Riverine transport of organic carbon (OC) is a significant flux in the global C budget, representing major terms for both terrestrial losses and marine inputs. Rivers and estuaries are also dynamic systems where terrestrial OC is transformed chemically and isotopically prior to its export to the ocean margins. The global C budget currently assumes that riverine flux represents the excess of terrestrial ecosystem net primary production over decomposition. New data, however, show that rivers discharging to the NW Atlantic margin exhibit a broad range in OC ages, from modern to >5,000 yrs B.P., with particulate OC (POC) often being much older than dissolved OC (DOC). Therefore, part of the OC residing in continental reservoirs on 103-108 year timescales is being remobilized, but the extremely limited dataset precludes quantitative estimates of the transfer of different-aged OC pools from land to the oceans. A major question to be addressed in this proposal is "How much OC of young, old, and ancient ages is mobilized in rivers, and is the processing of DOC and POC in specific rivers and estuaries prior to discharge to the ocean dependent on OC ages and source characteristics?" This will be addressed by examining a regional subset of river/estuarine systems using isotopic (D14C, d13C), organic biomarker (kerogen, microbial fatty acids, plant lignin, and hydrocarbons), and modeling approaches. The systems chosen for study will cover a spectrum of land-use, lithology, size and discharge in the northeastern U.S. Different stages of the rivers' hydrographs will be sampled to assess the key roles of runoff and residence times as controlling factors. Results from this project will be important for understanding how land-derived OC fuels the intense net heterotrophy of rivers and estuaries, for constraining the losses of soil OC via river/estuarine transport, and for interpreting the chemical and isotopic signatures of OC in ocean margins due to both OC sources and processing.
