Organic carbon is mainly produced via photosynthetic plants in the terrestrial realm and algae in the aquatic realm. During transport of terrestrial organic carbon to the oceans, the majority is lost or ‘mineralized’, within the land-ocean margin, as very little chemical signal of the newly fixed biological carbon can be found in the ocean (Hedges et al., 1997). Moreover, in freshwater and marine aquatic systems, much of the organic carbon fixed in the euphotic zone has been shown to be recycled in the water column such that a minor percentage of the primary production actually reaches the benthos (e.g. Laskov et al., 2002; Wakeham et al., 1997) and an even smaller portion becomes deeply buried and preserved. But, this residual portion of organic carbon that is preserved in aquatic sediments is extremely important as it determines the balance between CO2 and O2 in the atmosphere over time, forms fossil fuels, and ultimately regulates climate.
What is particularly confounding about the residual carbon is that the majority of it is, as yet, molecularly uncharacterized (Hedges et al., 2000). Furthermore, the sources and processes leading to its formation are poorly understood. A knowledge of the sources and processes leading to the formation of this refractory organic matter in coastal sediments would allow scientists to better understand the link between carbon fixation and long term preservation in this important terrestrial-marine transition zone as well as to understand the how future changes in climate and hydrological transport of this material may create climate feedbacks.
One idea that is currently gaining acceptance is that residual, and presumably refractory, organic carbon that is found in modern coastal sediments is highly aged and mainly derived from the upper catchments of river watersheds. This notion would constitute a shift in paradigm, suggesting that some of the OM exported by rivers is highly aged and/or refractory material, rather than “young” and biodegradable terrestrial organic matter, as was initially presumed (Berner, 1982; Ludwig et al., 1996). If that is indeed the case, then certain fundamental questions arise: 1) what exactly is the source of this aged fossil ROM carried by rivers, and 2) how do hydrological processes drive the flux of such ROM?
Sources and processes affecting the quantity and composition of labile and refractory OM fractions in riverine systems include soil and sediment erosion, atmospheric deposition, autotrophic production, microbial decomposition, and burial. Although we know the processes involved, we know neither the magnitude nor relative importance of each.
Carbon Cycling in Coastal Margins
Continental margin sediments are responsible for about 90% of the ocean’s ‘sequestered’ organic matter (OM), yet most of this carbon is difficult to characterize due to its chemical complexity. A large portion of this OM may be refractory and is likely to be preserved in the coastal zone, but, we have a poor understanding of its source and temporal and spatial variability. Understanding the composition of refractory carbon in coastal systems is important as some of it may represent recent transfers of carbon from the atmosphere to the geosphere (i.e. atmospheric CO2 drawdown: Fig 1) while some may be derived from C fixed much earlier.