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1. Dissolved Organic Radiocarbon in the Eastern Pacific and Southern Oceans

   https://doi.org/10.1029/2021GL092904  

   Druffel, Ellen R. M. ; Griffin, Sheila ; Lewis, Christian B. ; Rudresh, Megha ; Garcia, Noreen G. ; Key, Robert M. ; McNichol, Ann P. ; Hauksson, Niels E. ; Walker, Brett D. ( May 2021 , Geophysical Research Letters)

   We report marine dissolved organic carbon (DOC) concentrations, and DOC Δ¹⁴C and δ¹³C values in seawater collected from the Southern Ocean and eastern Pacific GOSHIP cruise P18 in 2016/2017. The aging of¹⁴C in DOC in circumpolar deep water northward from 69°S to 20°N was similar to that measured in dissolved inorganic carbon in the same samples, indicating that the transport of deep waters northward is the primary control of¹⁴C in DIC and DOC. Low DOC ∆¹⁴C and δ¹³C 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).

    

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2. Dissolved Organic Radiocarbon in the West Indian Ocean

   https://doi.org/10.1029/2023GL104732  

   Druffel, Ellen R. M. ; Lewis, Christian B. ; Griffin, Sheila ; Flaherty, Alessandra ; Rudresh, Megha ; Hauksson, Niels E. ; Key, Robert M. ; McNichol, Ann P. ; Hwang, Jeomshik ; Walker, Brett D. ( October 2023 , Geophysical Research Letters)

   We report marine dissolved organic carbon (DOC) concentrations, and DOC Δ¹⁴C and δ¹³C in seawater collected from the West Indian Ocean during the GO‐SHIP I07N cruise in 2018. We find bomb¹⁴C in DOC from the upper 1,000 m of the water column. There is no significant change in ∆¹⁴C of DOC in deep water northward, unlike that of dissolved inorganic carbon (DIC), suggesting that transport of deep water northward is not controlling the¹⁴C age of DOC. Variability of DOC ∆¹⁴C, including high values in the deep waters, is more pronounced than in other oceans, suggesting that dissolution of surface derived particulate organic carbon is a source of modern carbon to deep DOC in the West Indian Ocean. Low δ¹³C are present at two of the five stations studied, suggesting a source of low δ¹³C DOC, or additional microbial utilization of deep DOC.

    

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3. New Radiocarbon Constraints on the Global Cycling of Solid‐Phase Extractable Dissolved Organic Carbon

   https://doi.org/10.1029/2020GL090995  

   Lewis, Christian B. ; Walker, Brett D. ; Druffel, Ellen R. M. ( July 2021 , Geophysical Research Letters)

   Radiocarbon (∆¹⁴C) measurements suggest the deep ocean stores marine dissolved organic carbon (DOC) on millennial timescales. The mechanisms that mediate this residence time remain unconstrained. Solid‐phase extraction (SPE) has emerged as a widely used technique to isolate DOC for subsequent analyses. We present SPE‐DOC concentrations and ∆¹⁴C values for three GO‐SHIP Repeat Hydrography transects, spanning the Pacific, Southern and Indian Oceans. 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 ± 5%, 35 ± 6 μM), whereas RDOC in the surface ocean varies as a function of total DOC concentration. Our results fill in spatial SPE‐DOC ∆¹⁴C sampling gaps for the global ocean, and our mass‐balance RDOC estimates are consistent with previous observations.

    

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4. Refractory Dissolved Organic Matter has Similar Chemical Characteristics but Different Radiocarbon Signatures With Depth in the Marine Water Column

   https://doi.org/10.1029/2022GB007603  

   White, Margot E. ; Nguyen, Tran B. ; Koester, Irina ; Lardie Gaylord, Mary C. ; Beman, J. Michael ; Smith, Kenneth L. ; McNichol, Ann P. ; Beaupré, Steven R. ; Aluwihare, Lihini I. ( April 2023 , Global Biogeochemical Cycles)

   The >5,000‐year radiocarbon age (¹⁴C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result,¹⁴C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant¹⁴C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar¹⁴C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ¹⁴C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same¹⁴C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ¹⁴C gradient for RDOC. The observed Δ¹⁴C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean.

    

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5. Low Molecular Weight Dissolved Organic Carbon: Aging, Compositional Changes, and Selective Utilization During Global Ocean Circulation

   https://doi.org/10.1029/2020GB006547  

   Broek, Taylor A. B. ; Walker, Brett D. ; Guilderson, Thomas P. ; Vaughn, John S. ; Mason, Harris E. ; McCarthy, Matthew D. ( June 2020 , Global Biogeochemical Cycles)

   The composition and cycling dynamics of marine dissolved organic carbon (DOC) have received increased interest in recent years; however, little research has focused on the refractory, low molecular weight (LMW) component that makes up the majority of this massive C pool. We measured stable isotopic (δ¹³C), radioisotopic (Δ¹⁴C), and compositional (C/N,¹³C solid‐state NMR) properties of separately isolated high molecular weight (HMW) and LMW DOC fractions collected using a coupled ultrafiltration and solid phase extraction approach from throughout the water column in the North Central Pacific and Central North Atlantic. The selective isolation of LMW DOC material allowed the first investigation of the composition and cycling of a previously elusive fraction of the DOC pool. The structural composition of the LMW DOC material was homogeneous throughout the water column and closely matched carboxylic‐rich alicyclic material that has been proposed as a major component of the marine refractory DOC pool. Examination of offsets in the measured parameters between the deep waters of the two basins provides the first direct assessment of changes in the properties of this material with aging and utilization during ocean circulation. While our direct measurements largely confirm hypotheses regarding the relative recalcitrance of HMW and LMW DOC, we also demonstrate a number of novel observations regarding the removal and addition of DOC during global ocean circulation, including additions of fresh carbohydrate‐like HMW DOC to the deep ocean and large‐scale removal of both semilabile HMW and recalcitrant LMW DOC.

    

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