Small mountainous rivers (SMR) export globally significant quantities of particulate organic carbon (POC) to the oceans, the composition of which may be distinct from POC delivered by major world rivers. To better constrain the controls on the Δ¹⁴C signatures of POC suspended in SMR, we examined the temporal variability of Δ¹⁴C and δ¹³C of POC suspended in the Santa Clara River, a SMR in California, USA. Our data suggest mixing of bedrock and contemporary soil, which can be applied to other SMR in contrasting climate zones and with varying land cover. The data also suggest that chemical denudation is incomplete, but effective to the point that only the most recalcitrant moieties of the parent POC are exported. We further propose sediment yield as a potentially useful proxy for the Δ¹⁴C of riverine POC. A preliminary analysis of the existing data for a spectrum of rivers supports this hypothesis.

Small mountainous rivers (SMR) export globally significant quantities of particulate organic carbon (POC) to the oceans, the composition of which may be distinct from POC delivered by major world rivers. To better constrain the controls on the Δ¹⁴C signatures of POC suspended in SMR, we examined the temporal variability of Δ¹⁴C and δ¹³C of POC suspended in the Santa Clara River, a SMR in California, USA. Our data suggest mixing of bedrock and contemporary soil, which can be applied to other SMR in contrasting climate zones and with varying land cover. The data also suggest that chemical denudation is incomplete, but effective to the point that only the most recalcitrant moieties of the parent POC are exported. We further propose sediment yield as a potentially useful proxy for the Δ¹⁴C of riverine POC. A preliminary analysis of the existing data for a spectrum of rivers supports this hypothesis.

Surface sediments along a transect from an abyssal site in the northeastern Pacific (Station M, 34°50′N, 123°00′W) to a small mountainous river on the California coast (Santa Clara River) were studied to investigate the sources and cycling of organic matter on the continental margin. Sediment samples were separated into organic compound fractions (extractable lipids, amino acids (THAA), carbohydrates (TCHO), and the acid-insoluble fraction), and their carbon isotope ratios were measured. The Δ¹⁴C values of all the THAA and TCHO fractions were greater than −100‰, indicating relatively modern organic carbon (OC) source(s), and rapid cycling of these fractions. In contrast, the Δ¹⁴C values of extractable lipids and the acid-insoluble fraction were distinctly lower than those of the THAA and TCHO fractions. The Δ¹⁴C values of source OC to the sediments were estimated using a simple mixed layer model. These values were lower than the Δ¹⁴C signatures of pre-industrial plankton suggesting input of both old OC and contemporary plankton to the margin sediments. The source of old OC at the 2000-m site was likely from laterally transported coastal sediment. The estimated low Δ¹⁴C value of the transported OC suggests that old lipids and acid-insoluble material were selectively transported to the 2000-m site. The contribution of riverine POC to the margin sediments were estimated from Δ¹⁴C and δ¹³C values and indicate that relict OC exported by rivers was an important source of old lipids and acid-insoluble material to sedimentary OC on the shelf.

 Chemical and isotopic variability of particulate organic carbon (POC) was examined in samples from the Santa Clara River watershed and adjacent shelf to investigate the processing of fossil POC derived from bedrock. The Santa Clara is a small coastal river that drains mountainous terrain in southern California, United States. Organic carbon in shale, river suspended sediment, and coastal marine sediment was separated into three operationally defined organic compound classes: total extractable lipids, acid hydrolyzable material, and the nonhydrolyzable residue. In all samples, the nonhydrolyzable fraction was dominant (∼50% of POC), while lipids and acid hydrolyzable moieties were relatively minor (≤22 and ≤13%, respectively). The Δ¹⁴C and δ¹³C signatures of the isolated fractions varied dramatically, not only across different sample types, but also for a given sample. At the shale exposure, low Δ¹⁴C values (<−760‰) indicated dominance of ancient C in all three organic compound classes. In downstream samples, the extractable lipids displayed the lowest Δ¹⁴C values (<−500‰), while the acid hydrolyzable fraction was predominantly modern (Δ¹⁴C > −30‰). The nonhydrolyzable fraction displayed intermediate Δ¹⁴C values (<−190‰) that increased steadily downstream with decreasing δ¹³C values (−22.2 to −25.0‰), possibly from mixing of shale and surface soil POC. Our results indicate that most of the fossil POC discharged by the Santa Clara is composed of non-acid hydrolyzable material, but its elusive molecular structure and marine-like δ¹³C signature may render its detection in the ocean difficult. In contrast, fossil lipids may be more amenable to detection if their resistant components (e.g., asphaltic material) are unique to crustal sources.