The air-sea transfer of carbon dioxide can be viewed as a dynamical system through which atmospheric and oceanic processes push surface waters away from thermodynamic equilibrium, while diffusive gas transfer pulls them back towards local equilibrium. These push/pull processes drive significant sub-seasonal, seasonal, and interannual variability in air-sea carbon fluxes, the quantification of which is critical both for diagnosing the ocean response to fossil fuel emissions and for attempts to mitigate anthropogenic climate disruption through intentional modification of surface ocean biogeochemistry. In this study, we present a new approach for attributing air-sea carbon fluxes to specific mechanisms. The new framework is first applied to the two-box ocean nutrient and carbon cycle model as an illustrative example. Next, the outputs from a regional eddy-resolving model of the Southern Ocean are analyzed. The roles of multiple physical and biogeochemical processes are identified. Decomposition of the seasonal air-sea carbon flux shows the dominant role of biological carbon pumps that are partially compensated by the transport convergence. Finally, the framework is used to diagnose the response to mesoscale iron and alkalinity release, explicitly quantifying transport feedbacks and eventual impacts on net air-sea carbon flux. Ocean carbon transport have divergent influences between iron and alkalinity release, due to opposing near-surface gradients of dissolved inorganic carbon. More broadly, we suggest that our attribution framework may be a useful analytical technique for monitoring natural ocean carbon fluxes and quantifying the impacts of human intervention on the ocean carbon cycle.

A document by Yoshiki Kanzaki. Click on the document to view its contents.

Significant interest and capital are currently being channeled into techniques for durable carbon dioxide removal (CDR) from Earth’s atmosphere. A particular class of these approaches — referred to as enhanced weathering (EW) — seeks to modify the surface alkalinity budget to store CO2 as dissolved inorganic carbon species. Here, we use SCEPTER — a reaction-transport code designed to simulate EW in managed lands — to evaluate the throughput and storage timescales of anthropogenic alkalinity in agricultural soils. Through a series of alkalinity flux simulations, we explore the main controls on cation storage and export from surface soils in key U.S. agricultural regions. We find that lag times between alkalinity modification and climate-relevant CDR can span anywhere from years to many decades locally but can aggregate to shorter timescales depending on deployment region. Background soil cation exchange capacity, agronomic target pH, and fluid infiltration all impact the timescales of CDR relative to the timing of alkalinity input. There is likely scope for optimization of weathering-driven alkalinity transport through variation in land management practice. However, shifting management practices to reduce lag times will likely decrease total CDR from weathering and lead to non-optimal nutrient use efficiencies and soil nitrous oxide (N2O) fluxes. Although CDR lag times will be more of an issue in some regions than others, these results have significant implications for the technoeconomics of EW and the integration of EW into voluntary carbon markets, as there may often be a large temporal disconnect between deployment of EW and climate-relevant CDR.

The global-scale oxygenation of Earth’s surface represents one of the most fundamental chemical transformations in our planet’s history. There is empirical and theoretical evidence for at least two distinct and stable regimes of Earth surface oxygenation—a ‘low-O2 world’ characterized by pervasively reducing deep ocean waters, and a ‘high-O2 world’ with dominantly well-oxygenated deep ocean waters represented by our modern surface environment. Numerous biogeochemical processes and feedbacks control the redox state of the marine system, particularly when considered globally and on geologic timescales. It has therefore proven challenging to provide quantitative and internally consistent estimates of the atmospheric oxygen levels (and thereby, productivity, nutrient availability, and reductant consumption) necessary to oxygenate the deep seas. Here, we leverage an Earth-system biogeochemical model that tracks the carbon, nitrogen, oxygen, phosphorus, and sulfur cycles (CANOPS) to provide new quantitative constraints on this relationship. We explore ocean biogeochemistry and fluxes of reduced carbon to the seafloor across a wide range of atmospheric oxygen levels from 0.01 – 100% of the present atmospheric level (PAL), and implement a stochastic approach to provide formal estimates of uncertainty on our results. We find that deep ocean waters remain largely reducing, and ocean productivity remains significantly muted relative to the modern marine biosphere, until pO2 levels reach ~40% PAL. These results have major implications for quantitative constraints on atmospheric pO2 levels during the latest Proterozoic and Paleozoic, both in terms of environmental habitability for early animals and with respect to potential energetic constraints on growing and diversifying benthic communities.