Earth System Models generally predict increasing upper ocean stratification from 21st century planetary warming, which will cause a decrease in the vertical nutrient flux resulting in declining marine net primary productivity (NPP) and carbon export fluxes. Recent advances in quantifying marine ecosystem carbon to nutrient stoichiometry have identified large latitudinal and biome variability, with low-latitude oligotrophic systems harboring pico-sized phytoplankton exhibiting large phosphorus to carbon cellular plasticity. Climate forced changes in nutrient flux stoichiometry and phytoplankton community composition is thus likely to alter the ocean’s biogeochemical response and feedback with the carbon-climate system. We have added three pico-phytoplankton functional types within the Biogeochemical Elemental Cycling component of the Community Earth System Model while incorporating variable cellular phosphorus to carbon stoichiometry for all represented phytoplankton types. The model simulates Prochlorococcus and Synechococcus populations that dominate the productivity and sinking carbon export of the tropical and subtropical ocean, and pico-eukaryote populations that contribute significantly to productivity and export within the subtropical to mid-latitude transition zone, contributing a combined 50 – 70% of these fluxes. Pico-phytoplankton cellular stoichiometry and resulting sinking export patterns inversely track the distribution of surface phosphate, with the western subtropical regions of each basin supporting the most P-poor stoichiometries. Collectively, pico-phytoplankton contribute ~58% of global NPP and ~46% of global particulate organic carbon export below 100 meters. Subtropical gyre recirculation regions along the poleward flanks of surface western boundary currents are identified as regional hotspots of enhanced carbon export exhibiting C-rich/P-poor stoichiometry, preferentially inhabited by pico-eukaryotes and diatoms.

Under a high-end emission scenario to the year 2300, climate warming drives a drastic slowdown in the ocean’s meridional overturning circulation, with a cessation of Antarctic Bottom Water (AABW) production and North Atlantic Deep Water (NADW) formation reduced to 5 Sv. In conjunction with regionally enhanced biological production and upper-ocean nutrient trapping in the Southern Ocean, this deep circulation slowdown drives long-term sequestration of nutrients and dissolved inorganic carbon in the deep ocean, but also greatly reduces the ocean’s capacity to take up heat and anthropogenic CO from the atmosphere, prolonging peak warmth climate conditions. Surface nutrients (N, P, and Si) are steadily depleted driving down biological productivity and weakening the biological pump, which transfers carbon to the ocean interior. Ocean dissolved oxygen concentrations steadily decline, with the potential for anoxia eventually developing in some regions. This Community Earth System Model (CESM) simulation did not include active ice sheet dynamics, but the strong climate warming simulated would lead to large freshwater discharge from the Antarctic and Greenland ice sheets. This would further stratify the polar regions, potentially leading to complete shutdown of the meridional overturning circulation. The impacts of this would be catastrophic as the hothouse Earth climate conditions could be extended for thousands of years, with widespread ocean anoxia developing, driving a mass extinction event.

Dissolved iron (dFe) plays an important role in regulating marine biological productivity. In high nutrient, low chlorophyll (HNLC) regions (> 33% of the global ocean) iron is the primary growth limiting nutrient, and elsewhere can regulate nitrogen fixation and growth by diazotrophs. Overall, dFe supply potentially impacts half of global ocean productivity. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas, or cellular Fe:C ratios. This ratio can vary by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentration in sparse field observations. The Community Earth System Model (CESM) ocean component has been modified to simulate dynamic, group-specific, phytoplankton iron quotas (Fe:C) that vary as a function of ambient iron concentration. The simulated Fe:C ratios match the spatial trends in the observations and improve the correlation with global-scale, observed nutrient distributions. Acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition fluxes of soluble iron, compared to a model with fixed Fe:C. However, varying atmospheric soluble iron supply still has first order impacts on global carbon and nitrogen fluxes, and on the spatial patterns of nutrient limitation; both of which are strongly sensitive to changes in pyrogenic sources of iron. Accounting for dynamic, phytoplankton iron quotas is critical for capturing the ocean biogeochemical responses to varying atmospheric soluble iron inputs, including expected changes in both the mineral dust and pyrogenic sources with climate warming and anthropogenic activity.

Marine dimethyl sulfide (DMS) is important to climate due to the ability of DMS to alter Earth’s radiation budget. However, a knowledge of the global-scale distribution, seasonal variability, and sea-to-air flux of DMS is needed in order to understand the factors controlling surface ocean DMS and its impact on climate. Here we examine the use of an artificial neural network (ANN) to extrapolate available DMS measurements to the global ocean and produce a global climatology with monthly temporal resolution. A global database of 57,810 ship-based DMS measurements in surface waters was used along with a suite of environmental parameters consisting of lat-lon coordinates, time-of-day, time-of-year, solar radiation, mixed layer depth, sea surface temperature, salinity, nitrate, phosphate, silicate, and oxygen. Linear regressions of DMS against the environmental parameters show that on a global scale mixed layer depth and solar radiation are the strongest predictors of DMS, however, they capture 14% and 12% of the raw DMS data variance, respectively. The multi-linear regression can capture more (29%) of the raw data variance, but strongly underestimates high DMS concentrations. In contrast, the ANN captures ∼61% of the raw data variance in our database. Like prior climatologies our results show a strong seasonal cycle in DMS concentration and sea-to-air flux. The highest concentrations (fluxes) occur in the high-latitude oceans during the summer. We estimate a lower global sea- to-air DMS flux (17.90±0.34 Tg S yr−1) than the prior estimate based on a map interpolation method when the same gas transfer velocity parameterization is used.

The downward flux of organic carbon exported from the surface ocean is of great importance to the Earth’s climate because it represents the major pathway for transporting CO from the surface ocean and atmosphere into the deep ocean and sediments where it can be sequestered for a long time. Here we present global-scale estimates for the export fluxes of total, dissolved, and particulate organic carbon (TOC, DOC, and POC, respectively) constrained by observed thorium-234 (Th) activity and dissolved phosphorus (DIP) concentration in a global inverse biogeochemical model for the cycling of phosphorus and Th. We find that POC export flux is low in the subtropical oceans, indicating that a projected expansion of the subtropical gyres due to global warming will weaken the gravitational biological carbon pump. We also find that DOC export flux is low in the tropical oceans, intermediate in the upwelling Antarctic zone and subtropical south Pacific, and high in the subtropical Atlantic, subtropical north Pacific, and productive subantarctic zone (SAZ). The horizontal distribution of DOC export ratio (F/F) increases from tropical to polar regions, possibly due to the detrainment of DOC rich surface water during mixing events into subsurface waters (increasing the strength of the mixed layer pump poleward due to stronger seasonality). Large contribution to the export flux from DOC implies that the efficiency with which photosynthetically fixed carbon is exported as particles may not be as large as currently assumed by widely used global export algorithms.

The Marine Biogeochemistry Library (MARBL) is a prognostic ocean biogeochemistry model that simulates marine ecosystem dynamics and the coupled cycles of carbon, nitrogen, phosphorus, iron, silicon, and oxygen. MARBL is a component of the Community Earth System Model (CESM); it supports flexible ecosystem configuration of multiple phytoplankton and zooplankton functional types; it is also portable, designed to interface with multiple ocean circulation models. Here, we present scientific documentation of MARBL, describe its configuration in CESM2 experiments included in the Coupled Model Intercomparison Project version 6 (CMIP6), and evaluate its performance against a number of observational datasets. The model simulates an air-sea CO2 flux and many aspects of the carbon cycle in good agreement with observations. However, the simulated integrated uptake of anthropogenic CO2 is weak, which we link to poor thermocline ventilation, a feature evident in simulated chlorofluorocarbon distributions. This also contributes to larger-than-observed oxygen minimum zones. Moreover, radiocarbon distributions show that the simulated circulation in the deep North Pacific is extremely sluggish, yielding extensive oxygen depletion and nutrient trapping at depth. Surface macronutrient biases are generally positive at low latitudes and negative at high latitudes. CESM2 simulates globally-integrated net primary production (NPP) of 48 Pg C yr-1 and particulate export flux at 100 m of 7.1 Pg C yr-1. The impacts of climate change include an increase in globally-integrated NPP, but substantial declines in the North Atlantic. Particulate export is projected to decline globally, attributable to decreasing export efficiency associated with changes in phytoplankton community composition.