marine sediments may not sufficiently inform the extent of pyrite oxidation and its control on atmospheric oxygen and carbon dioxide levels.

Scientific and societal context

This work builds on ETH Zurich’s ongoing work in reconstructing pyrite oxidation’s role in the evolution of atmospheric conditions and its expertise in modeling biogeochemical processes while complementing IIT Kanpur’s efforts in studying the Himalayas’ impact on CO2 and climate.