NSF Award Abstract - #0222584

AWSFL008-DS3

Age Distribution of the Particulate Organic Carbon (POC) Discharged from Small
Mountainous Rivers-- the Influence of Sediment Yield and Soil Residence Time

Abstract

Abstract

Short, mountainous rivers dominate the fluvial supply of sediment to the western margin of North America and to the oceans in general. As a result of rapid rates of landscape denudation and minimal sediment storage in these active margin systems, the particulate organic carbon (POC) discharged differs from that transported by larger rivers on passive margins. Recent investigations of two rivers in California indicate they carry a bimodal mixture of modern and ancient particulate organic carbon (POC), derived almost equally from extant vegetation and from the kerogen of uplifted sedimentary rocks. To a first approximation, soils are eroded in these systems before significant aging occurs or before the kerogen can be completely oxidized and replaced by modern material. Based on these preliminary results, the balance between modern, ancient, and intermediate-aged POC carried by rivers is hypothesized to depend on the residence time of particles in the regolith. To test this hypothesis, this investigation will characterize the suspended POC loads of six rivers that exhibit a three-order range of magnitude in sediment yield. Four rivers (Siuslaw, Noyo, Russian, and Eel) drain portions of northern California and Oregon. The Waipaoa and Waiapu rivers on the north island of New Zealand were chosen because of their extremely high sediment yields. Bedrock POC will also be characterized in each of the watersheds. The principle measurements, OC content, 13C/12C(d13C), and 14C/12C (?14C), will be made on bulk samples, as well as density- and size-fractionated subsamples, to determine the kerogen and modern C contents. Additional studies of soil profiles in the Eel watershed will be conducted to further delineate weathering and OC replacement processes in the regolith. The evolution of the POC pool from bedrock C to modern surficial material will be studied in thick horizons, to contrast with previous results from thin soil profiles. The results of this study will further understanding of the processes that regulate the quantity and character of OC transported from drainage basins to the ocean. This study also addresses a key issue in the long-term biogeochemical cycle, namely how previously buried OC is recycled. Interpretation of the sedimentary organic geochemical record of past environments, particularly on active margins, could be significantly improved by this investigation.