Search

The ocean contains about 40 times more carbon than the atmosphere, storing 38,000 Pg C as dissolved inorganic carbon (DIC) versus 900 Pg C as carbon dioxide (CO2) in the present atmosphere. The biological carbon pump contributes to ocean carbon storage by moving organic carbon out of the surface ocean into deeper waters in sinking particles, vertically migrating organisms and physical circulation. Century-scale (≥100 years) storage of the resulting biogenic DIC is commonly assumed to occur exclusively in the deep ocean, typically below 1,000 m. However, recent work has shown that carbon can be sequestered at century scales above 1,000 m in many ocean regions, in what we call ‘continuous vertical sequestration’. Here we calculate the century-scale carbon sequestration flux driven by the biological pump throughout the water column by combining previously published estimates of organic carbon flux and modelled values of water-mass sequestration time distributions. We estimate that the flux of organic carbon that is sequestered for ≥100 years in the contemporary ocean by the combined action of various biological pump pathways is 0.9–2.6 Pg C yr−1, which is up to six times larger than previous estimates based on the organic carbon flux reaching the deep ocean.

We explore how the iron dependence of the Si:P uptake ratio RSi:Pof diatoms controls the response of the global silicon cycle and phytoplankton community structure to Southern Ocean iron fertilization. We use a data‐constrained model of the coupled Si‐P‐Fe cycles that features a mechanistic representation of nutrient colimitations for three phytoplankton classes and that is embedded in a data‐assimilated global ocean circulation model. We consider three parameterizations of the iron dependence of RSi:P, all of which are consistent with the available field data and allow equally good fits to the observed nutrient climatology but result in very different responses to iron fertilization: Depending on how sharply RSi:Pdecreases with increasing iron concentration, iron fertilization can either cause enhanced silicic acid leakage from the Southern Ocean or strengthened Southern Ocean silicon trapping. Enhanced silicic acid leakage occurs if decreases in RSi:Pwin over increases in diatom growth, while the converse causes strengthened Southern Ocean silicon trapping. Silicic acid leakage drives a floristic shift in favor of diatoms in the subtropical gyres and stimulates increased low‐latitude opal export. The diatom contribution to global phosphorus export increases, but the lower diatom silicon requirement under iron‐replete conditions reduces the global opal export. Regardless of RSi:Pparameterization, the global response of the biological phosphorus and silicon pumps is dominated by the Southern Ocean. The Si isotope signature of opal flux becomes systematically lighter with increasing iron‐induced silicic acid leakage, consistent with sediment records from iron‐rich glacial periods. Iron fertilization leads to silicic acid leakage if the Si:P uptake ratio decreases sharply enough to overwhelm increased diatom growthSilicic acid leakage increases low‐latitude opal export, while the Southern Ocean dominates increases in diatom organic matter exportSilicic acid leakage in response to iron fertilization produces isotopically lighter biogenic opal flux

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 ΓArand 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 ΓArby ∼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. 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

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. Dissolved Cd and PO43−show different correlations in the near‐surface and deep waters of the Eastern Tropical PacificThe Cd:PO43−regenerated:preformed ratio is ~1.8(plus/minus 0.06):1 in the deep Eastern Tropical Pacific but ~1.1(plus/minus 0.6):1 above 250 mDifferences between shallow and deep regenerated:preformed Cd:PO43−ratios may be explained by Cd speciation or plankton Cd requirements

Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo‐Pacific Oceans between 60°S and 70°N is (12 ± 3) × 1013m3yr−1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo‐Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon. Global magnitude of SGD is measured using a radium‐228 budgetGlobal SGD is 3 to 4 times greater than the river water fluxes into the oceansAbout 70% of SGD flows into the Indo‐Pacific Oceans