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Abstract All else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O₂) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO₂) concentrations of the ice ages. Vollmer et al. (2022,https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed¹³C/¹²C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO₂storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange. 

Abstract Measuring, reporting, and verification (MRV) of ocean-based carbon dioxide removal (CDR) presents challenges due to the dynamic nature of the ocean and the complex processes influencing marine carbonate chemistry. Given these challenges, finding the optimal sampling strategies and suite of parameters to be measured is a timely research question. While traditional carbonate parameters such as total alkalinity (TA), dissolved inorganic carbon (DIC), pH, and seawater pCO2 are commonly considered, exploring the potential of carbon isotopes for quantifying additional CO2 uptake remains a relatively unexplored research avenue. In this study, we use a coupled physical-biogeochemical model of the California Current System (CCS) to run a suite of Ocean Alkalinity Enhancement (OAE) simulations. The physical circulation for the CCS is generated using a nested implementation of the Regional Ocean Modeling System (ROMS) with an outer domain of 1/10 ̊ (~10 km) and an inner domain of 1/30 ̊ (~3 km) resolution. The biogeochemical model, NEMUCSC, is a customized version of the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO) that includes carbon cycling and carbon isotopes. The CCS is one of four global eastern boundary upwelling systems characterized by high biological activity and CO2 concentrations. Consequently, the CCS represents an essential test case for investigating the efficacy and potential side effects of OAE deployments. The study aims to address two key questions: (1) the relative merit of OAE to counter ocean acidification versus the additional sequestration of CO2 from the atmosphere, and (2) the footprint of potentially harmful seawater chemistry adjacent to OAE deployments. We plan to leverage these high-resolution model results to competitively evaluate different MRV strategies, with a specific focus on analyzing the spatiotemporal distribution of carbon isotopic signatures following OAE. In this talk, we will showcase our initial results and discuss challenges in integrating high-resolution regional modeling into models of the global carbon cycle. More broadly, this work aims to provide insights into the plausibility of OAE as a climate solution that maintains ocean health and to inform accurate quantification of carbon uptake for MRV purposes. https://agu.confex.com/agu/OSM24/prelim.cgi/Paper/1491096 

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. 

A compilation of radiocarbon measurements is used to characterize deep-sea overturning since the last ice age. 

Abstract Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state. 

Changes in the concentration and isotopic composition of the major constituents in seawater reflect changes in their sources and sinks. Because many of the processes controlling these sources and sinks are tied to the cycling of carbon, such records can provide insights into what drives past changes in atmospheric carbon dioxide and climate. Here, we present a stable strontium (Sr) isotope record derived from pelagic marine barite. Our δ^88/86Sr record exhibits a complex pattern, first declining between 35 and 15 million years ago (Ma), then increasing from 15 to 5 Ma, before declining again from ~5 Ma to the present. Numerical modeling reveals that the associated fluctuations in seawater Sr concentrations are about ±25% relative to present-day seawater. We interpret the δ^88/86Sr data as reflecting changes in the mineralogy and burial location of biogenic carbonates. 

Abstract Bulk sediment δ¹⁵N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ¹⁵N, then a deglacial δ¹⁵N maximum, followed by a gradual decline to a late Holocene δ¹⁵N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ¹⁵N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ¹⁵N) measurements for the two speciesNeogloboquadrina dutertreiandNeogloboquadrina incomptaover the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ¹⁵N. FB‐δ¹⁵N contrasts with bulk sediment δ¹⁵N by not indicating a lower δ¹⁵N during the LGM. Instead, the FB‐δ¹⁵N records are dominated by a deglacial δ¹⁵N maximum, with comparable LGM and Holocene values. The lower LGM δ¹⁵N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.