Award: OCE-1951073

Project Outcomes Report OCE-1951073

Most of the organic carbon in ocean water is dissolved, like the broth in chicken soup. Even though it is believed that dissolved organic carbon (DOC) is formed recently during photosynthesis in the surface ocean by plants using modern carbon, its radiocarbon (14C) age is thousands of years old. We studied the main controls on the cycling of DOC in the ocean. We measured C isotopes in DOC (13C and 14C) in the East Pacific, Southern, and West Indian Oceans. We hypothesized that circulation was the primary control on DOC cycling in the deep water of these ocean.

First, we found that the aging of 14C in DOC in deep water flowing northward from 69S to 20N was similar to that measured in dissolved inorganic C (DIC) in the same samples, indicating that the transport of deep waters northward is the primary control of 14C in DIC and DOC (Figure 1) (Druffel et al. 2021 Geophysical Research Letters). Low DOC 14C and 13C measurements between 1,200 and 3,400 m depth may be evidence of a source of DOC produced in nearby hydrothermal ridge systems (East Pacific Rise). However, in the Indian Ocean, there was no significant change in 14C of DOC in deep water northward, unlike that of dissolved inorganic C (DIC), suggesting that transport of deep water northward is not controlling the 14C age of DOC. 

Second, we present C isotopic results of solid phase extracted (SPE) DOC in these samples from the Pacific and Indian (Lewis et al. 2024 Geophysical Research Letters). Comparisons of SPE-DOC with total DOC 14C values are used with an isotopic mass-balance to estimate the size of the refractory DOC (RDOC) reservoir and changes in RDOC relative abundance in the global ocean. Estimated RDOC abundance is similar across the deep Pacific and Indian Oceans (average = 93 %, 35 μM), whereas RDOC in the surface ocean varies as a function of total DOC concentration.

Third, we studied river discharge as providing significant quantities of DOC to the ocean, yet biomarker and isotope studies suggest that terrigenous DOC makes up only a small amount of DOC in the ocean. One of the removal pathways proposed for riverine DOC is sorption to marine sediments. This process is chemically selective, but whether sorption alters the isotopic composition of riverine DOC is unknown. We examined phase partitioning and isotopic composition of a riverine DOC standard material in the presence of marine sediment particles (Hauksson et al. 2023 Limnology & Oceanography) (Figure 2). We found that sediment sorbed DOC, and DOC compounds with higher 14C and lower 13C values relative to the bulk DOC was preferentially removed from solution. These results show that sorption of DOC onto sediment can alter the isotopic content of riverine DOC.

Fourth, C isotopes in DIC serve as tracers for oceanic water masses, biogeochemical processes, and air-sea gas exchange. We presented a timeseries of surface DIC 13C and 14C values from 2011 to 2022 from Newport Beach, California (Hauksson et al 2023 Radiocarbon). These data show that DIC 14C and 13C levels decreased since 2004 (Figure 3). These long-term trends are likely the result of significant fossil fuel derived CO2 in surface DIC from air-sea gas exchange. The seasonal variation in 14C levels is likely driven by variations in upwelling, surface eddies, and mixed layer depth. The variation in 13C values appears to be driven by isotopic fractionation from marine primary producers. The DIC 13C and 14C levels record the influence of the drought that began in 2012, and a major upwelling event in 2016.

Fifth, we studied oceanic black C (BC), the residue that remains from incomplete combustion of fossil fuels and biomass on land, and enters the ocean by rivers and atmospheric deposition. The fate of BC is considered to reside in the marine DOC pool, where the oldest BC 14C ages have been measured (>20,000 14C y), implying long-term storage. We report 14C measurements on dissolved BC to constrain the sources and cycling of this material and its contributions to refractory DOC in a site in the North Pacific Ocean (Coppola et al, 2024 PNAS). Our results revealed that the cycling of DBC is more dynamic and heterogeneous than previously believed, though it does not comprise a single, uniformly old 14C age.

Last Modified: 02/26/2025
Modified by: Ellen R Druffel