Changes in the Atlantic Meridional Overturning Circulation (AMOC) are believed to have affected the cycling of carbon isotopes (δ13C) in both the ocean and the atmosphere. However, understanding how AMOC changes δ13CDIC of Dissolved Inorganic Carbon (DIC) distributions in the ocean is limited, since models do not typically decompose the various processes that affect δ13CDIC. 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 δ13CDIC and separates between physical and biological effects. An AMOC collapse leads to a large and rapid decrease in δ13CDIC in the North Atlantic, which is due to, in about equal parts, accumulation of remineralized organic matter and changes in preformed δ13CDIC, both in glacial and preindustrial simulations. In the Pacific, Indian, and Southern Oceans δ13CDIC increases by a smaller magnitude. This increase is dominated by changes in preformed δ13CDIC in the glacial simulation and remineralized δ13CDIC 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.

jabbrv-ltwa-all.ldf jabbrv-ltwa-en.ldf Previous attempts to explain the distribution of Neodymium (Nd) and its isotopes in the ocean, a tracer for past global Meridional Overturning Circulation (MOC) changes, primarily focused on a top-down approach using reversible scavenging. This approach assumes that Nd is supplied to the ocean through terrestrial sources at the surface, and reversible particulate scavenging and sinking of Nd on particles explains observations of linearly increasing dissolved Nd concentrations with depth. Here, we explore a conceptually different model: the bottom-up hypothesis and irreversible scavenging. A benthic flux (BF) is assumed to be the sole source of marine Nd at the seafloor; vertical diffusion leads to upward transport and adsorption onto particles, with a constant adsorption coefficient (kp) as the only loss term. This model is explored in a simple one-dimensional (vertically resolved) version and a three-dimensional global general circulation numerical model. Results indicate that the observed distribution of Nd in the ocean is well reproduced globally using a constant BF of 20 pmol cm-2 yr -1 and irreversible scavenging. Our results suggest that bathymetry and lateral transport play important roles in determining Nd water column distribution in the pelagic ocean.

The Atlantic Meridional Overturning Circulation (AMOC) impacts temperatures, ecosystems, and the carbon cycle. However, how AMOC affects the carbon cycle remains poorly understood, in part because the respective 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 an AMOC shutdown on Dissolved Inorganic Carbon (DIC) in the ocean by applying a new decomposition that explicitly calculates preformed and regenerated DIC components and separates physical and biological contributions. An extensive evaluation of the method in transient simulations finds that, in most cases, it is accurate and reliable, 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, ocean carbon increases and then decreases, leading to opposite changes in atmospheric 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, preformed components decrease in the other ocean basins and dominate the long-term DIC decrease. 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.

Realistic model representation of ocean phytoplankton is important for simulating nutrient cycles and the biological carbon pump, which affects atmospheric carbon dioxide (pCO2) concentrations and, thus, climate. Until recently, most models assumed constant ratios (or stoichiometry) of phosphorous (P), nitrogen (N), silicon (Si), and carbon (C) in phytoplankton, despite observations indicating systematic variations. Here, we investigate the effects of variable stoichiometry on simulated nutrient distributions, plankton community compositions, and the C cycle in the preindustrial (PI) and glacial oceans. Using a biogeochemical model, a linearly increasing P:N relation to increasing PO4 is implemented for ordinary phytoplankton (PO), and a nonlinearly decreasing Si:N relation to increasing Fe is applied to diatoms (PDiat). C:N remains fixed. Variable P:N affects modeled community composition through enhanced PO4 availability, which increases N-fixers in the oligotrophic ocean, consistent with previous research. This increases the NO3 fertilization of PO, the NO3 inventory, and the total plankton biomass. Surface nutrients are not significantly altered. Conversely, variable Si:N shifts south the Southern Ocean’s meridional surface silicate gradient, which aligns better with observations, but depresses PDiat growth globally. In Last Glacial Maximum simulations, PO respond to more oligotrophic conditions by increasing their C:P. This strengthens the biologically mediated C storage such that dissolved organic (inorganic) C inventories increase by 34-40 (38-50) Pg C and 0.7-1.2 Pg yr-1 more particulate C is exported into the interior ocean. Thus, an additional 13-14 ppm of pCO2 difference from PI levels results, improving model agreement with glacial observations.

Iron is a key micronutrient controlling phytoplankton growth in vast regions of the global ocean. Despite its importance, uncertainties remain high regarding external iron source fluxes and internal cycling on a global scale. In this study, we used a global dissolved iron dataset, including GEOTRACES measurements, to constrain source and scavenging fluxes in the marine iron component of a global ocean biogeochemical model. Our model simulations tested three key uncertainties: source inputs of atmospheric soluble iron deposition (varying from 1.4–3.4 Gmol/yr), reductive sedimentary iron release (14–117 Gmol/yr), and compared a variable ligand parameterization to a constant distribution. In each simulation, scavenging rates were tuned to reproduce the observed global mean iron inventory for consistency. The variable ligand parameterization improved the global model-data misfit the most, suggesting that heterotrophic bacteria are an important source of ligands to the ocean. Model simulations containing high source fluxes of atmospheric soluble iron deposition (3.4 Gmol/yr) and reductive sedimentary iron release (114 Gmol/yr) further improved the model most notably in the surface ocean. High scavenging rates were then required to maintain the iron inventory resulting in relatively short surface and global ocean residence times of 0.83 and 7.5 years, respectively. The model simulates a tight spatial coupling between source inputs and scavenging rates, which may be too strong due to underrepresented ligands near source inputs, contributing to large uncertainties when constraining individual fluxes with dissolved iron concentrations. Model biases remain high and are discussed to help improve global marine iron cycle models.

The Atlantic meridional overturning circulation (AMOC) is a key component of the global climate system. Many models predict a weakening or even a collapse of the AMOC under future climate change. Recent studies suggested a 20th century weakening of the AMOC of unprecedented amplitude ( 15%) over the last millennium. Here, we present δ18O of benthic foraminifera in a sediment core from the Laurentian Channel and demonstrate that the δ18O trend is linked to the strength of the AMOC. In this 100-year record, the AMOC signal decrease steadily to reach its minimum value in the late 1970’s. The weakest AMOC signal is constant until 2000. We present a longer δ18O record of 1,500 years and highlight the uniqueness of these high δ18O values over that period. Moreover, the long record is also characterized by statistically heavier δ18O during the Little Ice age suggesting a relatively weak AMOC.