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While a reinvigoration of ocean circulation and CO 2 marine geologic carbon release over the last 20,000 years. Much of this evidence points to outgassing is the leading explanation for atmospheric CO rise since the Last Glacial Maximum (LGM), there is also evidence of regions of the mid-depth Pacific Ocean, where multiple radiocarbon (1 4 C) records show anomalously low 14 C/C values, potentially caused by the addition of carbon [1,2]. To better constrain this geologic carbon release hypothesis, we aim to place 14 C-free geologic an upper bound limit on the amount of carbon that may have been added, in addition to the geochemical pathway of that carbon. To do so, we numerical invert a carbon cycle model based on observational atmospheric CO 2 and 14 C records. Given these observational constraints, we use data assimilation techniques and an optimization algorithm to calculate the rate of carbon addition and its alkalinity-to-carbon ratio (R ) over the last 20,000 A/C years. Using the modeled planetary radiocarbon budget calculated in Hain et al. [3], we find observations allow for only ~300 Pg of carbon to be added, as a majority of the deglacial atmospheric 14 C decline is already explained by magnetic field strength changes and ocean circulation changes [3]. However, when we adjust the initial state of the model by increasing C by 75‰ to match the observational C records, we find that observations 14 14 allow for ~3500 Pg of carbon addition with an average R of ~1.4. A/C These results allow for the possibility of a large release of 14C-free geologic carbon, which could provide local and regional 14C anomalies, as the records have in the Pacific [1,2]. As this geological carbon was added with a RA/C of ~1.4, these results also imply that 14C evidence for significant geologic carbon release since the LGM may not be taken as contributing to deglacial CO2 rise, unless there is evidence for significant local acidification and corrosion of seafloor sediments. If the geologic carbon cycle is indeed more dynamic than previously thought, we may also need to rethink the approach to estimate the land/ocean carbon repartitioning from the deglacial stable carbon isotope budget. [1] Rafter et al. (2019), GRL 46(23), 13950–13960. [2] Ronge et al. (2016), Nature Communications 7(1), 11487. [3] Hain et al. (2014), EPSL 394, 198–208.
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PP33D-1547 Characterizing the spatial distribution and chemistry of geologic carbon input to the Eastern Tropical North Pacific before and after the last ice age
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.
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- PAR ID:
- 10507252
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Transactions American Geophysical Union
- ISSN:
- 2379-6723
- Format(s):
- Medium: X
- Location:
- San Francisco, CA, USA
- Sponsoring Org:
- National Science Foundation
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