- Award ID(s):
- 2032340
- PAR ID:
- 10320212
- Date Published:
- Journal Name:
- AGU Fall Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The observation of extremely low radiocarbon content / old radiocarbon ages (>4000 years old) in the intermediate-depth ocean during the last ice age draws attention to our incomplete understanding of ocean carbon cycling. For example, glacial-interglacial seawater 14C anomalies near the Gulf of California have been explained by both the advection from a 14C-depleted abyssal source and local geologic carbon flux. To provide insight to this the origin of the seawater 14C anomalies, we have produced several new records of glacial-interglacial intermediate water (i.e., 14C, δ11B, δ18O, and δ13C) in waters that are “upstream” and “downstream” of the Gulf of California. These observations plus geochemical modeling allow us to: (1) Answer whether the old seawater 14C ages are advected or produced locally; (2) Identify the approximate chemical make-up of this carbon; and (3) Consider the role of known sedimentary processes in this carbon flux to the ocean. (Note that several sites have age model controls based on terrestrial plant 14C ages, providing more confidence in our results.) Our new measurements and modeling indicate that the well-established >4000-year-old seawater 14C anomalies observed near known seafloor volcanism in the Gulf of California are not present “upstream,” indicating that this carbon flux results from a “local” geologic carbon. Furthermore, based on our new benthic foraminifera δ11B measurements, this local carbon Blux does not appear to affect seawater pH. Finally, we suggest several potential geologic carbon source(s) that could explain the anomalously old seawater 14C ages, the relatively unremarkable changes in seawater δ13C, and the essentially negligible change in seawater pH.more » « less
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Radiocarbon (14C) is a critical tool for understanding the global carbon cycle. During the Anthropocene, two new processes influenced14C in atmospheric, land and ocean carbon reservoirs. First,14C-free carbon derived from fossil fuel burning has diluted14C, at rates that have accelerated with time. Second, ‘bomb’14C produced by atmospheric nuclear weapon tests in the mid-twentieth century provided a global isotope tracer that is used to constrain rates of air–sea gas exchange, carbon turnover, large-scale atmospheric and ocean transport, and other key C cycle processes. As we write, the14C/12C ratio of atmospheric CO2is dropping below pre-industrial levels, and the rate of decline in the future will depend on global fossil fuel use and net exchange of bomb14C between the atmosphere, ocean and land. This milestone coincides with a rapid increase in14C measurement capacity worldwide. Leveraging future14C measurements to understand processes and test models requires coordinated international effort—a ‘decade of radiocarbon’ with multiple goals: (i) filling observational gaps using archives, (ii) building and sustaining observation networks to increase measurement density across carbon reservoirs, (iii) developing databases, synthesis and modelling tools and (iv) establishing metrics for identifying and verifying changes in carbon sources and sinks.
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