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

Abstract. Rivers are the primary means by which sediments and carbon are transported from the terrestrial biosphere to the oceans but gaps remain in our understanding of carbon associations from source to sink. Bed sediments from the Sacramento-San Joaquin River Delta (CA) were fractionated according to density and analyzed for sediment mass distribution, elemental (C and N) composition, mineral surface area, and stable carbon and radiocarbon isotope compositions of organic carbon (OC) and fatty acids to evaluate the nature of organic carbon in river sediments. OC was unevenly distributed among density fractions. Mass and OC were in general concentrated in mesodensity (1.6–2.0 and 2.0–2.5 g cm⁻³) fractions, comprising 84.0 ± 1.3 % of total 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 only 17.3 ± 12.8 % of TOC. High-density (> 2.5 g cm⁻³) organic-poor, mineral-rich material made-up 13.7 ± 1.4 % of sediment mass and 2.0 ± 0.9 % of TOC. Stable carbon isotope compositions of sedimentary OC were relatively uniform across bulk and density fractions (δ¹³C −27.4 ± 0.5 ‰). Radiocarbon content varied from Δ¹⁴C values of −382 (radiocarbon age 3800 yr BP) to +94 ‰ (modern) indicating a mix of young and old OC. Fatty acids were used to further constrain the origins of sedimentary OC. Short-chain n-C₁₄–n-C₁₈ fatty acids of algal origin were depleted in ¹³C (δ¹³C −37.5 to −35.2 ‰) but were enriched in ¹⁴C (Δ¹⁴C > 0) compared to long-chain n-C₂₄–n-C₂₈ acids of vascular plant origins with higher δ¹³C (−33.0 to −31.0 ‰) but variable Δ¹⁴C values (−180 and 61 ‰). These data demonstrate the potentially complex source and age distributions found within river sediments and provide insights about sediment and organic matter supply to the Delta.