Mark Holzer

The Miocene Climatic Optimum (MCO; 16.9–14.7 Ma) was a warm interval with atmospheric comparable to end‐of‐21st‐century projections under intermediate emission scenarios. Proxy records indicate the Oxygen Minimum Zone (OMZ) in the Eastern Tropical Pacific (ETP) was small or non‐existent during the MCO, expanding only when declined after 15 Ma. Conversely, the modern ETP OMZ is expanding under anthropogenic warming. The mechanisms behind these opposing trends remain unclear. Here, we use the Geophysical Fluid Dynamics Laboratory coupled model (GFDL CM2.1) with the offline biogeochemistry PCO2 model to simulate Miocene ocean oxygen across multiple levels and two paleogeographic configurations. Increasing produces widespread oxygen loss at shallow and intermediate depths. One configuration develops pervasive hypoxia throughout the water column in the Atlantic and Arctic, driven by stagnation and stratification associated with a collapsed Atlantic Meridional Overturning Circulation (AMOC). In the same configuration, the ETP OMZ contracts at higher due to weaker trade winds, reduced upwelling, and water mass transport changes through the open Central American Seaway (CAS) associated with the shutdown of North Atlantic Deep Water and Tethys Intermediate Water formation. The same circulation reorganization responsible for Atlantic deoxygenation thus contributes to ETP oxygenation, highlighting the spatial complexity of the marine oxygen response to warming. These results indicate Miocene ETP oxygen dynamics are not a perfect analog to future changes: the boundary conditions permitting ETP OMZ contraction (including an open CAS and collapsed AMOC) are absent from the modern ocean, limiting direct extrapolation from proxy evidence.

We present measurements of silicon isotopes in silicic acid, δ ³⁰ Si, from GEOTRACES GP17 _OCE between 20°S and 60°S along 152°W. The section sampled key water masses, allowing assessment of controls on large‐scale δ ³⁰ Si features. Observational data paralleled results of a data‐constrained Si isotope model and are consistent with nutrient trapping and partial nutrient consumption in polar surface waters leading to the accumulation of light silicon isotopes in the deep Southern Ocean (SO) and transport of the residually heavy fraction northward within Sub‐Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). Deep‐water δ ³⁰ Si values were nearly constant along isopycnals in the South Pacific, with vertical gradients driven by an increasing fractional contribution of isotopically light regenerated silicic acid with depth. Isotope values in deep waters from the SO to Tahiti exhibited a greater range than previously observed in the South Pacific or predicted by models, with δ ³⁰ Si falling between 1.19‰ and 1.45‰. SAMW and AAIW exhibited relatively heavy, uniform δ ³⁰ Si. Model results suggest that these mode waters are composed of preformed silicic acid subducted in the outcropping region and regenerated silicic acid sourced in roughly equal measure from polar waters and northern distal sources. Distal input is mainly through entrainment of underlying Upper Circumpolar Deep Water into AAIW.

Investigating changes in ocean oxygenation during past warm climates advances our process understanding of biogeochemical and physical dynamics in the ocean and may inform our predictions of future changes. The Miocene Climatic Optimum (MCO) was a warm climate episode ~15, million years ago (Ma), with high atmospheric CO2 concentrations that are comparable to end-of-century predictions for mid-range future emission scenarios. Proxy records suggest that the Oxygen Minimum Zone (OMZ) in the Eastern Tropical Pacific (ETP) was small or non-existent during the high-CO2 MCO, and only expanded when CO2 declined after 15 Ma. In contrast, the OMZ in the Eastern Pacific was already extensive in the recent preindustrial era, and is currently expanding further with increasing CO2, due to ocean warming and stratification. Despite the importance of understanding the controls on Pacific OMZ extent under warm conditions, there are no existing model investigations of these opposing OMZ dynamics. Here, we use a climate model with an offline biogeochemical framework to investigate ocean oxygen concentrations during the Miocene for a range of CO2 concentrations and two different topographic configurations. We compare results to available physical and biogeochemical proxies and assess which combination of boundary conditions best replicates recorded proxy trends. We find that for higher CO2 concentrations, oxygen declines globally and OMZs expand, particularly in the Atlantic Ocean. However, for one of the topographic configurations, OMZs in the ETP contract under higher CO2 concentrations. This contraction can be attributed to regionally reduced export production and remineralization rates, which are caused by weaker upwelling due to a southward shifted Hadley cell and correspondingly weaker southern hemisphere trade winds. This atmospheric response is driven by hemispheric asymmetries in warming due to changes in large scale ocean circulation. These results emphasize the complexity and spatial heterogeneity of the marine oxygen response to climate change.

Silicon isotopes within silicic acid, δ ³⁰ Si, were measured on US GEOTRACES section GP15 from Alaska to Tahiti along 152°W. The distribution of silicic acid, Si(OH) ₄ , exhibited a double Si(OH) ₄ maximum dominated by a midwater maximum that extended from 55°N latitude to the equator together with a bottom maximum north of 40°N latitude. The midwater maximum is dominated by North Pacific Deep Water and is consistent with the existence of a poorly ventilated North Pacific Shadow Zone between 1,500 and 3,000 m with an ideal mean age in excess of 1,300 years where regenerated Si(OH) ₄ accumulates. A data‐constrained model of the marine silicon cycle is used to assess the contribution of the Southern Ocean, North Pacific, and central latitudes to the preformed and regenerated forms of both Si(OH) ₄ and δ ³⁰ Si along GP15. Regenerated Si(OH) ₄ from the North Pacific and Southern Ocean are the dominant inputs to the midwater maximum. Silicic acid within the midwater maximum becomes lighter toward the north with a δ ³⁰ Si minimum at about 50°N. The model indicates that this feature is shaped by ready access to successive fractionations in the productive high‐latitude oceans of both hemispheres. Silicon trapping within the North Pacific accounts for the upper part the δ ³⁰ Si minimum, while the Shadow Zone's interhemispheric connectivity with the Southern Ocean accounts for the deeper part. Isotope values in the bottom Si(OH) ₄ maximum are slightly elevated relative to adjacent waters consistent with a sediment source, although this isotopic constraint lies within statistical uncertainty.

Ocean ventilation, or the transfer of tracers from the surface boundary layer into the ocean interior, is a critical process in biogeochemical cycles and the climate system. Here, we assess steady‐state ventilation patterns and timescales in three models of ocean transport: a 1 global configuration of the Nucleus for European Modeling of the Ocean (NEMO), a recent 2 solution of the Ocean Circulation Inverse Model (OCIM), and a 2 solution of the Total Matrix Intercomparison (TMI). We release artificial dyes in six surface regions of each model and compare equilibrium dye distributions as well as ideal age distributions. We find good qualitative agreement in large‐scale dye distributions across the three models. However, the distributions indicate that TMI and OCIM are more diffusive than NEMO. A shallow bias of North Atlantic ventilation in NEMO contributes to a stronger presence of the North Atlantic dye in the mid‐depth Southern Ocean and Pacific. This isopycnal communication between the North Atlantic surface and the mid‐depth Pacific is very slow, however, and NEMO simulates a maximum age in the North Pacific (NP) about 900 years higher than the data‐constrained models. Overly slow NP ventilation persists across NEMO sensitivity experiments encompassing our current best knowledge of diapycnal and isopycnal mixing, pointing to biases in subarctic Pacific dynamics. This study provides a synoptic picture of deep ocean ventilation and a framework for assessing its representation in general circulation models.

Ocean ventilation, or the transfer of tracers from the surface boundary layer into the ocean interior, is a critical process in the climate system. Here, we assess steady-state ventilation patterns and rates in three models of ocean transports: a 1° global configuration of the Nucleus for European Modelling of the Ocean (NEMO), version 2 of the Ocean Circulation Inverse Model (OCIM), and the Total Matrix Intercomparison (TMI). We release artificial dyes in six surface regions of each model and compare equilibrium dye distributions as well as ideal age distributions. We find good qualitative agreement in large-scale dye distributions across the three models. However, the distributions indicate that TMI is more diffusive than OCIM, itself more diffusive than NEMO. NEMO simulates a sharp separation between bottom and intermediate water ventilation zones in the Southern Ocean, leading to a weaker influence of the latter zone on the abyssal ocean. A shallow bias of North Atlantic ventilation in NEMO contributes to a stronger presence of the North Atlantic dye in the mid-depth Southern Ocean and Pacific. This isopycnal communication between the North Atlantic surface and the mid-depth Pacific is very slow, however, and NEMO simulates a maximum age in the North Pacific about 900 years higher than the data-constrained models. Possible causes of this age bias are interrogated with NEMO sensitivity experiments. Implementation of an observation-based 3D map of isopycnal diffusivity augments the maximum age, due to weaker isopycnal diffusion at depths. We suggest that tracer upwelling in the subarctic Pacific is underestimated in NEMO and a key missing piece in the representation of global ocean ventilation in general circulation models.

Ocean ventilation, or the transfer of tracers from the surface boundary layer into the ocean interior, is a critical process in biogeochemical cycles and the climate system. Here, we assess steady-state ventilation patterns and timescales in three models of ocean transport: a 1º global configuration of the Nucleus for European Modelling of the Ocean (NEMO), a recent 2º solution of the Ocean Circulation Inverse Model (OCIM), and a 2º solution of the Total Matrix Intercomparison (TMI). We release artificial dyes in six surface regions of each model and compare equilibrium dye distributions as well as ideal age distributions. We find good qualitative agreement in large-scale dye distributions across the three models. However, the distributions indicate that TMI and OCIM are more diffusive than NEMO. A shallow bias of North Atlantic ventilation in NEMO contributes to a stronger presence of the North Atlantic dye in the mid-depth Southern Ocean and Pacific. This isopycnal communication between the North Atlantic surface and the mid-depth Pacific is very slow, however, and NEMO simulates a maximum age in the North Pacific about 900 years higher than the data-constrained models. Overly slow North Pacific ventilation persists across NEMO sensitivity experiments encompassing our current best knowledge of diapycnal and isopycnal mixing, pointing to biases in subarctic Pacific dynamics. This study provides a synoptic picture of global ocean ventilation and a framework for assessing its representation in general circulation models.

Ocean deoxygenation is an important consequence of climate change that poses an imminent threat to marine life and global food security. However, our understanding of the complex interactions between changes in circulation, solubility, and respiration that drive global‐scale deoxygenation is incomplete. Here, we consider idealized biogeochemical steady states in equilibrium with perpetually slower and warmer oceans constructed from climate‐model simulations of the 2090s that we hold constant in time. In contrast to simulations of the end‐of‐century transient state, our idealized states are intensely deoxygenated in the abyss, consistent with perpetually reduced ventilation and throttled Antarctic Bottom Water formation. We disentangle the effects of the deoxygenation drivers on preformed oxygen and true oxygen utilization (TOU) using the novel concept of upstream exposure time, which precisely connects TOU to oxygen utilization rates and preformed oxygen to ventilation. For our idealized steady states, deoxygenation below 2,000 m depth is due to increased TOU, driven dominantly by slower circulations that allow respiration to act roughly 2–3 times longer thereby overwhelming the effects of reduced respiration rates. Above 500 m depth, decreased respiration and slower circulation closely compensate, resulting in little expansion of upper‐ocean hypoxia. The bulk of preformed oxygen loss is driven by ventilation shifting equatorward to where warmer surface waters hold less oxygen. Warming‐driven declines in solubility account for less than 10% of the total oxygen loss. Although idealized, our analysis suggests that long‐term changes in the marine oxygen cycle could be driven dominantly by changes in circulation rather than by thermodynamics or biology.