Cycling of marine dissolved organic carbon: New insights from radiocarbon distribution within carboxylic-rich alicyclic molecules and acylated heteropolysaccharides

Dissolved organic carbon (DOC) is the largest reservoir of reduced carbon in the ocean with a carbon inventory similar to that of the atmosphere. Natural-abundance radiocarbon (¹⁴C) and stable carbon (¹³C) isotopic measurements have been used to constrain DOC biogeochemistry. However, most of these measurements were performed on bulk-, ultrafiltered-, or hydrophobic DOC and represent the average isotopic composition of all respective DOC constituents. There is, however, good evidence that the constituent molecules span a large range of ¹⁴C and ¹³C values [1, 2, 3], and accordingly cycle on a variety of timescales. In this study, we used a detailed size and chemical fractionation approach to comprehensively examine the isotopic distribution of carboxylic-rich alicyclic molecules (CRAM) and acylated heteropolysaccharides (APS) within marine DOC. Seawater samples from the North Central Pacific ocean (22^o 45’N, 158^oW) were separated into APS and CRAM by ultrafiltration and solid phase extraction. We used a thermal serial oxidation approach to link the isotopic diversity of DOC to its major structural components. Our data revealed isotopic heterogeneity in the DOC pool, with radiocarbon-enriched APS (1000 ¹⁴C yr to modern) mixed with relatively radiocarbon-depleted CRAM (ca. 1000 - 2500 ¹⁴C yr) in surface waters of the North Central Pacific ocean. In the deep ocean, a major component of APS had a radiocarbon value similar to that of ambient dissolved inorganic carbon (DIC) indicating conservative transport, or perhaps, a chemosynthetic origin. In contrast, deep ocean CRAM (~3000-8000 years) was much older than ambient DIC and the APS fraction (2000 – 3500 years), but also exhibited considerable isotopic diversity in its constituent organic compounds. Our data constrain the minimum age distribution of the majority of DOC, and allows for better modeling of DOC in the marine carbon cycle.

Cited work: [1] Beaupre et al. 2007. Limnology and Oceanography: Methods 5: 174-184. [2] Beaupre and Druffel 2012. Geophysical Research Letters 39, L18602, doi:10.1029/2012GL052974. [3] Follet et al. 2014. PNAS 111: 16706-16711.