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

Abstract

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 D 14 C, d 13 C, 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 14 C-depleted than streams draining forested, shale-free watersheds. However, this is not sufficient to account for the significantly lower D 14 C-POC measured in the river mainstem. Dissolved OM [DOM] D 14 C 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 d 13 C signatures were similar across all watersheds. Based on isotope mass balance, 14 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 D 14 C-POC. Thus export of aged, particle-associated OM may be a feature of river systems along both passive and active continental margins.

Erosion, deposition and replacement of soil organic carbon in Mediterranean catchments: a geomorphological, isotopic and land use change approach

“…These sediments presented OC characteristics ( 14 C, δ 13 C) that matched those of deeper soil layers (i.e. >30 cm) of the drainage area in the subcatchment, an observation that is in agreement with other studies that report transport of old OC in rivers when the dominant erosion processes deliver material from deeper soil layers and bedrock in the source areas into river channels (Gómez et al, 2010;Longworth et al, 2007;Masiello and Druffel, 2001). …”

“…The occurrence of erosion processes that transport deep soil in C51 was related to land use change and lithology, both of which can greatly influence the characteristics of the mobilized OC in catchments (Longworth et al, 2007). Lithology, a key component in sediment delivery (de Vente et al, 2011;Haregeweyn et al, 2006) was sensitive to the formation of gullies and mass movements in the slope-streambed connection of C51.…”

Wetland‐driven shifts in suspended particulate organic matter composition of the Hudson River estuary, New York

“…However, the d 13 C POC values for both wetlands were more depleted (231.8% 6 1.6%; Table 1) than typical for sedimentary rocks (229.3% 6 0.6%, Petsch 2000; Table 2), as well as soils and surface sediments (228.8% 6 0.7%; Table 2, Longworth et al 2007) that might be supplied from the upper HRE. Moreover, the d 13 C POC was even more depleted than that of bed sediments within the wetlands (227.0% 6 1.5%; Table 2).…”

Controls on the origin and cycling of riverine dissolved inorganic carbon in the Brazos River, Texas

“…These soils have been shown to be rich in charcoal (Skjemstad et al 2002), leading us to conclude that lignite is much less than 3.8% of the riverine POM. Longworth et al (2007) observed young dissolved organic matter (DOM) in watersheds draining OM-rich shale, suggesting little or no contribution of shale organic carbon to riverine DOM. Assuming that contributions of lignite black carbon to the DOM pool are also negligible in the lower Brazos River, we conclude that lignite decomposition has a trivial impact on 14 C content of riverine DIC.…”