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).
Dissolved Organic Radiocarbon in the West Indian Ocean
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.
more » « less- NSF-PAR ID:
- 10468828
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 19
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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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Abstract We report marine dissolved organic carbon (DOC) concentrations, and DOC ∆14C and δ13C values in seawater collected from the central Pacific. Surface ∆14C values are low in equatorial and polar regions where upwelling occurs and high in subtropical regions dominated by downwelling. A core feature of these data is that14C aging of DOC (682 ± 8614C years) and dissolved inorganic carbon (643 ± 4014C years) in Antarctic Bottom Water between 54.0°S and 53.5°N are similar. These estimates of aging are minimum values due to mixing with deep waters. We also observe minimum ∆14C values (−550‰ to −570‰) between the depths of 2,000 and 3,500 m in the North Pacific, though the source of the low values cannot be determined at this time.
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Abstract 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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Abstract Rivers discharge significant quantities of dissolved organic carbon (DOC) to the ocean, yet biomarker and isotope studies suggest that terrigenous DOC makes up only a small amount 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. Because there is isotopic variability across different organic compound classes, sorptive removal of DOC could also alter the isotopic composition of DOC. As a first step in addressing this question, we examined phase partitioning and isotopic composition of a riverine DOC standard in the presence of marine sediment particles. In a series of controlled experiments, the standard was mixed with marine sediment in 35‰ NaCl solution, then separated into particulate and dissolved phases for analyses of mass, δ13C, and ∆14C of organic carbon (OC). Across a range of sediment OC to DOC mass ratios (from < 0.1 to ~ 3), we found that: (1) sediment sorbed 0.8
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