Stable organic carbon formation accelerated by manganese oxide mineral reduction and the implications for the global carbon cycle

Abstract

Abstract Marine and terrestrial sediments stabilise organic carbon (OC) against microbial respiration, influencing the climate by slowing the release of CO2 into the atmosphere. Redox-active element-bearing minerals, such as the manganese oxide mineral birnessite (δMnO2) play an important role in OC stabilisation. However, the mechanism for birnessite-OC stabilisation is unclear, limiting further understanding of the role of birnessite in the carbon cycle. We hypothesise that bond formation is promoted on δMnO2 surfaces, ultimately producing a complex assembly of stabilised OC. We test our hypothesis by investigating δMnO2 samples spanning < 1 year to 2.5 billion years old, using X-ray photoelectron spectroscopy and thermogravimetric analysis to synergistically observe OC thermal decomposition. We propose that thermally stable OC is generated during δMnO2 reduction. We speculate that thermally stable OC is unsuitable for rapid microbial respiration and is therefore chronologically stable. We conclude that the OC stabilisation described here could be an important but hitherto unrecognised process that removes OC from the readily exchangeable OC pool, thereby helping to modulate the climate.