Glacial runoff exports large amounts of carbon (C) to the oceans, but major uncertainty remains regarding sources, seasonality, and magnitude. We apportioned C exported by five rivers from glacial and periglacial sources in northwest Greenland by monitoring discharge, water sources (δ18O), concentration and composition of dissolved organic carbon (DOC), and ages (14C) of DOC and particulate organic C over three summers (2010–2012). We found that particulate organic C (
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.
more » « less- NSF-PAR ID:
- 10409476
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 37
- Issue:
- 4
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract F = 1.0366–0.2506) was generally older than DOC in glacial sourced rivers and likely sourced from the physical erosion of aged C pools. Most exported DOC showed strong seasonal variations in sources and discharge. In summer, mean DOC ages ranged from modern to 4,750 cal years BP (F = 1.0022–0.6291); however, the annual C flux from glacially sourced rivers was dominated by young, plant‐derived DOC (F = 0.9667–1.002) exported during the spring freshet. The most aged DOC (F = 0.6891–0.8297) was exported in middle to late summer at lower concentrations and was glacial in origin. Scaled to the whole of Greenland using model‐estimated runoff, we estimate a total riverine DOC flux of 0.29% to 0.45% ± 20% Tg C/year. Our flux results indicate that the highest C fluxes occur during the time of year when the majority of C is modern in age. However, higher melt rates from the Greenland ice sheet and longer growing seasons could result in increasing amounts of ancient C from the Greenland ice sheet and from the periglacial landscape to the ocean. -
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