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1. CM44B-1175 Using Carbon Isotopes and Regional Ocean Modeling to Trace the Impact of Ocean Alkalinity Enhancement in the California Current System

   Green, Ryan A; Rafter, Patrick A; Edwards, Christopher A; Fiechter, Jerome; Hain, Mathis (February 2024, Transactions American Geophysical Union)

   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 

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2. Monitoring, reporting, and verification for ocean alkalinity enhancement

   https://doi.org/10.5194/sp-2-oae2023-12-2023  

   Ho, David T; Bopp, Laurent; Palter, Jaime B; Long, Matthew C; Boyd, Philip W; Neukermans, Griet; Bach, Lennart T (January 2023, State of the Planet)

   Oschlies, Andreas (Ed.)

   Abstract. Monitoring, reporting, and verification (MRV) refers to the multistep process of monitoring the amount of greenhouse gas removed by a carbon dioxide removal (CDR) activity and reporting the results of the monitoring to a third party. The third party then verifies the reporting of the results. While MRV is usually conducted in pursuit of certification in a voluntary or regulated CDR market, this chapter focuses on key recommendations for MRV relevant to ocean alkalinity enhancement (OAE) research. Early stage MRV for OAE research may become the foundation on which markets are built. Therefore, such research carries a special obligation toward comprehensiveness, reproducibility, and transparency. Observational approaches during field trials should aim to quantify the delivery of alkalinity to seawater and monitor for secondary precipitation, biotic calcification, and other ecosystem changes that can feed back on sources or sinks of greenhouse gases where alkalinity is measurably elevated. Observations of resultant shifts in the partial pressure of CO2 (pCO2) and ocean pH can help determine the efficacy of OAE and are amenable to autonomous monitoring. However, because the ocean is turbulent and energetic and CO2 equilibration between the ocean and atmosphere can take several months or longer, added alkalinity will be diluted to perturbation levels undetectable above background variability on timescales relevant for MRV. Therefore, comprehensive quantification of carbon removal via OAE will be impossible through observational methods alone, and numerical simulations will be required. The development of fit-for-purpose models, carefully validated against observational data, will be a critical part of MRV for OAE. 

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3. A decade of marine inorganic carbon chemistry observations in the northern Gulf of Alaska – insights into an environment in transition

   https://doi.org/10.5194/essd-16-647-2024  

   Monacci, Natalie M; Cross, Jessica N; Evans, Wiley; Mathis, Jeremy T; Wang, Hongjie (January 2024, Earth System Science Data)

   Abstract. As elsewhere in the global ocean, the Gulf of Alaska is experiencing the rapid onset of ocean acidification (OA) driven by oceanic absorption of anthropogenic emissions of carbon dioxide from the atmosphere. In support of OA research and monitoring, we present here a data product of marine inorganic carbon chemistry parameters measured from seawater samples taken during biannual cruises between 2008 and 2017 in the northern Gulf of Alaska. Samples were collected each May and September over the 10 year period using a conductivity, temperature, depth (CTD) profiler coupled with a Niskin bottle rosette at stations including a long-term hydrographic survey transect known as the Gulf of Alaska (GAK) Line. This dataset includes discrete seawater measurements such as dissolved inorganic carbon and total alkalinity, which allows the calculation of other marine carbon parameters, including carbonate mineral saturation states, carbon dioxide (CO2), and pH. Cumulative daily Bakun upwelling indices illustrate the pattern of downwelling in the northern Gulf of Alaska, with a period of relaxation spanning between the May and September cruises. The observed time and space variability impart challenges for disentangling the OA signal despite this dataset spanning a decade. However, this data product greatly enhances our understanding of seasonal and interannual variability in the marine inorganic carbon system parameters. The product can also aid in the ground truthing of biogeochemical models, refining estimates of sea–air CO2 exchange, and determining appropriate CO2 parameter ranges for experiments targeting potentially vulnerable species. Data are available at https://doi.org/10.25921/x9sg-9b08 (Monacci et al., 2023). 

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4. A Numerical reassessment of the Gulf of Mexico carbon system in connection with the Mississippi River and global ocean

   https://doi.org/10.5194/bg-19-4589-2022  

   Zhang, Le; Xue, Z. George (January 2022, Biogeosciences)

   Abstract. Coupled physical–biogeochemical models can fill thespatial and temporal gap in ocean carbon observations. Challenges ofapplying a coupled physical–biogeochemical model in the regional oceaninclude the reasonable prescription of carbon model boundary conditions,lack of in situ observations, and the oversimplification of certainbiogeochemical processes. In this study, we applied a coupledphysical–biogeochemical model (Regional Ocean Modelling System, ROMS) to theGulf of Mexico (GoM) and achieved an unprecedented 20-year high-resolution(5 km, 1/22∘) hindcast covering the period of 2000 to 2019. Thebiogeochemical model incorporated the dynamics of dissolved organic carbon(DOC) pools and the formation and dissolution of carbonate minerals. Thebiogeochemical boundaries were interpolated from NCAR's CESM2-WACCM-FV2solution after evaluating the performance of 17 GCMs in the GoM waters. Modeloutputs included carbon system variables of wide interest, such aspCO2, pH, aragonite saturation state (ΩArag), calcitesaturation state (ΩCalc), CO2 air–sea flux, and carbon burialrate. The model's robustness is evaluated via extensive model–datacomparison against buoys, remote-sensing-based machine learning (ML)products, and ship-based measurements. A reassessment of air–sea CO2flux with previous modeling and observational studies gives us confidencethat our model provides a robust and updated CO2 flux estimation, andNGoM is a stronger carbon sink than previously reported. Model resultsreveal that the GoM water has been experiencing a ∼ 0.0016 yr−1 decrease in surface pH over the past 2 decades, accompanied by a∼ 1.66 µatm yr−1 increase in sea surfacepCO2. The air–sea CO2 exchange estimation confirms in accordance with severalprevious models and ocean surface pCO2 observations that theriver-dominated northern GoM (NGoM) is a substantial carbon sink, and theopen GoM is a carbon source during summer and a carbon sink for the rest ofthe year. Sensitivity experiments are conducted to evaluate the impacts ofriver inputs and the global ocean via model boundaries. The NGoM carbonsystem is directly modified by the enormous carbon inputs (∼ 15.5 Tg C yr−1 DIC and ∼ 2.3 Tg C yr−1 DOC) from theMississippi–Atchafalaya River System (MARS). Additionally,nutrient-stimulated biological activities create a ∼ 105 timeshigher particulate organic matter burial rate in NGoM sediment than in thecase without river-delivered nutrients. The carbon system condition of theopen ocean is driven by inputs from the Caribbean Sea via the Yucatan Channeland is affected more by thermal effects than biological factors. 

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5. Skillful multiyear predictions of ocean acidification in the California Current System

   https://doi.org/10.1038/s41467-020-15722-x  

   Brady, Riley X.; Lovenduski, Nicole S.; Yeager, Stephen G.; Long, Matthew C.; Lindsay, Keith (December 2020, Nature Communications)

   null (Ed.)

   Abstract The California Current System (CCS) sustains economically valuable fisheries and is particularly vulnerable to ocean acidification, due to its natural upwelling of carbon-enriched waters that generate corrosive conditions for local ecosystems. Here we use a novel suite of retrospective, initialized ensemble forecasts with an Earth system model (ESM) to predict the evolution of surface pH anomalies in the CCS. We show that the forecast system skillfully predicts observed surface pH variations a year in advance over a naive forecasting method, with the potential for skillful prediction up to five years in advance. Skillful predictions of surface pH are mainly derived from the initialization of dissolved inorganic carbon anomalies that are subsequently transported into the CCS. Our results demonstrate the potential for ESMs to provide skillful predictions of ocean acidification on large scales in the CCS. Initialized ESMs could also provide boundary conditions to improve high-resolution regional forecasting systems. 

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