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The contribution of carbonate‐producing benthic organisms to the global marine carbon budget has been overlooked, the prevailing view being that calcium carbonate (CaCO3) is predominantly produced and exported by marine plankton in the “biological pump.” Here, we provide the first estimation of the global contribution of echinoderms to the marine inorganic and organic carbon cycle, based on organism‐level measurements from species of the five echinoderm classes. Echinoderms' global CaCO3 contribution amounts to ~0.861 Pg CaCO3/yr (0.102 Pg C/yr of inorganic carbon) as a production rate, and ~2.11 Pg CaCO3 (0.25 Pg C of inorganic carbon) as a standing stock from the shelves, slopes, and abyssal depths. Echinoderm inorganic carbon production (0.102 Pg C/yr) is less than the global pelagic production (0.4–1.8 Pg C/yr) and similar to the estimates for carbonate shelves globally (0.024–0.120 Pg C/yr). Echinoderm CaCO3 production per unit area is ~27.01 g CaCO3·m−2·yr−1 (3.24 g C·m−2·yr−1 as inorganic carbon) on a global scale for all areas, with a standing stock of ~63.34 g CaCO3/m2 (7.60 g C/m2 as inorganic carbon), and ~7.97 g C/m2 as organic carbon. The shelf production alone is 77.91 g CaCO3·m−2·yr−1 (9.35 g C·m−2·yr−1 as inorganic carbon) in contrast to 2.05 g CaCO3·m−2·yr−1 (0.24 g C·m−2·yr−1 as inorganic carbon) for the slope on a global scale. The biogeography of the CaCO3 standing stocks of echinoderms showed strong latitudinal variability. More than 80% of the global CaCO3 production from echinoderms occurs between 0 and 800 m, with the highest contribution attributed to the shelf and upper slope. We provide a global distribution of echinoderm populations in the context of global calcite saturation horizons, since undersaturated waters with respect to mineral phases are surfacing. This shallowing is a direct consequence of ocean acidification, and in some places it may reach the shelf and upper slope permanently, where the highest CaCO3 standing stocks from echinoderms originate. These organism‐level data contribute substantially to the assessment of global carbonate inventories, which at present are poorly estimated. Additionally, it is desirable to include these benthic compartments in coupled global biogeochemical models representing the “biological pump” and its feedbacks, since at present all efforts have focused on pelagic processes, dominated by coccolithophores. The omission of the benthic processes from modeling will only diminish the understanding of elemental fluxes at large scales and any future prediction of climate change scenarios. [ABSTRACT FROM AUTHOR]

Understanding the risks posed by the presence of microplastics in the marine environment requires fundamental and robust knowledge of the exposure levels to this contaminant and the associated harms. However, these terms are poorly constrained for the open ocean which is largely undersampled for microplastics, especially for those in a smaller size range. The mass balance between plastic entering the ocean from land and the amounts found within remains open. With large uncertainties, we can account for at most 10% the total global plastic waste discarded into the ocean every year with <1% constituted by small plastic debris down to 300 µm in size floating in the surface waters. Answering the basic questions of what and where microplastics in the ocean are, how long they persist in this environment and where they go is critical to define the scale and impacts of microplastic pollution. We quantified and characterised the abundance and, for the first time, the vertical distribution of microplastics down to 25 µm in size in the Atlantic Ocean on a passage of the Atlantic Meridional Transect cruise from the UK to the Falklands in 2016. Depth-resolved particle samples were collected in the top 300 m via in situ filtration following a strict contamination prevention control. Microplastics in the bulk particle mixture were identified and characterised for polymer type and size using the state-of-the-art Fourier Transform infrared imaging system. Microplastics were present at all sampled stations and depths as polymer resins, synthetic fibres and additives. Our study is focused on polyethylene, polypropylene and polystyrene which were the most dominant non-fibre polymer groups in terms of numerical abundance and mass concentration, consistent with the trends in the global polymer-specific waste generation and disposal. All three polymer groups showed elevated abundance in the North Atlantic subtropical gyre (45°N-25°N) and in the Southern Subtropical Convergence Zone (40°S-55°S). Polyethylene microplastics were the most abundant and ubiquitously dispersed over the examined area and depth layer. The mass concentrations of polypropylene and polystyrene microplastics were up to several orders of magnitude lower compared to polyethylene. Their distribution with depth was also patchier suggesting different residence times of these polymers in the water column. The polymer-specific particle sizes also varied latitudinally and with depth. They were mostly in a range between 70 and 120 µm and rarely exceeded 600 µm. Our depth-resolved data thus provide new insights on abundance and distribution of marine plastic debris in a lower micro-size range, which is unprecedented for marine microplastics studies. Our findings now also allow for more accurate estimates of the inventory of microplastics in the ocean, their source-to-sink budgets and environmental persistence, resolving to a degree the issue of 'missing plastics' in the ocean. [ABSTRACT FROM AUTHOR]

Acantharian cysts were discovered in sediment trap samples from spring 2007 at 2000 m in the Iceland Basin. Although these single-celled organisms contribute to particulate organic matter flux in the upper mesopelagic, their contribution to bathypelagic particle flux has previously been found negligible. Four time-series sediment traps were deployed and all collected acantharian cysts, which are reproductive structures. Across all traps, cysts contributed on average 3-22%, and 4-24% of particulate organic carbon and nitrogen (POC and PON) flux, respectively, during three separate collection intervals (the maximum contribution in any one trap was 48% for POC and 59% for PON). Strontium (Sr) flux during these 6 weeks reached 3 mg m-2 d-2. The acantharian celestite (SrSO4) skeleton clearly does not always dissolve in the mesopelagic as often thought, and their cysts can contribute significantly to particle flux at bathypelagic depths during specific flux events. Their large size (~ 1 mm) and mineral ballast result in a sinking rate of ~ 500 m d-1; hence, they reach the bathypelagic before dissolving. Our findings are consistent with a vertical profile of salinity-normalized Sr concentration in the Iceland Basin, which shows a maximum at 1700 m. Profiles of salinity-normalized Sr concentration in the subarctic Pacific reach maxima at ≤ 1500 m, suggesting that Acantharia might contribute to the bathypelagic particle flux there as well. We hypothesize that Acantharia at high latitudes use rapid, deep sedimentation of reproductive cysts during phytoplankton blooms so that juveniles can exploit the large quantity of organic matter that sinks rapidly to the deep sea following a bloom. [ABSTRACT FROM AUTHOR]

Abstract: Residual flow, barotropic tides and internal (baroclinic) tides interact in a number of ways with kilometer-scale seafloor topography such as abyssal hills and seamounts. Because of their likely impact on vertical mixing such interactions are potentially important for ocean circulation and the mechanisms and the geometry of these interactions are a matter of ongoing studies. In addition, very little is known about how these interactions are reflected in the sedimentary record. This multi-year study investigates if flow/topography interactions are reflected in distributional patterns of the natural short-lived (half-life: 24.1d) particulate-matter tracer 234Th relative to its conservative (non-particle-reactive) and very long-lived parent nuclide 238U. The sampling sites were downstream of, or surrounded by, fields of short seamounts and, therefore, very likely to be influenced by nearby flow/topography interactions. At the sampling sites between about 200 and 1000m above the seafloor recurrent ‘fossil’ disequilibria were detected. ‘Fossil’ disequilibria are defined by clearly detectable 234Th/238U disequilibria (total 234Th radioactivity <238U radioactivity, indicating a history of intense particulate 234Th scavenging and particulate-matter settling from the sampled parcel of water) and conspicuously low particle-associated 234Th activities. ‘Fossil’ disequilibria were centered at levels in the water column that correspond to the average height of the short seamounts near the sampling sites. This suggests the ‘fossil’ disequilibria are formed on the seamount slopes. Moreover, the magnitude of the ‘fossil’ disequilibria suggests that the slopes of the short seamounts in the study region are characterized by particularly vigorous fluid dynamics. Since ‘fossil’ disequilibria already occurred at ∼O(1–10km) away from the seamount slopes it is likely that these vigorous fluid dynamics rapidly decay away from the slopes on scales of O(1–10km). These conclusions are supported by the horizontal distribution and magnitude of the modeled total (barotropic+baroclinic) tidal current velocities of the predominating tidal M2 constituent: on (near-)critical seamount slopes baroclinic tides lead to localized [∼O(1km)] increases of the overall tidal current velocity by a factor of ∼ 2, thereby pushing the total current velocity well above the threshold for sediment erosion. The results of this and a previous study [Turnewitsch, R., Reyss, J.-L., Chapman, D.C., Thomson, J., Lampitt, R.S., 2004. Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount. Earth and Planetary Science Letters 222(3–4), 1023–1036] show that kilometer-scale flow/topography interactions leave a marine geochemical imprint. This imprint may help develop new sediment proxies for the reconstruction of past changes of fluid dynamics in the deep sea, including residual and tidal flow. Sedimentary records controlled by kilometer-scale seafloor elevations are promising systems for the reconstruction of paleo-changes of deep-ocean fluid dynamics. For the sediment-based reconstruction of paleo-parameters other than physical oceanographic ones it may be advisable to avoid kilometer-scale topography altogether. [Copyright &y& Elsevier]