The role of iron bound carbon for organic matter preservation and global climate systems: an experimental approach.

Burial of organic carbon (OC) in marine sediments is a crucial process for the drawdown of atmospheric CO2 over geological timescales. Association of OC with reactive iron (FeR) phases represents the largest described mechanism by which OC is preserved in marine sediments (accounting for ~22% of preservation). Despite its importance, little is known about which FeR phases are involved in OC uptake, or the binding mechanism of OC to these reactive iron minerals. Moreover, the effect of different OC moieties on the stability and preservation of Fe-bound OC is not fully understood. To determine the importance of the OC-FeR mechanism, a citrate-dithionite-bicarbonate (CDB) extraction is used to dissolve the ‘easily reducible iron oxide’ fraction and release associated OC from sediments. However, natural samples contain a range of FeR phases extractable by CDB, and phases are defined by their susceptibility to chemical reduction. Therefore, factors affecting mineral stability, including association with OC, may influence estimates of the amount of OC bound to FeR phases. Here, OC-Fe composites were synthesised with known FeR phases and OC moieties and spiked into OC-free marine sediment followed by treatment with CDB to determine the impact of OC moiety on Fe release. It is shown that CDB treatment results in only partial dissolution of the most susceptible Fe phase and that greater losses of Fe occur for carboxyl rich organominerals due to increased structural disorder. Further findings indicate that the strength of OC bound to Fe, determined by structure, can affect quantification of the OC-Fe pool and effort is made to reconstruct the CDB method in order to improve its accuracy. A developed understanding of this molecular level characterisation will improve understandings of OC-Fe quantifications from sediment analysis, and subsequently in determining the extent to which Fe bound OC preservation is important for global carbon cycling.