Abstract

As one of the largest pools of carbon on the planet, organic matter (OM) in aquatic environments plays a major role in balancing the global cycles of carbon and oxygen. Continued sequestration of OM in sediments is required to maintain the Earth’s oxidizing atmosphere and the mechanisms responsible for the preservation of OM in sediments are of the utmost importance for understanding and modelling the long-term repercussions of global climate change and ocean acidification. A common approach used to track OM in such environments involves stable carbon isotope analysis, exploiting the variability in 13C content of natural OM. For stable isotope analysis to be useful, the difference in carbon stable isotope content of the OM pools of interest must be larger than the sum of the analytical uncertainty and natural spread in 13C content. Here we present several examples where differences in 13C content of natural OM can be used to track the fate of OM, while also providing a more realistic representation of the analytical uncertainty associated to these measurements. In addition, carbon stable isotopes were used in a long-term incubation study aimed at following the incorporation of a 13C depleted dissolved algal OM tracer from solution into the sediment mineral matrix. The incorporation of this algal tracer was enhanced through interactions with redox sensitive iron oxides, especially when there is co-precipitation of iron alongside OM. The effect of iron oxide precipitation on the preservation of OM is clear, increasing the quantity of OM sequestered in sediments while also slowing its degradation. Combined with a series of C and Fe K-edge X-ray studies, we demonstrate the importance of inner-sphere covalent complexation for the sequestration of OM via interactions with reactive iron in a series of samples from around the globe including marine and lactustrine sediments with varying oxygen exposure regimes. These strong molecular interactions stabilize iron and OM, allowing for their persistence in sediments via a synergistic ferric carbon shuttle. This allows OM to be stabilized by iron while the reduction of ferric iron is hampered by the presence of OM, leading to their persistence even in reducing environments.