Linking lithology and land use to sources of dissolved and particulate organic matter in headwaters of a temperate, passive-margin river system

A number of rivers have been found to transport highly aged organic matter [OM]; however, the sources of this aged material remain a matter of debate. One potential source may be erosion and weathering of headwater lithologies rich in ancient sedimentary OM. In this study, waters, suspended particulates, streambed sediments, rocks and soils from fourteen small headwater watersheds of a mid-size, temperate, passive margin river were sampled and characterized by DC, dC, and POC/TPN ratios to identify sources of particulate and dissolved OM delivered to the river mainstem. These headwater sites encompass a range in lithology (OM-rich shales, OM-lean carbonate/mudstone facies, and OM-free crystalline rocks) and land use types (forested and agricultural), and allow investigation of the influence of agriculture and bedrock types on stream OM characteristics. Streams draining large areas of both agricultural land use and OM-rich lithology contain particulate OM [POM] that is more C-depleted than streams draining forested, shale-free watersheds. However, this is not sufficient to account for the significantly lower DC-POC measured in the river mainstem. Dissolved OM [DOM] DC are in all cases enriched compared to POM from the same stream, but are otherwise highly variable and unrelated to either land use or lithology. POC/TPN ratios were likewise highly variable. POC and DOC dC signatures were similar across all watersheds. Based on isotope mass balance, C-free fossil OM sources contribute 0–12% of total stream POM. Although these results do not unequivocally separate the influences of land use and lithology, watershed coverage by shale and agriculture are both important controls on stream DC-POC. Thus export of aged, particle-associated OM may be a feature of river systems along both passive and active continental margins. 2007 Elsevier Ltd. All rights reserved.