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1. PP12B-05 and CO2 14C - Constraints on the Deglacial Release of Geologic Carbon Using Atmospheric Records

   RYAN GREEN, MATHIS HAIN (January 2021, AGU Fall Meeting)

   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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2. The Deep Ocean's Carbon Exhaust

   https://doi.org/10.1029/2021GB007156  

   Chen, Haidi; Haumann, F. Alexander; Talley, Lynne D.; Johnson, Kenneth S.; Sarmiento, Jorge L. (July 2022, Global Biogeochemical Cycles)

   Abstract The deep ocean releases large amounts of old, pre‐industrial carbon dioxide (CO₂) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO₂release is relevant to the global climate because its changes could alter atmospheric CO₂levels on long time scales, and also affects the present‐day potential of the Southern Ocean to take up anthropogenic CO₂. Here, year‐round profiling float measurements show that this CO₂release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea‐ice edge. This band of high CO₂subsurface water coincides with the outcropping of the 27.8 kg m⁻³isoneutral density surface that characterizes Indo‐Pacific Deep Water (IPDW). It has a potential partial pressure of CO₂exceeding current atmospheric CO₂levels (∆PCO₂) by 175 ± 32 μatm. Ship‐based measurements reveal that IPDW exhibits a distinct ∆PCO₂maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO₂decreases. Most of this vertical ∆PCO₂decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO₂outgassing from the high‐carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO₂fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles. 

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3. Biogenic carbon pool production maintains the Southern Ocean carbon sink

   https://doi.org/10.1073/pnas.2217909120  

   Huang, Yibin; Fassbender, Andrea J.; Bushinsky, Seth M. (April 2023, Proceedings of the National Academy of Sciences)

   Through biological activity, marine dissolved inorganic carbon (DIC) is transformed into different types of biogenic carbon available for export to the ocean interior, including particulate organic carbon (POC), dissolved organic carbon (DOC), and particulate inorganic carbon (PIC). Each biogenic carbon pool has a different export efficiency that impacts the vertical ocean carbon gradient and drives natural air–sea carbon dioxide gas (CO₂) exchange. In the Southern Ocean (SO), which presently accounts for ~40% of the anthropogenic ocean carbon sink, it is unclear how the production of each biogenic carbon pool contributes to the contemporary air–sea CO₂exchange. Based on 107 independent observations of the seasonal cycle from 63 biogeochemical profiling floats, we provide the basin-scale estimate of distinct biogenic carbon pool production. We find significant meridional variability with enhanced POC production in the subantarctic and polar Antarctic sectors and enhanced DOC production in the subtropical and sea-ice-dominated sectors. PIC production peaks between 47°S and 57°S near the “great calcite belt.” Relative to an abiotic SO, organic carbon production enhances CO₂uptake by 2.80 ± 0.28 Pg C y⁻¹, while PIC production diminishes CO₂uptake by 0.27 ± 0.21 Pg C y⁻¹. Without organic carbon production, the SO would be a CO₂source to the atmosphere. Our findings emphasize the importance of DOC and PIC production, in addition to the well-recognized role of POC production, in shaping the influence of carbon export on air–sea CO₂exchange. 

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4. Large Igneous Province Sulfur Emissions Have Long‐Term (>1000 Years) Effects on the Ocean Carbon Cycle

   https://doi.org/10.1029/2024GC011893  

   Cheong, Hee_Jun; Mittal, Tushar; Sprain, Courtney_Jean; Fendley, Isabel_M (March 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Large Igneous Province (LIP) eruptions are thought to have driven environmental and climate change over wide temporal scales ranging from a few to thousands of years. Since the radiative effects and atmospheric lifetime of carbon dioxide (CO₂, warming) and sulfur dioxide (SO₂, cooling) are very different, the conventional assumption has been to analyze the effects of CO₂and SO₂emissions separately and add them together afterward. In this study, we test this assumption by analyzing the joint effect of CO₂and SO₂on the marine carbonate cycle using a biogeochemical carbon cycle box model (Long‐term Ocean‐atmosphere‐Sediment CArbon cycle Reservoir Model). By performing model runs with very fine temporal resolution (∼0.1‐year timestep), we analyze the effects of LIP carbon and sulfur gas emissions on timescales ranging from an individual eruption (hundreds to thousands of years) to the entire long‐term carbon cycle (>100,000 years). We find that, contrary to previous work, sulfur emissions have significant long‐term (>1,000 years) effects on the marine carbon cycle (dissolved inorganic carbon, pH, alkalinity, and carbonate compensation depth). This is due to two processes: the strongly temperature‐dependent equilibrium coefficients for marine carbonate chemistry and the few thousand‐year timescale for ocean overturning circulation. Thus, the effects of volcanic sulfur are not simply additive to the impact of carbon emissions. We develop a causal mechanistic framework to visualize the feedbacks associated with combined carbon and sulfur emissions and the associated timescales. Our results provide a new perspective for understanding the complex feedback mechanisms controlling the environmental effects of large volcanic eruptions over Earth history. 

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5. Making the case for an International Decade of Radiocarbon

   https://doi.org/10.1098/rsta.2023.0081  

   Eglinton, Timothy I.; Graven, Heather D.; Raymond, Peter A.; Trumbore, Susan E.; Aluwihare, Lihini; Bard, Edouard; Basu, Sourish; Friedlingstein, Pierre; Hammer, Samuel; Lester, Joanna; et al (November 2023, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   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. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'. 

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