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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The atmospheric bridge communicated the <i>δ</i><sup>13</sup>C decline during the last deglaciation to the global upper ocean
Abstract. During the early part of the last glacial termination (17.2–15 ka) and coincident with a ∼35 ppm rise in atmospheric CO2, a sharp 0.3‰–0.4‰ decline in atmospheric δ13CO2 occurred, potentially constraining the key processes that account for the early deglacial CO2 rise. A comparable δ13C decline has also been documented in numerous marine proxy records from surface and thermocline-dwelling planktic foraminifera. The δ13C decline recorded in planktic foraminifera has previously been attributed to the release of respired carbon from the deep ocean that was subsequently transported within the upper ocean to sites where the signal was recorded (and then ultimately transferred to the atmosphere). Benthic δ13C records from the global upper ocean, including a new record presented here from the tropical Pacific, also document this distinct early deglacial δ13C decline. Here we present modeling evidence to show that rather than respired carbon from the deep ocean propagating directly to the upper ocean prior to reaching the atmosphere, the carbon would have first upwelled to the surface in the Southern Ocean where it would have entered the atmosphere. In this way the transmission of isotopically light carbon to the global upper ocean was analogous to the ongoing ocean invasion of fossil fuel CO2. The model results suggest that thermocline waters throughout the ocean and 500–2000 m water depths were affected by this atmospheric bridge during the early deglaciation.
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- Award ID(s):
- 1736771
- PAR ID:
- 10298572
- Date Published:
- Journal Name:
- Climate of the Past
- Volume:
- 17
- Issue:
- 4
- ISSN:
- 1814-9332
- Page Range / eLocation ID:
- 1507 to 1521
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/cp-17-1507-2021
