ALBERT

Description: Correlations between particulate organic carbon (POC) and mineral fluxes in the deep ocean have inspired the inclusion of “ballast effect” parameterizations in carbon cycle models. A recent study demonstrated regional variability in the effect of ballast minerals on the flux of POC in the deep ocean. We have undertaken a similar analysis of shallow export data from the Arctic, Atlantic, and Southern Oceans. Mineral ballasting is of greatest importance in the high-latitude North Atlantic, where 60% of the POC flux is associated with ballast minerals. This fraction drops to around 40% in the Southern Ocean. The remainder of the export flux is not associated with minerals, and this unballasted fraction thus often dominates the export flux. The proportion of mineral-associated POC flux often scales with regional variation in export efficiency (the proportion of primary production that is exported). However, local discrepancies suggest that regional differences in ecology also impact the magnitude of surface export. We propose that POC export will not respond equally across all high-latitude regions to possible future changes in ballast availability.

Description: The spatial distribution, biogeochemical cycle and external sources of dissolved cobalt (DCo) were investigated in the southeastern Atlantic and the Southern Ocean between 33°58′S and 57°33′S along the Greenwich Meridian during the austral summer 2008 in the framework of the International Polar Year. DCo concentrations were measured by flow-injection analysis and chemiluminescence detection in filtered (0.2 μm), acidified and UV-digested samples at 12 deep stations in order to resolve the several biogeochemical provinces of the Antarctic Circumpolar Current and to assess the vertical and frontal structures in the Atlantic sector of the Southern Ocean. We measured DCo ranging from 5.73 ± 1.15 pM to 72.9 ± 4.51 pM. The distribution of DCo was nutrient-like in surface waters of the subtropical domain with low concentrations in the euphotic layer due to biological uptake. The biological utilization of dissolved cobalt was proportional to that of phosphate in the subtropical domain with a DCo:HPO42− depletion ratio of ~ 44 μM M−1. In deeper waters the distribution indicated remineralization of DCo and inputs from the margins of South Africa with lateral advection of enriched intermediate and deep waters to the southeastern Atlantic Ocean. In contrast the vertical distribution of DCo changed southward, from a nutrient-like distribution in the subtropical domain to scavenged-type behavior in the domain of the Antarctic Circumpolar Current and conservative distribution in the Weddell Gyre. There the cycle of DCo featured low biological removal by Antarctic diatoms with input to surface waters by snow, removal in oxygenated surface waters, and dissolution and stabilization in the low-oxygenated Upper Circumpolar Deep Waters. DCo distributions and physical hydro-dynamics features also suggest inputs from the Drake Passage and the southwestern Atlantic to the 0° meridian along the eastward flow of the Antarctic Circumpolar Current. Bottom enrichment of DCo in the Antarctic Bottom Waters was also evident, together with increasing water-mass pathway and aging, possibly due to sediment resuspension and/or mixing with North Atlantic Deep waters in the Cape Basin. Overall atmospheric input of soluble Co by dry aerosols to the surface waters was low but higher in the ACC domain than in the northern part of the section. At the highest latitudes, it is possible that snowfall could be a source of DCo to surface waters. Tentative budgets for DCo in the mixed layer of the subtropical and the ACC domains have been constructed for each biogeochemical region encountered during the cruise. The estimated DCo uptake flux was found to be the dominant cobalt flux along the section. This flux decreases southward, which is consistent with the observations that DCo shows a southward transition from nutrient-like towards conservative distribution in the mixed layer.

Description: In this study we first evaluate the small-scale spatial variability of particulate export, using a set of synoptic thorium-234 activity observations sampled within a one-degree radius. These data show significant variability of surface thorium activity on scales of the order of 100 km (∼270–550 dpm m−3). This patchiness of export potentially affects the robustness of point observations and our interpretation of them. Motivated by these observations we subsequently couple an explicit model of thorium-234 dynamics to a coupled physical–biogeochemical basin model capable of resolving these small-scales. The model supports the observations in displaying marked thorium variability on spatial scales of the order of 100 km and smaller, with highest values in the regions of large eddy kinetic energy and large primary productivity. The model is also used to quantify the impact of small-scale variability on export estimates. Our model shows that the primary source of error associated with the presence of small-scale spatial variability is related to the standard assumptions of steady state and non-steady state (〉40% during bloom condition). The non-steady state method can misinterpret variations due to patchiness in thorium activity as temporal changes and lead to errors larger than those introduced by the simpler steady state approach. We show that the non-steady state approach could improve the flux estimates in some cases if the sampling was conducted in a Lagrangian framework. Undersampling the spatial variability results in further bias (〉20%) that can be reduced when the sampling density is increased. Finally, errors due to the dynamical transport of thorium associated with small-scale structures are relatively low (〈20%) except in regions of high eddy kinetic energy.

Description: The simultaneous estimation of particulate organic carbon (POC), particulate inorganic carbon (PIC) and biogenic silica (BSi) export fluxes is key to the study of carbon export due to the hypothesized role of biominerals in the sinking of organic particles. This paper presents of the first attempts to measure downward fluxes of POC, PIC and BSi from the surface ocean using both the 234Th-238U and the 210Po-210Pb disequilibria and drifting sediments trap synchronously at the Porcupine Abyssal Plain in summer 2009. The combined use of the three techniques allowed us to analyze their suitability not only for POC flux estimates, but also as tracers of PIC and BSi fluxes. POC and biomineral/radionuclide ratios were measured in two size fractions to better understand differences between 234Th derived export and 210Po derived export. 210Po derived POC and biomineral fluxes were unexpectedly closer to POC and biomineral fluxes recorded by sediment traps than 234Th derived POC and biomineral fluxes which were higher than obtained from the other two approaches. We suggest that 210Po, because of its biogeochemical behavior, is a better proxy for POC and mineral fluxes than is 234Th in post bloom conditions. The contribution of smaller (1–53 μm) particles to flux is also considered in order to explain the differences in derived fluxes.

Description: Estimates of the amount of carbon sequestered in the ocean interior per unit iron (Fe) supplied, as quantified by the sequestration efficiency (Ceffx), vary widely. Such variability in Ceffx has frequently been attributed to estimate uncertainty rather than intrinsic variability. Here we derive new estimates of Ceffx for the subpolar North Atlantic, where Fe stressed conditions have recently been demonstrated. Derived values of Ceffx from across the region, including areas subject to atypical external Fe fertilization events during the year of sample collection (2010), ranged from 17 to 19 kmol C (mol Fe−1). Comparing these estimates with values from other systems, considered in the context of variable bloom durations in the different oceanographic settings, we suggest that apparent variability in Ceffx may be related to the mode of Fe delivery.

Description: The biological carbon pump (BCP) transfers carbon from the surface ocean into the oceans' interior, mainly in the form of sinking particles with an organic component, and thereby keeps atmospheric CO2 at significantly lower levels than if the oceans were abiotic. The depth at which these sinking particles are remineralized is a key control over atmospheric CO2. Particle sinking speed is likely to be a critical parameter over remineralization depth. Carbon export is usually controlled by large, rapidly sinking particles (〉150 m·d−1); however, under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly (〈10 m·d−1) sinking particles. Therefore, there is an interest in determining sinking particle velocities and their variations with depth, as well as in understanding the interplay between sinking velocity distributions and carbon export. Here, we use profiles of total and particulate concentrations of the naturally occurring radionuclide pair 210Po-210Pb from the Porcupine Abyssal Plain (PAP) site (48°N, 16.5°W) to estimate depth variation in particle sinking speed using a one-box model and inverse techniques. Average sinking speeds increase from 60 ± 30 m·d−1 at 50 m, to 75 ± 25 m·d−1 and 90 ± 20 m·d−1 at 150 and 500 m. Furthermore, a sensitivity analysis suggests that at the PAP site the measured 210Po profiles are inconsistent with the usually assumed sinking velocities of 200 m·d−1. We hypothesize that a trend of increasing velocity with depth might be caused by a gradual loss of slow-sinking material with depth, a factor with significant implications for regional carbon budgets.

Description: Upwelling ocean currents associated with oxygen minimum zones (OMZs) supply nutrients fuelling intense marine productivity. Perturbations in the extent and intensity of OMZs are projected in the future, but it is currently uncertain how this will impact fluxes of redox‐sensitive trace metal micronutrients to the surface ocean. Here we report seawater concentrations of Fe, Mn, Co, Cd, and Ni alongside the redox indicator iodide/iodate in the Peruvian OMZ during the 2015 El Niño event. The El Niño drove atypical upwelling of oxygen‐enriched water over the Peruvian Shelf, resulting in oxidized iodine and strongly depleted Fe (II), total dissolved Fe, and reactive particulate Fe concentrations relative to non‐El Niño conditions. Observations of Fe were matched by the redox‐sensitive micronutrients Co and Mn, but not by non‐redox‐sensitive Cd and Ni. These observations demonstrate that oxygenation of OMZs significantly reduces water column inventories of redox‐sensitive micronutrients, with potential impacts on ocean productivity. Plain Language Summary Some trace metals, including iron, are essential micronutrients for phytoplankton growth. However, the solubility of iron is very low under oxygenated conditions. Consequently, restricted iron availability in oxygen‐rich seawater can limit phytoplankton growth in the ocean, including in the Eastern Tropical South Pacific. Under typical conditions, depleted oxygen on the South American continental shelf is generally thought to enhance iron supply to the ocean, fuelling phytoplankton productivity in overlying waters. However, the impact of changes in oxygenation, which are predicted to occur in the future, are not known. The 2015 El Niño event led to unusually high oxygen on the Peruvian shelf, offering a system‐scale test on how oxygen influences seawater iron concentrations. We show that El Niño‐driven oxygenation resulted in marked decreases in iron and other metals sensitive to oxygen (cobalt and manganese), whilst metals not sensitive to oxygen (cadmium and nickel) were unaffected. The measured reductions in iron may have led to decreased phytoplankton productivity.

Description: Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes' Law estimating particle SV from density and size has been widely used. But while Stokes' Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes' models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes' Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes' assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes' models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.