Spatiotemporal variation of the quality, origin, and age of particulate organic matter transported by the Yangtze River (Changjiang)

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2018-09-15
Authors
Wu, Ying
Eglinton, Timothy I.
Zhang, Jing
Montlucon, Daniel B.
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10.1029/2017JG004285
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Organic carbon
Changjiang
Radiocarbon
Suspended particulate matter
Three Gorges Dam
Abstract
Information on the age dynamics of particulate organic matter (POM) in large river systems is currently sparse and represents an important knowledge gap in our understanding of the global carbon cycle. Here we examine variations in organic geochemical characteristics of suspended sediments from the Changjiang (Yangtze River) system collected between 1997 and 2010. Higher particulate organic carbon content (POC%) values were observed in the middle reach, especially after 2003, and are attributed to the increase of in situ (aquatic) primary production associated with decreased total suspended matter concentrations. Corresponding Δ14C values from depth profiles taken in 2009 and 2010 indicate spatial and temporal variations in POC sources within the basin. Two isotopic mass balance approaches were explored to quantitatively apportion different sources of Changjiang POM. Results indicate that contributions of biomass and pre‐aged soil organic matter are dominant, regardless of hydrological conditions, with soil‐derived organic carbon comprising 17–56% of POC based on a Monte Carlo three‐end‐member mixing model. In contrast, binary mixing model calculations suggest that up to 80% of POC (2009 samples only) derived from biospheric sources. The emplacement of the Three Gorges Dam and resulting trapping of sediment from the upper reach of the watershed resulted in a modification of POM 14C ages in the reservoir. With the resulting decline in sediment load and increase in the proportion of modern POC in the lower reach, these changes in POM flux and composition of the Changjiang have significant implications for downstream carbon cycle processes.
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Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 123 (2018): 2908-2921, doi:10.1029/2017JG004285.
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Journal of Geophysical Research: Biogeosciences 123 (2018): 2908-2921
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