Using Radiocarbon to Assess the Abundance, Distribution, and Nature of Labile Organic Carbon in Marine Sediments

• Published in: Global biogeochemical cycles 2021-06, Vol.35 (6), p.n/a
• Main Authors: DeMaster, D. J., Taylor, R. S., Smith, C. R., Isla, E., Thomas, C. J.
• Format: Article
• Language: English
• Subjects: Abundance; Biodegradation; Bioturbation; Carbon; Carbon 14; Coefficients; Continental margins; Cores; Depth; Diagenesis; Folding; Gradients; labile organic carbon; Marine sediments; Mixed layer; Nutrition; Ocean floor; Organic carbon; Organic matter; Radiocarbon dating; reactive organic matter; seabed radiocarbon modeling; Sediment; sedimentary organic carbon degradation rates; Sediments; Shelves; Stations; Steady state models; Surface boundary layer; Surface layers; Surface mixed layer; Total organic carbon
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2020GB006676

Positive 14C gradients have recently been observed within the surface mixed layer of several continental‐margin sediments. The best explanation for these positive 14C gradients is the occurrence and rapid degradation of labile organic carbon (LOC) in the upper 5–10 cm of the seabed. Based on a two‐component model for sedimentary organic matter (i.e., a planktonic labile component and an older refractory component), bulk 14Corg data were used to determine the abundances of LOC within the surface mixed layers of three cores from the West Antarctic Peninsula (WAP) shelf and one core from San Clemente Basin (California Borderland). LOC contents in surface samples from the four stations varied from 0.5 to 1.1 mg/cm3, comprising 20% (San Clemente Basin) to 80% (WAP, Sta. G) of the total organic carbon. By incorporating a steady state diagenetic model and particle‐mixing bioturbation coefficients, the LOC profiles were used to determine LOC turnover times (LOC τ) and LOC e‐folding depths. The LOC τ values for the West Antarctic Peninsula sediments varied from 0.09 to 0.59 years, whereas the LOC τ value from the San Clemente Basin core was 63 years. The LOC e‐folding depths for the WAP stations varied from 0.8 to 3.4 cm, in contrast to the LOC e‐folding depth in San Clemente Basin, which was 4.0 cm. LOC characteristics from the four cores examined in this study were compared to LOC data in the literature as a means of substantiating the overall 14Corg‐based approach and justifying model assumptions. Plain Language Summary A new C‐14 based technique for measuring the abundance of Labile (or reactive) Organic Carbon in marine sediments is presented. Based on this new approach and a model for sedimentary organic matter, Labile Organic Carbon depth profiles from West Antarctic Peninsula and California Borderland sediment cores were used to determine Labile Organic Carbon turnover times, which ranged from 0.1 to 0.6 years (Antarctic shelf sediments) to 63 years (California Borderland deposits). Within the bioturbated surface layer of the seabed, Labile Organic Carbon makes up only 6%–12% of the Total Organic Carbon, however, the Labile Organic Carbon is the source of nearly all nutrition and energy for bottom‐dwelling fauna and bacteria. The Labile Organic Carbon abundances and turnover times are in good agreement with values reported in the literature, confirming the efficacy of the new C‐14 based technique. Key Points A new analytical technique, based on C‐14, is pre