Explore the vast range of titles available.

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications.

Photosynthesis in the surface ocean converts atmospheric CO2 into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Major d‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms. Plain Language Summary Phytoplankton photosynthesis in the surface ocean plays a critical role in stabilizing atmospheric CO2. It converts CO2 into organic particles that sink and are reworked by colonizing bacteria. Bacteria respire most particles back to CO2 while also transforming some into their cell components. Although the involvement of bacteria can replace the plankton‐derived particles to bacterial material, most past organic geochemical data have suggested that the deep‐sea particles are still comprised mainly of plankton remnants. This renders the contribution of bacterial material to total particle export an unresolved and yet important question, because the source and composition of particles are important to their fate in the ocean. Here, we analyzed bacteria‐specific molecules in deep‐sea sinking particles and found that bacterial organic matter actually made up a large fraction of the particles. In addition, the relative contribution of bacterial material to the sinking particles increased as the total carbon export decreased. This has important implications for the future ocean carbon cycle, because modeling work predicts a scenario of lower carbon export to the deep sea as the ocean warms. In this context, our findings imply a greater importance of bacteria in marine organic matter export and sequestration in a warming ocean. Key Points We exploit d‐amino acid biomarkers in multi‐year deep‐sea sediment trap time series to evaluate bacterial contribution to total export Bacterial detritus accounts for up to 19 ± 8% of sinking POC and up to 36 ± 14% of PN, making up a large unrecognized part of biological pump The relative contribution of bacterial detritus to sinking particles increases with decreased export production

We use observations from the Southern Ocean (SO) biogeochemical profiling float array to quantify the meridional pattern of particle export efficiency (PEeff) during the austral productive season. Float estimates reveal a pronounced latitudinal gradient of PEeff, which is quantitatively supported by a compilation of existing ship‐based measurements. Relying on complementary float‐based estimates of distinct carbon pools produced through biological activity, we find that PEeff peaks near the region of maximum particulate inorganic carbon sinking flux in the polar antarctic zone, where net primary production (NPP) is the lowest. Regions characterized by intermediate NPP and low PEeff, primarily in the subtropical and seasonal ice zones, are generally associated with a higher fraction of dissolved organic carbon production. Our study reveals the critical role of distinct biogenic carbon pool production in driving the latitudinal pattern of PEeff in the SO. Plain Language Summary Microbial organisms in seawater transform carbon dioxide into different types of carbon through photosynthesis and food web cycling. These carbon types include particulate and dissolved phases, with particles being more efficiently transferred out of the sunlit ocean via gravitational sinking. The ratio of sinking particulate organic carbon to total organic carbon production, commonly referred to as the particle export efficiency, is a metric used to describe how efficiently carbon moves from the surface to the deep ocean. Using observations from a large array of robots in the Southern Ocean, we find that the different types of biogenic carbon produced control the latitudinal gradient in particle export efficiency, which is highest in regions where particulate inorganic carbon export is greatest, even when photosynthetically fixed carbon is minimal. In other areas where phytoplankton carbon production is moderate but largely comprised of dissolved organic carbon, the particle export efficiency is lower. Key Points Meridional pattern of particle export efficiency (PEeff) estimated from BGC‐Argo aligns with ship‐based observations in the Southern Ocean Low PEeff in subtropical and ice‐covered regions and high PEeff in subpolar regions is linked to the biogenic carbon pools produced Most global models struggle to reproduce the meridional pattern of PEeff in the Southern Ocean

Diatoms sustain the marine food web and contribute to the export of carbon from the surface ocean to depth. They account for about 40% of marine primary productivity and particulate carbon exported to depth as part of the biological pump. Diatoms have long been known to be abundant in turbulent, nutrient-rich waters, but observations and simulations indicate that they are dominant also in meso- and submesoscale structures such as fronts and filaments, and in the deep chlorophyll maximum. Diatoms vary widely in size, morphology and elemental composition, all of which control the quality, quantity and sinking speed of biogenic matter to depth. In particular, their silica shells provide ballast to marine snow and faecal pellets, and can help transport carbon to both the mesopelagic layer and deep ocean. Herein we show that the extent to which diatoms contribute to the export of carbon varies by diatom type, with carbon transfer modulated by the Si/C ratio of diatom cells, the thickness of the shells and their life strategies; for instance, the tendency to form aggregates or resting spores. Model simulations project a decline in the contribution of diatoms to primary production everywhere outside of the Southern Ocean. We argue that we need to understand changes in diatom diversity, life cycle and plankton interactions in a warmer and more acidic ocean in much more detail to fully assess any changes in their contribution to the biological pump.

Although plastic is ubiquitous in marine systems, our current knowledge of transport mechanisms is limited. Much of the plastic entering the ocean sinks; this is intuitively obvious for polymers such as polystyrene (PS), which have a greater density than seawater, but lower density polymers like polyethylene (PE) also occur in sediments. Biofouling can cause large plastic objects to sink, but this phenomenon has not been described for microplastics <5 mm. We incubated PS and PE microplastic particles in estuarine and coastal waters to determine how biofouling changes their sinking behavior. Sinking velocities of PS increased by 16% in estuarine water (salinity 9.8) and 81% in marine water (salinity 36) after 6 weeks of incubation. Thereafter sinking velocities decreased due to lower water temperatures and reduced light availability. Biofouling did not cause PE to sink during the 14 weeks of incubation in estuarine water, but PE started to sink after six weeks in coastal water when sufficiently colonized by blue mussels Mytilus edulis, and its velocity continued to increase until the end of the incubation period. Sinking velocities of these PE pellets were similar irrespective of salinity (10 vs. 36). Biofilm composition differed between estuarine and coastal stations, presumably accounting for differences in sinking behavior. We demonstrate that biofouling enhances microplastic deposition to marine sediments, and our findings should improve microplastic transport models.

The sinking of organic particles formed in the photic layer is a main vector of carbon export into the deep ocean. Although sinking particles are heavily colonized by microbes, so far it has not been explored whether this process plays a role in transferring prokaryotic diversity from surface to deep oceanic layers. Using Illumina sequencing of the 16S rRNA gene, we explore here the vertical connectivity of the ocean microbiome by characterizing marine prokaryotic communities associated with five different size fractions and examining their compositional variability from surface down to 4,000 m across eight stations sampled in the Atlantic, Pacific, and Indian Oceans during the Malaspina 2010 Expedition. Our results show that the most abundant prokaryotes in the deep ocean are also present in surface waters. This vertical community connectivity seems to occur predominantly through the largest particles because communities in the largest size fractions showed the highest taxonomic similarity throughout the water column, whereas free-living communities were more isolated vertically. Our results further suggest that particle colonization processes occurring in surface waters determine to some extent the composition and biogeography of bathypelagic communities. Overall, we postulate that sinking particles function as vectors that inoculate viable particle-attached surface microbes into the deep-sea realm, determining to a considerable extent the structure, functioning, and biogeography of deep ocean communities.

Representing glacial‐interglacial changes in ocean carbon sequestration remains a major challenge for Earth System Models (ESMs). Uncertainties in ocean circulation and biological carbon export are essential causes for model‐data mismatch. We quantify the impact of these factors by calibrating the Max Planck Institute‐ESM. A shallower and weaker glacial Atlantic Meridional Overturning Circulation (AMOC) than the present day, achieved by decreasing background vertical diffusivity, enables capturing the main features in observed δ13C${\\delta }^{13}\\mathrm{C}$ , 14C$\\mathrm{C}$and CO32−${\\mathrm{C}\\mathrm{O}}_{3}^{2-}$ . A prognostic sinking scheme responding to changing dust deposition and biological production enhances glacial ocean organic matter export efficiency in high latitudes. Together, the shallower AMOC and a comprehensive sinking scheme substantially improve model‐data comparison and carbon storage in the glacial Atlantic but not in the Pacific and Indian oceans, mainly due to reduced primary production. Our results yield that representing glacial‐interglacial ocean carbon storage in ESMs requires both constraining ocean circulation and improved biogeochemical processes. Plain Language Summary The ocean contains about 60 times as much carbon as the atmosphere. The exchange of carbon between the ocean and the atmosphere is a major driver of global climate change. During the last ice age, atmospheric CO2${\\text{CO}}_{2}$concentrations were much lower, and the climate was much colder than today. Much evidence suggests that the ocean sequestered more carbon at the last ice age. Understanding this behavior of the ocean is vital to gaining knowledge about how the natural processes change atmospheric CO2${\\text{CO}}_{2}$and to better predict future atmospheric CO2${\\text{CO}}_{2}$and climate. Here, we use an Earth System Model to examine how the physical and biogeochemical processes in the ocean affected deep ocean carbon storage during the last ice age. We showcase that simulating a shallower Atlantic Meridional Overturning Circulation at the last ice age than it is today is crucial to reproducing proxy data observed from marine sediment cores. A more comprehensive representation of marine snow, a shower of biological debris from the upper ocean to the depth, and its sinking speeds also have a significant impact on the deep ocean carbon storage at the last ice age. Key Points A shallow and weak glacial Atlantic Meridional Overturning Circulation in Max Planck Institute Earth System Model allows the representation of the main features in δ13C, 14C$\\mathrm{C}$ ‐based ventilation age and CO32−${\\mathrm{C}\\mathrm{O}}_{3}^{2-}$data A prognostic computation of marine aggregate sinking speeds enhances the efficiency of organic matter export in high latitudes at the Last Glacial Maximum The shallow AMOC and prognostic marine aggregate sinking speeds enhance carbon storage in the glacial Atlantic

It is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes. However, the observational evidence that the strong turbulent fluxes generated by small-scale mixing in the stratified ocean interior are more vigorous close to the ocean bottom boundary than above implies that small-scale mixing converts light waters into denser ones, thus driving a net sinking of abyssal waters. Using a combination of theoretical ideas and numerical models, it is argued that abyssal waters upwell along weakly stratified boundary layers, where small-scale mixing of density decreases to zero to satisfy the no density flux condition at the ocean bottom. The abyssal ocean meridional overturning circulation is the small residual of a large net sinking of waters, driven by small-scale mixing in the stratified interior above the bottom boundary layers, and a slightly larger net upwelling, driven by the decay of small-scale mixing in the boundary layers. The crucial importance of upwelling along boundary layers in closing the abyssal overturning circulation is the main finding of this work.

Dead zooplankton, including crustaceans, are increasingly recognized as important agents of vertical carbon export from surface waters and in marine food webs. Quantifying the contribution of passively sinking copepods (PSCs) to vertical fluxes of total particulate organic carbon (POC) is important for understanding marine ecosystem carbon budgets. Information on this is limited because identifying PSCs in sediment trap samples is difficult. Generally, swimmers (undecomposed metazoans, including PSCs, caught in sediment traps) are removed from a trap sample before the POC content is measured, although ignoring PSCs causes the total POC flux to be significantly underestimated. We quantified temporal and regional variability in PSC flux and con tribution of PSCs to total POC flux (PSCs + detrital sinking particles, generally analyzed to estimate detrital POC flux) at the Mackenzie Shelf margins in the Beaufort Sea. Six datasets were used to examine PSC flux variability at ~100 m depth, which is deeper than the winter pycnocline depth (30−50 m), at the continental margin. The average (±SD) annual PSC flux (1378 ± 662 mg C m−2 yr−1, n = 6 [datasets]) and PSC contribution to the total POC flux (21 ± 10%, n = 6) suggested that PSCs, especially Pareuchaeta glacialis, were important agents of POC export from the surface layer (~100 m) to deeper water at the inter-regional and multiyear scales. We propose a hypothesis that processes controlling PSC flux variability may vary seasonally, perhaps relating to life cycle (reproduction) in winter (February) and osmotic stress in July−October when the PSC flux is relatively high.

Particulate organic carbon (POC) generated by primary production and exported to depth, is an important pathway for carbon transfer to the abyss, where it is stored over climatically significant timescales. These processes constitute the biological carbon pump. A spectrum of particulate sinking velocities exists throughout the water column, however numerical models often simplify this spectrum into suspended, fast and slow sinking particles. Observational studies suggest the spectrum of sinking speeds in the ocean is strongly bimodal with >85% POC flux contained within two pools with sinking speeds of <10 m day−1 and >350 m day−1. We deployed a Marine Snow Catcher (MSC) to estimate the magnitudes of the suspended, fast and slow sinking pools and their fluxes at the Porcupine Abyssal Plain site (48°N, 16.5°W) in summer 2009. The POC concentrations and fluxes determined were 0.2 μg C L−1 and 54 mg C m−2 day−1 for fast sinking particles, 5 μg C L−1 and 92 mg C m−2 day−1 for slow sinking particles and 97 μg C L−1 for suspended particles. Our flux estimates were comparable with radiochemical tracer methods and neutrally buoyant sediment traps. Our observations imply: (1) biomineralising protists, on occasion, act as nucleation points for aggregate formation and accelerate particle sinking; (2) fast sinking particles alone were sufficient to explain the abyssal POC flux; and (3) there is no evidence for ballasting of the slow sinking flux and the slow sinking particles were probably entirely remineralised in the twilight zone. Key Points Fast and slow sinking POC contributes 40% and 60% of export Slow sinking POC is not ballasted and remineralized in the twilight zone Fast sinking POC supplies the abyss and is at least partially ballasted