Climate warming is likely resulting in ocean deoxygenation, but models still cannot fully explain the observed decline in oxygen. One unconstrained parameter is the oxygen demand for respiring particulate organic carbon and nitrogen (i.e., the total respiration quotient, rΣ-O2:C). It is untested if rΣ-O2:C systematically declines with depth. Here, we tested for such depth variance by quantifying particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphorus (POP), particulate chemical oxygen demand (PCOD, the oxygen demand for respiring POC), and total oxygen demand (-O2 = PCOD + 2PON) concentrations down to a depth of 1000 m in the Sargasso Sea. C:N and -O2:N changed with depth, but values at the surface were similar to those at 1000 m. C:P, N:P, and -O2:P exponentially decreased with depth. The respiration quotient (r-O2:C = PCOD:POC) and total respiration quotient (rΣ-O2:C = ‑O2:POC) were both higher below the euphotic zone. We hypothesize that rΣ-O2:C is linked to multiple environmental factors that change with depth, such as phytoplankton community structure and the preferential production/removal of biomolecules. Using a global model, we show that the global distribution of dissolved oxygen is sensitive to changes in the PCOD surface production (PPCOD) and depth attenuation (bPCOD). These variables mostly affect oxygen in the tropical and North Pacific Ocean, where deoxygenation rates and model discrepancy are the highest. This study aims to improve our understanding of biological oxygen demand as warming-induced deoxygenation continues.

This study characterized ocean biological carbon pump metrics in the second iteration of the REgional Carbon Cycle Assessment and Processes (RECCAP2) project, a coordinated, international effort to constrain contemporary ocean carbon air-sea fluxes and interior carbon storage trends using a combination of observation-based estimates, inverse models, and global ocean biogeochemical models. The analysis here focused on comparisons of global and biome-scale regional patterns in particulate organic carbon production and sinking flux from the RECCAP2 model ensemble against observational products derived from satellite remote sensing, sediment traps, and geochemical methods. There was generally encouraging model-data agreement in large-scale spatial patterns, though with substantial spread across the model ensemble and observational products. The global-integrated, model ensemble-mean export production, taken as the sinking particulate organic carbon flux at 100 m (6.41 ± 1.52 Pg C yr–1), and export ratio defined as sinking flux divided by net primary production (0.154 ± 0.026) both fell at the lower end of observational estimates. Comparison with observational constraints also suggested that the model ensemble may have underestimated regional biological CO2 drawdown and air-sea CO2 flux in high productivity regions. Reasonable model-data agreement was found for global-integrated, ensemble-mean sinking particulate organic carbon flux into the deep ocean at 1000 m (0.95 ± 0.64 Pg C yr–1) and the transfer efficiency defined as flux at 1000m divided by flux at 100m (0.121 ± 0.035), with both variables exhibiting considerable regional variability. Future modeling studies are needed to improve system-level simulation of interaction between model ocean physics and biogeochemical response.

The ocean has many underwater light niches, but the selection pressure for chromatic acclimaters (generalists) compared to blue or green-specialists is not well understood. Here, we tested the hypothesis that changes in ocean spectra brought about by mixing on the order of days preferentially selects for generalists within a Synechococcus population. We investigated ocean conditions that led to high proportions of Synechococcus generalists versus specialists in a model ocean column, and compared simulations with in situ metagenomic and physical oceanographic data from major Bio-GO-SHIP cruises, supplemented with GEOTRACES and TARA Oceans, as well as the GOOS Argo Program and sea surface height from AVISO. We found that greater mixed layer depths selected for generalists in simulated Synechococcus populations, but explained only 14% of the partitioning between strategies in situ. Rather, variability due to upwelling and ocean fronts had larger effects, explaining ~40% of the partitioning between Synechococcus generalists and specialists in the ocean. Physical oceanographic drivers therefore offer a significant selection pressure on marine Synechococcus light-harvesting strategies. Our results motivate further study of the in situ light environments of upwelling zones and ocean fronts, which are currently understudied as potential light-driven niche habitats.

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.

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.

We develop a hierarchy of simplified ocean models for coupled ocean, atmosphere, and sea ice climate simulations using the Community Earth System Model version 1 (CESM1). The hierarchy has four members: a slab ocean model, a mixed-layer model with entrainment and detrainment, an Ekman mixed-layer model, and an ocean general circulation model (OGCM). Flux corrections of heat and salt are applied to the simplified models ensuring that all hierarchy members have the same climatology. We diagnose the needed flux corrections from auxiliary simulations in which we restore the temperature and salinity to the daily climatology obtained from a target CESM1 simulation. The resulting 3-dimensional corrections contain the interannual variability fluxes that maintain the correct vertical gradients of temperature and salinity in the tropics. We find that the inclusion of mixed-layer entrainment and Ekman flow produces sea surface temperature and surface air temperature fields whose means and variances are progressively more similar to those produced by the target CESM1 simulation. We illustrate the application of the hierarchy to the problem of understanding the response of the climate system to the loss of Arctic sea ice. We find that the shifts in the positions of the mid-latitude westerly jet and of the Inter-tropical Convergence Zone (ITCZ) in response to sea-ice loss depend critically on upper ocean processes. Specifically, heat uptake associated with the mixed-layer entrainment influences the shift in the westerly jet and ITCZ. Moreover, the shift of ITCZ is sensitive to the form of Ekman flow parameterization.

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.

Carbonyl sulfide (COS) was measured in firn air collected during seven different field campaigns carried out at four different sites in Greenland and Antarctica between 2001 and 2015. A Bayesian probabilistic statistical model is used to conduct multi-site inversions and to reconstruct separate atmospheric histories for Greenland and Antarctica. The firn air inversions cover most of the 20 century over Greenland and extend back to the 19 century over Antarctica. The derived atmospheric histories are consistent with independent surface air time-series data from the corresponding sites and the Antarctic ice core COS records during periods of overlap. Atmospheric COS levels began to increase over preindustrial levels starting in the 19 century and the increase continued for much of the 20 century. Atmospheric COS peaked at higher than present-day levels around 1975 CE over Greenland and around 1987 CE over Antarctica. An atmosphere/surface ocean box model is used to investigate the possible causes of observed variability. The results suggest that changes in the magnitude and location of anthropogenic sources have had a strong influence on the observed atmospheric COS variability.