Organic Carbon Burial With Reactive Iron Across Global Environments

Preservation of organic carbon (OC) in marine and terrestrial deposits is enhanced by bonding with reactive iron (Fe_R). Association of OC with Fe_R (OC-Fe_R) provides physical protection and hinders microbiological degradation. Roughly 20% of all OC stored in unconsolidated marine sediments and 40% of all OC present in Quaternary terrestrial deposits is preserved as OC-Fe_R, but this value varies from 10% to 80% across global depositional environments. Here, we provide a new assessment of global OC-Fe_R burial rates in both marine and terrestrial environments, using published estimates of OC associated with Fe_R, carbon burial, and probabilistic modeling. We estimate the marine OC-Fe_R sink between 31 and 70 Mt C yr⁻¹ (average 52 Mt C yr⁻¹), and the terrestrial OC-Fe_R sink at between 146 and 917 Mt C yr⁻¹ (average 446 Mt C yr⁻¹). In marine environments, continental shelves (average 17 Mt C yr⁻¹) and deltaic/estuarine environments (average 11 Mg C yr⁻¹) are the primary settings of OC-Fe_R burial. On land, croplands (279 Mt C yr⁻¹) and grasslands (121 Mt C yr⁻¹) dominate the OC-Fe_R burial budget. Changes in the Earth system through geological time impact the OC-Fe_R pools, particularly in marine settings. For example, periods of intense explosive volcanism may lead to increased net OC-Fe_R burial in marine sediments. Our work highlights the importance of OC-Fe_R in marine carbon burial and demonstrates how OC-Fe_R burial rates may be an order of magnitude greater in terrestrial environments, but here OC-Fe_R stocks are most sensitive to the anthropogenic impacts of climatic change.