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Rivers are the primary means by which sediments and carbon are transportedfrom the terrestrial biosphere to the oceans but gaps remain in ourunderstanding of carbon associations from source to sink. Bed sediments fromthe Sacramento-San Joaquin River Delta (CA) were fractionated according todensity and analyzed for sediment mass distribution, elemental (C and N)composition, mineral surface area, and stable carbon and radiocarbon isotopecompositions of organic carbon (OC) and fatty acids to evaluate the nature oforganic carbon in river sediments. OC was unevenly distributed among densityfractions. Mass and OC were in general concentrated in mesodensity (1.6–2.0and 2.0–2.5 g cm−3) fractions, comprising 84.0 ± 1.3 % oftotal sediment mass and 80.8 ± 13.3 % of total OC (TOC). Low-density (< 1.6 g cm−3) material, although rich in OC(34.0 ± 2.0 % OC) due to woody debris, constituted only17.3 ± 12.8 % of TOC. High-density(> 2.5 g cm−3) organic-poor, mineral-rich material made-up13.7 ± 1.4 % of sediment mass and 2.0 ± 0.9 % of TOC.Stable carbon isotope compositions of sedimentary OC were relatively uniformacross bulk and density fractions (δ13C−27.4 ± 0.5 ‰). Radiocarbon content varied fromΔ14C values of −382 (radiocarbon age 3800 yr BP) to+94 ‰ (modern) indicating a mix of young and old OC. Fatty acidswere used to further constrain the origins of sedimentary OC. Short-chainn-C14–n-C18 fatty acids of algal origin were depleted in13C (δ13C −37.5 to −35.2 ‰) but wereenriched in 14C (Δ14C > 0) compared to long-chainn-C24–n-C28 acids of vascular plant origins with higherδ13C (−33.0 to −31.0 ‰) but variable Δ14Cvalues (−180 and 61 ‰). These data demonstrate the potentiallycomplex source and age distributions found within river sediments and provideinsights about sediment and organic matter supply to the Delta

The nature of organic carbon in density-fractionated sediments in the Sacramento-San Joaquin River Delta (California)