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Abstract Anthropogenic perturbations from fossil fuel burning, nuclear bomb testing, and chlorofluorocarbon (CFC) use have created useful transient tracers of ocean circulation. The atmospheric¹⁴C/C ratio (∆¹⁴C) peaked in the early 1960s and has decreased now to pre‐industrial levels, while atmospheric CFC‐11 and CFC‐12 concentrations peaked in the early 1990s and early 2000s, respectively, and have now decreased by 10%–20%. We present the first analysis of a decade of new observations (2007 to 2018–2019) and give a comprehensive overview of the changes in ocean ∆¹⁴C and CFC concentration since the WOCE surveys in the 1990s. Surface ocean ∆¹⁴C decreased at a nearly constant rate from the 1990–2010s (20‰/decade). In most of the surface ocean ∆¹⁴C is higher than in atmospheric CO₂while in the interior ocean, only a few places are found to have increases in ∆¹⁴C, indicating that globally, oceanic bomb¹⁴C uptake has stopped and reversed. Decreases in surface ocean CFC‐11 started between the 1990 and 2000s, and CFC‐12 between the 2000–2010s. Strong coherence in model biases of decadal changes in all tracers in the Southern Ocean suggest ventilation of Antarctic Intermediate Water was enhanced from the 1990 to the 2000s, whereas ventilation of Subantarctic Mode Water was enhanced from the 2000 to the 2010s. The decrease in surface tracers globally between the 2000 and 2010s is consistently stronger in observations than in models, indicating a reduction in vertical transport and mixing due to stratification. 

Changes in the Atlantic Meridional Overturning Circulation (AMOC) are believed to have affected the cycling of carbon isotopes in both the ocean and the atmosphere. However, understanding how AMOC changes of Dissolved Inorganic Carbon (DIC) distributions in the ocean is limited, since models do not typically decompose the various processes that affect . Here, a new decomposition is applied to idealized simulations of an AMOC collapse, both for glacial and preindustrial conditions. The decomposition explicitly calculates the preformed and regenerated components of and separates between physical and biological effects. An AMOC collapse leads to a large and rapid decrease in in the North Atlantic, which is due to, in about equal parts, accumulation of remineralized organic matter and changes in preformed , both in glacial and preindustrial simulations. In the Pacific, Indian, and Southern Oceans increases by a smaller magnitude. This increase is dominated by changes in preformed in the glacial simulation and remineralized in the preindustrial simulation. An extensive evaluation of the decomposition shows that its errors are small in most cases, especially for large basin‐wide changes, whereas for small, local or global changes errors can be substantial. In contrast, approximations of the remineralized component based on Apparent Oxygen Utilization have large errors in most cases and are generally unreliable because they include contributions from oxygen disequilibrium. 

The Atlantic Meridional Overturning Circulation (AMOC) impacts temperatures, ecosystems, and the carbon cycle. However, AMOC effects on Earth's carbon cycle remains poorly understood, in part because contributions of different physical and biological mechanisms that impact carbon storage in the ocean are not typically diagnosed in climate models. Here, we explore modeled effects of AMOC shutdowns on ocean Dissolved Inorganic Carbon (DIC) by applying a new decomposition that explicitly calculates preformed and regenerated DIC components and separates physical and biological contributions. An extensive evaluation in transient simulations finds that the method is accurate, especially for basin‐wide changes, whereas errors can be significant at global and local scales. In contrast, estimates of respired carbon based on Apparent Oxygen Utilization lead to large errors and are generally not reliable. In response to a shutdown of the AMOC under Last Glacial Maximum (LGM) background climate, ocean carbon increases and then decreases, leading to opposite changes in atmospheric carbon dioxide (CO2). DIC changes are dominated by opposing changes in biological carbon storage. Whereas regenerated components increase in the Atlantic and dominate the initial increase in global ocean DIC until model year 1000, preformed components decrease in the other ocean basins and dominate the long‐term DIC decrease until year 4000. Biological disequilibrium is an important contribution to preformed carbon changes. Biological saturation carbon decreases in the Pacific, Indian, and Southern Oceans due to a decrease in surface alkalinity. The spatial patterns of the DIC components and their changes in response to an AMOC collapse are presented. 

his directory contains model code, input, output, and scripts from a hosing (freshwater forcing in the North Atlantic) simulation with the OSU-UVic climate model (version 2.9.10) to investigate the effect of changes in the Atlantic Meridional Overturning Circulation (AMOC) on carbon and carbon-13 components in the ocean as described in Schmittner and Boling (2025) and Schmittner (2025). Model code is in the code/ subdirectory. Model input data is in the data/ subdirectory and in the control.in and mk.in files. Model output data is in the tavg*nc and tsi*nc files. Ferret scripts used to produce the figures are in the ferret/ subdirectory. Andreas Schmittner (andreas.schmittner@oregonstate.edu) References: Schmittner, A. and M. Boling (2025) Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon Components in the Ocean, Global Biogeochemical Cycles, 39, e2025GB008526 doi: 10.1029/2025GB008526. Schmittner, A. (2025) Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon-13 Components in the Ocean, Global Biogeochemical Cycles, 39, e2025GB008527 doi: 10.1029/2025GB008527. 

This directory contains model code, input, output, and scripts from a hosing (freshwater forcing in the North Atlantic) simulation with the OSU-UVic climate model (version 2.9.10) to investigate the effect of changes in the Atlantic Meridional Overturning Circulation (AMOC) on carbon and carbon-13 components in the ocean as described in Schmittner and Boling (2025) and Schmittner (2025). Model code is in the code/ subdirectory. Model input data is in the data/ subdirectory and in the control.in and mk.in files. Model output data is in the tavg*nc and tsi*nc files. Ferret scripts used to produce the figures are in the ferret/ subdirectory. A more detailed description about the OSU-UVic climate model is available at https://github.com/OSU-CEOAS-Schmittner/UVic2.9 and https://doi.org/10.5281/zenodo.11224826. Andreas Schmittner (andreas.schmittner@oregonstate.edu) References: Schmittner, A. and M. Boling (2025) Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon Components in the Ocean, Global Biogeochemical Cycles, 39, e2025GB008526 doi: 10.1029/2025GB008526. Schmittner, A. (2025) Impact of Atlantic Meridional Overturning Circulation Collapse on Carbon-13 Components in the Ocean, Global Biogeochemical Cycles, 39, e2025GB008527 doi: 10.1029/2025GB008527. 

The performance of global ocean biogeochemical models can be quantified as the misfit between modeled tracer distributions and observations, which is sought to be minimized during parameter optimization. These models are computationally expensive due to the long spin‐up time required to reach equilibrium, and therefore optimization is often laborious. To reduce the required computational time, we investigate whether optimization of a biogeochemical model with shorter spin‐ups provides the same optimized parameters as one with a full‐length, equilibrated spin‐up over several millennia. We use the global ocean biogeochemical model MOPS with a range of lengths of model spin‐up and calibrate the model against synthetic observations derived from previous model runs using a derivative‐free optimization algorithm (DFO‐LS). When initiating the biogeochemical model with tracer distributions that differ from the synthetic observations used for calibration, a minimum spin‐up length of 2,000 years was required for successful optimization due to certain parameters which influence the transport of matter from the surface to the deeper ocean, where timescales are longer. However, preliminary results indicate that successful optimization may occur with an even shorter spin‐up by a judicious choice of initial condition, here the synthetic observations used for calibration, suggesting a fruitful avenue for future research. 

Despite their importance for Earth’s climate and paleoceanography, the cycles of carbon (C) and its isotope¹³C in the ocean are not well understood. Models typically do not decompose C and¹³C storage caused by different physical, biological, and chemical processes, which makes interpreting results difficult. Consequently, basic observed features, such as the decreased carbon isotopic signature (δ¹³C_DIC) of the glacial ocean remain unexplained. Here, we review recent progress in decomposing Dissolved Inorganic Carbon (DIC) into preformed and regenerated components, extend a precise and complete decomposition to δ¹³C_DIC, and apply it to data-constrained model simulations of the Preindustrial (PI) and Last Glacial Maximum (LGM) oceans. Regenerated components, from respired soft-tissue organic matter and dissolved biogenic calcium carbonate, are reduced in the LGM, indicating a decrease in the active part of the biological pump. Preformed components increase carbon storage and decrease δ¹³C_DICby 0.55 ‰ in the LGM. We separate preformed into saturation and disequilibrium components, each of which have biological and physical contributions. Whereas the physical disequilibrium in the PI is negative for both DIC and δ¹³C_DIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ¹³C_DIC, a pattern that is magnified in the LGM. The biological disequilibrium is the dominant driver of the increase in glacial ocean C and the decrease in δ¹³C_DIC, indicating a reduced sink of biological carbon. Overall, in the LGM, biological processes increase the ocean’s DIC inventory by 355 Pg more than in the PI, reduce its mean δ¹³C_DICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO₂. Spatial distributions of the δ¹³C_DICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors. 

Marine and terrestrial biogeochemical models are key components of the Earth System Models (ESMs) used to project future environmental changes. 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 equilibrium. Here, a solution to this spin-up problem based on sequence acceleration, is shown to accelerate equilibration of state-of-the-art marine biogeochemical models by over an order of magnitude. The technique can be applied in a black box fashion to existing models. Even under the challenging spin-up protocols used for Intergovernmental Panel on Climate Change (IPCC) simulations, this algorithm is 5 times faster. Preliminary results suggest that terrestrial models can be similarly accelerated, enabling a quantification of major parametric uncertainties in ESMs, improved estimates of metrics such as climate sensitivity, and higher model resolution than currently feasible. 

OSU-UVic Climate Model of Intermediate Complexity with Model of Ocean Biogeochemistry and Isotopes (MOBI2.2). New features in this release include Nathaniel Fillman's carbon and C13 decomposition code and Samar Khatiwala's Pa/Th code. 

Input data required for a simulation of the Last Glacial Maximum (LGM) simulation with the OSU version of the University of Victoria climate model (version 2.9.10) with the Model of Ocean Biogeochemistry and Isotopes (MOBI2.2).