Riverine Export of Aged Carbon Driven by Flow Path Depth and Residence Time
Barnes, R.T., Butman, D.E., Wilson, H.F., Raymond, P.A. (2018). Riverine Export of Aged Carbon Driven by Flow Path Depth and Residence Time. Environmental Science & Technology, [online] 52(3), 1028-1035. http://dx.doi.org/10.1021/acs.est.7b04717
Plain language summary
Dissolved organic carbon (DOC) gives water colour and can be thought of as the tea of the soil and vegetation that water passes over before reaching a river. This research shows that the age and the chemistry of DOC in rivers can be used to identify whether it came from flow over the soil surface or from deeper paths of flow. Catchments with more precipitation, that hold water for shorter periods of time, and that have less removal or disturbance of surface soils younger and more highly coloured DOC. Soil disturbance and dry conditions lead to adding old carbon back into the modern carbon cycle where it may be used by microorganisms.
The flux of terrestrial C to rivers has increased relative to preindustrial levels, a fraction of which is aged dissolved organic C (DOC). In rivers, C is stored in sediments, exported to the ocean, or (bio)chemically processed and released as CO2. Disturbance changes land cover and hydrology, shifting potential sources and processing of DOC. To investigate the likely sources of aged DOC, we analyzed radiocarbon ages, chemical, and spectral properties of DOC and major ions from 19 rivers draining the coterminous U.S. and Arctic. DOC optics indicated that the majority is exported as aromatic, high molecular weight, modern molecules while aged DOC tended to consist of smaller, microbial degradation products. Aged DOC exports, observed regularly in arid basins and during base flow in arctic rivers, are associated with higher proportion of mineral weathering products, suggesting deeper flows paths. These patterns also indicate potential for production of microbial byproducts as DOC ages in soil and water with longer periods of time between production and transport. Thus, changes in hydrology associated with landscape alteration (e.g., tilling or shifting climates) that can result in deeper flow paths or longer residence times will likely lead to a greater proportion of aged carbon in riverine exports.