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39Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of 39Ar using a steady ocean circulation inverse model. Our estimates match available 39Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that 39Ar traces out the world ocean's ventilation pathways and that the 39Ar age ΓAr and the ideal mean age have broadly similar large‐scale patterns. At the surface, 39Ar is close to saturated except at high latitudes. Undersaturation imparts a finite 39Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating ΓAr by ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of 39Ar, thus providing guidance for future measurements. Key Points: We estimate the global three‐dimensional 39Ar distribution in the ocean using a data‐assimilated circulation modelThe 39Ar age correlates with the ideal mean age, encodes TTD‐shape information, and traces out the world ocean's ventilation pathwaysHigh‐latitude surface waters are undersaturated in 39Ar imparting a reservoir age of order 100 years to Antarctic waters [ABSTRACT FROM AUTHOR]

Biologically fixed carbon is transferred from the surface to deep ocean as sinking particles or dissolved organic carbon (DOC). DOC is estimated to account for ~20% of global export production, but the degree to which this varies regionally has not been assessed at a global scale. Here we present the first observationally based global-scale assessment of DOC production and export, obtained by combining an artificial neural network estimate of the global DOC distribution, and a data-constrained ocean circulation model. Our results demonstrate that the efficiency of DOC production and export varies more than threefold across oceanographic regions. DOC production and export display a pronounced peak in the oligotrophic subtropical oceans, where DOC accounts for roughly half of the total organic carbon export. These stratified nutrient-depleted regions are expected to expand with future warming, amplifying the role of DOC in the biological pump, and magnifying the need to improve DOC cycling in climate models. [ABSTRACT FROM AUTHOR]

The relationship between dissolved cadmium (Cd) and phosphate (PO4−3) can elucidate a biological role for Cd in the ocean and help to evaluate the usefulness of Cd as a tracer of past ocean circulation and nutrient distributions. Here we determine and analyze this relationship in the poorly studied region of the tropical South Pacific. The dissolved Cd distribution is generally similar to PO43−, but a plot of Cd versus PO43− shows a clear concavity resulting from distinct Cd:PO43− ratios in waters local to our transect and in waters formed distally in higher latitudes. To determine the factors affecting the subsurface Cd:PO43− ratio along our transect, we used an ocean circulation model and a multilinear regression model to determine the preformed and regenerated components of dissolved Cd and PO43−. We found that both the preformed and regenerated Cd:PO43− ratios are low in the shallow, locally formed water masses along the transect and significantly higher in the deeper and older water masses. Overall, the regenerated:preformed Cd:PO43− ratio in the deep waters (>1,000 m) along our transect is ~1.8:1, reflecting the basin-wide average Cd:PO43− 'fractionation factor' during biological uptake. However, we find a lower fractionation factor in local waters of 1.1 (± 0.6). We suggest that this locally lower biological fractionation factor is due to either the chemical speciation of Cd or to a lower efficiency of Cd assimilation by the picoplankton and nanoplankton species found in our study region. [ABSTRACT FROM AUTHOR]

Hydrothermal vents along the ocean's tectonic ridge systems inject superheated water and large amounts of dissolved metals that impact the deep ocean circulation and the oceanic cycling of trace metals. The hydrothermal fluid contains dissolved mantle helium that is enriched in 3 He relative to the atmosphere, providing an isotopic tracer of the ocean's deep circulation and a marker of hydrothermal sources. This work investigates the potential for the 3 He/ 4 He isotope ratio to constrain the ocean's mantle 3 He source and to provide constraints on the ocean's deep circulation. We use an ensemble of 11 data-assimilated steady-state ocean circulation models and a mantle helium source based on geographically varying sea-floor spreading rates. The global source distribution is partitioned into 6 regions, and the vertical profile and source amplitude of each region are varied independently to determine the optimal 3 He source distribution that minimizes the mismatch between modeled and observed δ 3 He. In this way, we are able to fit the observed δ 3 He distribution to within a relative error of ∼15%, with a global 3 He source that ranges from 640 to 850 mol yr −1 , depending on circulation. The fit captures the vertical and interbasin gradients of the δ 3 He distribution very well and reproduces its jet-sheared saddle point in the deep equatorial Pacific. This demonstrates that the data-assimilated models have much greater fidelity to the deep ocean circulation than other coarse-resolution ocean models. Nonetheless, the modelled δ 3 He distributions still display some systematic biases, especially in the deep North Pacific where δ 3 He is overpredicted by our models, and in the southeastern tropical Pacific, where observed westward-spreading δ 3 He plumes are not well captured. Sources inferred by the data-assimilated transport with and without isopycnally aligned eddy diffusivity differ widely in the Southern Ocean, in spite of the ability to match the observed distributions of CFCs and radiocarbon for either eddy parameterization. [ABSTRACT FROM AUTHOR]

The article presents a study which established several benchmarks for Atlantic, Southern, and Pacific deep-sea circulation and ventilation since the last ice age. Topics discussed include a flipped Pacific and slower deep overturning circulation during the Last Glacial Maximum (LGM), Southern, Atlantic, and Pacific deep-sea ventilation during the LGM, and deep-sea reorganization and carbon ventilation during the deglaciation.