NSFGEO-NERC: Quantifying the Modern and Glacial Ocean's Carbon Cycle Including Isotopes

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences

Abstract

Data-constrained process-based models of the modern and glacial ocean's carbon cycle will be developed and analyzed using a novel method. The method decomposes Dissolved Inorganic Carbon (DIC = Cpre + Creg) accurately into preformed (Cpre = Csat + Cdis) and regenerated (Creg = Corg + Ccaco3) components, where Csat = Csat,phy + Csat,bio is the equilibrium saturation and Cdis = Cdis,phy + Cdis,bio the disequilibrium, each with physical and biological contributions, and Csoft and Ccaco3 are organic (soft tissue) and calcium carbonate (hard tissue) components. DIC = Cphy + Cbio can thus be separated into physical Cphy = Csat,phy + Cdis,phy and biological Cbio = Csat,bio + Cdis,bio + Csoft + Ccaco3 parts. Perturbation experiments will be used to attribute the change of each component, DIC and atmospheric CO2 to changes in individual variables (circulation, sea ice, temperature, sea level and iron fluxes). Different viable equilibrium states will be produced for the modern and glacial ocean incorporating recent innovations in ocean physics, such as different mixing parameterizations and ventilation diagnostics, and in biogeochemistry, such as variable elemental (C:P) stoichiometry, dissolved iron fluxes, sediment interactions, cycling of Pa/Th, and land carbon changes. This approach will allow quantitative, process-based understanding of glacial-interglacial changes in ocean carbon storage including uncertainty estimates. It will also elucidate the response of carbon components to circulation changes. The decomposition will be extended to carbon isotopes (d13CDIC).

Planned Impact

Previous decompositions of the ocean's carbon cycle were incomplete and plagued by the use of simplifications such as the AOU-based approximation of Csoft that can lead to large errors. The accurate and comprehensive method proposed here has the potential to set a new, transformative standard for ocean carbon cycle analysis and aid future interpretations of ocean carbon models. Significant results will be disseminated through peer-reviewed publications and presentations at scientific conferences. Results of potential interest to the greater public will be disseminated to the press by working with our institution's press offices. The continuing development of computational tools such as the Transport Matrix Method and the Model of Ocean Biogeochemistry and Isotopes will benefit the scientific community. Both are freely available through an open source distribution site and used by a large international and interdisciplinary community. This project will support training of a graduate student in oceanography, ocean biogeochemical and physical modeling. The student will have the opportunity to gain teaching experience at Oregon State University. In partnership with OSU's Science & Math Investigative Learning Experiences (SMILE) program the project will support more than 25 afterschool Science Technology Engineering and Mathematics (STEM) clubs in rural Oregon middle and high schools designed to facilitate the pathway for underrepresented students towards higher education and careers in STEM fields. Specifically, we will work with the teachers to develop and implement an ocean carbon cycle and climate science curriculum into their program. This will foster diversity and climate literacy by advance STEM education for communities that need it most.

Publications

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Description Paleoceanographic proxies archived in sediments suggest that the the deep ocean was deoxygenated during glacial times. This has been used to support the hypothesis that a lower atmospheric carbon dioxide was due to an increase in the strength of the ocean's biological pump during that period. This relies on the assumption that surface ocean oxygen (O2) is equilibrated with the atmosphere such that any O2 deficiency observed in deep waters is a result of organic matter respiration, which consumes O2 and produces dissolved inorganic carbon. However, this assumption has been shown to be imperfect because of disequilibrium. We recently used an Earth system model tuned to a suite of observations, which reproduces the pattern of glacial-to-Holocene oxygenation change seen in proxy data, to show that disequilibrium plays an important role in glacial deep ocean deoxygenation. Using a novel decomposition method to track O2, we found a whole-ocean loss of 33 Pmol O2 from the preindustrial to the Last Glacial Maximum despite a 27 Pmol gain from the increased solubility due to cooler temperatures. This loss was driven by a biologically mediated O2 disequilibrium, which contributed 10% of the reduction of the O2 inventory from the solubility equilibrium in the preindustrial compared with 27% during the Last Glacial Maximum. Sea ice and iron fertilization were found to be the largest contributors to the Last Glacial Maximum deoxygenation, which occurs despite overall reduced production and respiration of organic matter in the glacial ocean. Our results challenge the notion that deep ocean glacial deoxygenation was caused by a stronger biological pump or more sluggish circulation, and instead highlight the importance and previously underappreciated role of O2 disequilibrium.

A second important result is a new computational method to accelerate the spin-up to equilibrium of radiocarbon and other geochemical tracers. These have provided great insight into the workings of the ocean but are prohibitively expensive to simulate in climate models. This project has led to the development of a new computational method and software that can be applied to any model to greatly speed-up simulations of such tracers, enabling their routine inclusion in climate models and thus more effective use of those tracers.
Exploitation Route Results might inform assessments of climate change (e.g., deoxygenation) and policymaking. The new fast spin-up method and associated (freely available) software will greatly benefit the ocean and climate modeling community.
Sectors Environment

 
Title Fast spin-up of Earth System Models (algorithm and software) 
Description The ocean and land carbon cycles plays a critical role in the climate system and are key components of the Earth System Models (ESMs) used to project future changes in the environment. However, their slow adjustment time also hinders effective use of ESMs because of the enormous computational resources required to integrate them to a pre-industrial quasi-equilibrium, a prerequisite for performing any simulations with these models and, in particular, identifying the human impact on climate. A novel solution to this ``spin-up'' problem, regarded as one of the grand challenges in climate science, has been developed and shown to accelerate the equilibration of state-of-the-art marine biogeochemical models typical of those embedded in ESMs by over an order of magnitude. Based on a ``sequence acceleration'' method originally developed in the context of electronic structure problems, the new technique can be applied in a ``black box'' fashion to any existing model. Even under the challenging protocols used to spin-up ESMs for the IPCC Coupled Model Intercomparison Project, which can take up to two years on even some of the most powerful supercomputers, the new algorithm can reduce simulation times by a factor of 5, with preliminary results suggesting that complex land surface models can be similarly accelerated. The ability to efficiently spin-up ESMs would enable for the first time a quantification of major parametric uncertainties in these models, lead to more accurate estimates of metrics such as climate sensitivity, and allow increased model resolution beyond what is currently feasible. 
Type Of Material Computer model/algorithm 
Year Produced 2023 
Provided To Others? Yes  
Impact The new algorithm could dramatically reduce spin-up times (and associated resource and energy costs) of Earth System Models and enable new science. I'm working with the UK Met Office and other climate modeling groups to implement this algorithm in their Earth System Models. 
 
Title New method to estimate biologically respired CO2 in the ocean 
Description A new method that used measurements of noble gases to yield improved estimates of biologically respired CO2 in the ocean 
Type Of Material Data analysis technique 
Year Produced 2021 
Provided To Others? Yes  
Impact Paper in Geophys. Res. Lett., Cassar et al. (2021) 
 
Description New collaboration with Prof. Nicolas Cassar at Duke University, USA 
Organisation Duke University
Country United States 
Sector Academic/University 
PI Contribution We initiated a new line of inquiry applying some of the results and methods developed in this grant to the problem of estimating the strength of the ocean biological pump in the modern ocean. The work was published in a paper in Geophys. Res. Lett. (Cassar et al.).
Collaborator Contribution We initiated a new line of inquiry applying some of the results and methods developed in this grant to the problem of estimating the strength of the ocean biological pump in the modern ocean. The work was published in a paper in Geophys. Res. Lett. (Cassar et al.).
Impact Paper in Geophys. Res. Lett.: Cassar et al. (2021)
Start Year 2020