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.

Automated summary

Bonding organic carbon with reactive iron (OC-Fe-R) enhances its preservation in marine and terrestrial deposits. The proportion of OC stored as OC-Fe-R varies globally, with marine sediments containing roughly 20% and Quaternary terrestrial deposits containing roughly 40% of their OC as OC-Fe-R. This study provides a new assessment of global OC-Fe-R burial rates, estimating the marine sink to be between 31 and 70 Mt C yr(-1) and the terrestrial sink to be between 146 and 917 Mt C yr(-1). The primary settings of OC-Fe-R burial are continental shelves and deltaic/estuarine environments in marine settings, and croplands and grasslands on land.