The climatology of the deep particle flux in the oligotrophic western North Atlantic gyre, 1978–2022

Pedrosa‐Pamies R., Conte M.H., Weber J.C., Andersson A.J.

AbstractTropical cyclones erode and remobilize coastal sediments but their impact on the deep ocean remains unclear. Hurricane‐driven transport of carbonates and associated materials from reef carbonate platforms to the deep ocean has important implications for carbon storage, deep ecosystems and ocean chemistry as carbonate platform reef‐sourced aragonite and high‐Mg calcite (HMC) may dissolve and contribute to deep water total alkalinity. Here we describe two hurricane‐driven resuspension events where deep sediment plumes from the Bermuda Pedestal (NW Atlantic) were advected to deep waters surrounding the Oceanic Flux Program (OFP) mooring site, ∼75 km southeast of Bermuda. Hurricanes Fabian (Cat. 3, 2003) and Igor (Cat. 1, 2010) generated large near‐inertial waves propagating to >750 m depths, leading to widespread sediment resuspension from the Pedestal. Following Fabian, carbonate fluxes at the OFP site increased 15‐fold, 32‐fold, and 6‐fold at 500, 1,500 and 3,200 m, respectively, with the 1,500 m flux equivalent to the total annual carbonate flux. OFP traps similarly captured a large detrital carbonate plume following Igor; here, the plume was shallower and persisted longer. Microscopy, geochemistry, and mineralogy confirmed that both plumes consisted of fine‐grained shallow‐water detrital carbonates alongside other materials accumulated on the Pedestal including phosphorus, lithogenic, authigenic, and pollutant elements. Clay‐sized particles (<4 μm) in both plumes exhibited high contents of lithogenic and authigenic elements, and Zn, Cd, and V, facilitating their transport over long distances. Grain‐size, elemental, and lipid composition indicated that plumes intercepted at different depths originated from different source areas on the Pedestal.

Dunne J.P.

AbstractAs ocean Carbon Dioxide Removal (CDR) techniques are being considered, it is critical that they be evaluated against our scientific understanding of the global biological carbon pump. In a recent paper in Global Biogeochemical Cycles entitled, "Quantifying the Carbon Export and Sequestration Pathways of the Ocean's Biological Carbon Pump", Nowicki et al (2022, GBC) provide an innovative and comprehensive breakdown of the different mechanistic pathways of carbon sequestration through the present‐day biological pump but then speculate that "These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.” Essentially, the authors combine their steady state result that oligotrophic subtropical gyres have lower residence times than other areas with the climate change result of these areas increasing under climate warming and extrapolate ‐assuming “all else is equal” – that the overall ocean will suffer a reduction in carbon sequestration efficiency. Expressing global changes in carbon sequestered by the ocean’s biological pump as the summation of local changes in the sequestered carbon, timescale of return to the surface, and biogeographical area, I discuss how all three terms are tightly coupled, and summarize decades of climate change modeling consistently indicating that the global scale physical sequestration response is an increase ‐ in opposition of what one would infer from changes in subtropical area alone.This article is protected by copyright. All rights reserved.

Lampitt R.S., Briggs N., Cael B.B., Espinola B., Hélaouët P., Henson S.A., Norrbin F., Pebody C.A., Smeed D.

The time series of downward particle flux at 3000 m at the Porcupine Abyssal Plain Sustained Observatory (PAP-SO) in the Northeast Atlantic is presented for the period 1989 to 2018. This flux can be considered to be sequestered for more than 100 years. Measured levels of organic carbon sequestration (average 1.88 gm−2 y−1) are higher on average at this location than at the six other time series locations in the Atlantic. Interannual variability is also greater than at the other locations (organic carbon flux coefficient of variation = 73%). We find that previously hypothesised drivers of 3,000 m flux, such as net primary production (NPP) and previous-winter mixing are not good predictors of this sequestration flux. In contrast, the composition of the upper ocean biological community, specifically the protozoan Rhizaria (including the Foraminifera and Radiolaria) exhibit a close relationship to sequestration flux. These species become particularly abundant following enhanced upper ocean temperatures in June leading to pulses of this material reaching 3,000 m depth in the late summer. In some years, the organic carbon flux pulses following Rhizaria blooms were responsible for substantial increases in carbon sequestration and we propose that the Rhizaria are one of the major vehicles by which material is transported over a very large depth range (3,000 m) and hence sequestered for climatically relevant time periods. We propose that they sink fast and are degraded little during their transport to depth. In terms of atmospheric CO2 uptake by the oceans, the Radiolaria and Phaeodaria are likely to have the greatest influence. Foraminifera will also exert an influence in spite of the fact that the generation of their calcite tests enhances upper ocean CO2 concentration and hence reduces uptake from the atmosphere.

Belcher A., Henley S.F., Hendry K., Wootton M., Friberg L., Dallman U., Wang T., Coath C., Manno C.

Abstract. The biological carbon pump is responsible for much of the decadal variability in the ocean carbon dioxide (CO2) sink, driving the transfer of carbon from the atmosphere to the deep ocean. A mechanistic understanding of the ecological drivers of particulate organic carbon (POC) flux is key both to the assessment of the magnitude of the ocean CO2 sink and for accurate predictions as to how this will change with changing climate. This is particularly important in the Southern Ocean, a key region for the uptake of CO2 and the supply of nutrients to the global thermocline. In this study we examine sediment-trap-derived particle fluxes and stable isotope signatures of carbon (C), nitrogen (N), and biogenic silica (BSi) at a study site in the biologically productive waters of the northern Scotia Sea in the Southern Ocean. Both deep (2000 m) and shallow (400 m) sediment traps exhibited two main peaks in POC, particulate N, and BSi flux: one in austral spring and one in summer, reflecting periods of high surface productivity. Particulate fluxes and isotopic compositions were similar in both deep and shallow sediment traps, highlighting that most remineralisation occurred in the upper 400 m of the water column. Differences in the seasonal cycles of isotopic compositions of C, N, and Si provide insights into the degree of coupling of these key nutrients. We measured increasing isotopic enrichment of POC and BSi in spring, consistent with fractionation during biological uptake. Since we observed isotopically light particulate material in the traps in summer, we suggest physically mediated replenishment of lighter isotopes of key nutrients from depth, enabling the full expression of the isotopic fractionation associated with biological uptake. The change in the nutrient and remineralisation regimes, indicated by the different isotopic compositions of the spring and summer productive periods, suggests a change in the source region of material reaching the traps and associated shifts in phytoplankton community structure. This, combined with the occurrence of advective inputs at certain times of the year, highlights the need to make synchronous measurements of physical processes to improve our ability to track changes in the source regions of sinking particulate material. We also highlight the need to conduct particle-specific (e.g. faecal pellets, phytoplankton detritus, zooplankton moults) isotopic analysis to improve the use of this tool in assessing particle composition of the sinking material and to develop our understanding of the drivers of biogeochemical fluxes.

Couret M., María Landeira J., Santana del Pino Á., Hernández-León S.

Mesozooplankton have been widely used as a bioindicator of marine ecosystems due to their key position in ocean food webs, rapid response to environmental changes, and ubiquity. Here, we show mesozooplankton biomass values in the Canary Current System from 1971 to 2021 in three different areas in relation to mesoscale activity: (1) scarcely affected by mesoscales structures (North of the Canary Islands), (2) affected by mesoscale activity and the presence of the islands (South and around the islands), and (3) close to the Northwest African coastal upwelling system (Upwelling influenced). A Generalized Additive Mixed Model (GAMM) was used to analyze the general mesozooplankton biomass trend throughout the studied period discriminating differences in biomass between the areas, annual cycle, and day-nighttime periods. The GAMM showed a significant negative biomass tendency North of the Canary Islands over the 50-year time-series compared to the South and around the islands, and significant differences between day and nighttime periods (p < 0.001) and the annual cycle (p < 0.0001). Linear regression analyses showed different tendencies depending on the area, season, and period. When comparing biomass data of the most oligotrophic zone (north of the islands) with other tropical-subtropical time-series stations in Hawaii (HOTS) and Bermuda (BATS), we obtained increasing biomass tendencies for both fixed time stations but decreasing tendency for our time-series.

Salter I., Bauerfeind E., Fahl K., Iversen M.H., Lalande C., Ramondenc S., Von Appen W.-., Wekerle C., Nöthig E.-.

The Fram Strait connects the Atlantic and Arctic Oceans and is a key conduit for sea ice advected southward by the Transpolar Drift and northward inflow of warm Atlantic Waters. Continued sea ice decline and “Atlantification” are expected to influence pelagic–benthic coupling in the Fram Strait and Arctic as a whole. However, interannual variability and the impact of changing ice conditions on deepwater particle fluxes in the Arctic remain poorly characterized. Here, we present long-term sediment trap records (2000–2013) from mesopelagic (200 m) and bathypelagic (2,300 m) depths at two locations (HGIV and HGN) in the Fram Strait subjected to variable ice conditions. Sediment trap catchment areas were estimated and combined with remote sensing data and a high-resolution model to determine the ice cover, chlorophyll concentration, and prevailing stratification regimes. Surface chlorophyll increased between 2000 and 2013, but there was no corresponding increase in POC flux, suggesting a shift in the efficiency of the biological carbon pump. A decrease in particulate biogenic Si flux, %opal, Si:POC, and Si:PIC at mesopelagic depths indicates a shift away from diatom-dominated export as a feasible explanation. Biogenic components accounted for 72% ± 16% of mass flux at 200 m, but were reduced to 34% ± 11% at 2,300 m, substituted by a residual (lithogenic) material. Total mass fluxes of biogenic components, including POC, were higher in the bathypelagic. Biomarkers and ∂13C values suggest both lateral advection and ice-rafted material contribute to benthic carbon input, although constraining their precise contribution remains challenging. The decadal time series was used to describe two end-members of catchment area conditions representing the maximum temperatures of Atlantic inflow water in 2005 at HGIV and high ice coverage and a meltwater stratification regime at HGN in 2007. Despite similar chlorophyll concentrations, bathypelagic POC flux, Si flux, Si:POC, and Si:PIC were higher and POC:PIC was lower in the high-ice/meltwater regime. Our findings suggest that ice concentration and associated meltwater regimes cause higher diatom flux. It is possible this will increase in the future Arctic as meltwater regimes increase, but it is likely to be a transient feature that will disappear when no ice remains.

Henson S.A., Briggs N., Carvalho F., Manno C., Mignot A., Thomalla S.

The biological carbon pump (BCP) contributes to the oceanic CO2 sink by transferring particulate organic carbon (POC) into the deep ocean. The magnitude and efficiency of the BCP is likely to vary on timescales of days to seasons, however characterising this variability from shipboard observations is challenging. High resolution, sustained observations of primary production and particle fluxes by autonomous vehicles offer the potential to fill this knowledge gap. Here we present a 4 month, daily, 1 m vertical resolution glider dataset, collected in the high productivity bloom, downstream of South Georgia, Southern Ocean. The dataset reveals substantial temporal variability in primary production, POC flux and attenuation. During the pre-bloom peak phase we find high export efficiency, implying minimal heterotrophic POC consumption, i.e. productivity is decoupled from upper ocean remineralisation processes. As the bloom progresses from its peak through its declining phase, export flux decreases, but transfer efficiency within the upper 100 m of the mesopelagic increases. Conversely, transfer efficiency in the lower mesopelagic decreases in the post-bloom phase, implying that the flux attenuation processes operating in the upper and lower mesopelagic are effectively decoupled. This finding underscores an important limitation of using a single parameter, such as Martin's ‘b’, to characterise POC flux attenuation in a given location or season. Frequent pulses of export flux are observed throughout the deployment, indicating decoupling between primary production and the processes driving export of material from the upper ocean. The mechanisms underlying the observed seasonal changes in BCP magnitude and efficiency are unclear, as temperature and oxygen concentration changed minimally, although the nature of the sinking particles changed substantially as the bloom progressed. Our results highlight the difficulty of capturing temporal variability and episodic flux events with traditional shipboard observations, which affects our conceptual understanding of the BCP. The increasing use of autonomous vehicles to observe particle fluxes will be essential to characterising the temporal variability in magnitude and functioning of the BCP.

Siegel D.A., DeVries T., Cetinić I., Bisson K.M.

The biological pump transports organic matter, created by phytoplankton productivity in the well-lit surface ocean, to the ocean's dark interior, where it is consumed by animals and heterotrophic microbes and remineralized back to inorganic forms. This downward transport of organic matter sequesters carbon dioxide from exchange with the atmosphere on timescales of months to millennia, depending on where in the water column the respiration occurs. There are three primary export pathways that link the upper ocean to the interior: the gravitational, migrant, and mixing pumps. These pathways are regulated by vastly different mechanisms, making it challenging to quantify the impacts of the biological pump on the global carbon cycle. In this review, we assess progress toward creating a global accounting of carbon export and sequestration via the biological pump and suggest a potential path toward achieving this goal. Expected final online publication date for the Annual Review of Marine Science, Volume 15 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Wang H.-., Merz R., Yang S., Tarasova L., Basso S.

Flow events with low frequency often cause severe damage, especially if their magnitudes are higher than suggested by historical observations. The heavier right tail of streamflow distribution indicates the increasing probability of high flows. In this paper, we investigate the role played by spatially variable rainfall in enhancing the tail heaviness of streamflow distributions. We synthetically generated a wide range of spatially variable rainfall inputs and fed them to a continuous probabilistic model of the catchment water transport to simulate streamflow in five German catchments with distinct properties in size and topography. Meanwhile, we used a comparable approach to analyze rainfall and runoff records from 175 German catchments. We identified the effects of spatially variable rainfall on the tails of streamflow distributions from both simulation scenarios and data analyses. Our results show that the tail of streamflow distribution becomes heavier with increasing spatial rainfall variability only beyond a certain threshold. This finding indicates the capability of catchments to buffer growing heterogeneities of rainfall, which we term catchment resilience to increasing spatial rainfall variability. The analyses suggest that the runoff routing through the river network controls this property. In fact, both small and elongated catchments are less resilient to increasing spatial rainfall variability due to their intrinsic runoff routing characteristics. We show the links between spatial rainfall characteristics and catchment geometry and the possible occurrence of high flows. The data analyses we performed on a large set of case studies confirm the simulation results and provide confidence for the transferability of these findings.

Merz B., Basso S., Fischer S., Lun D., Blöschl G., Merz R., Guse B., Viglione A., Vorogushyn S., Macdonald E., Wietzke L., Schumann A.

Macdonald E., Merz B., Guse B., Wietzke L., Ullrich S., Kemter M., Ahrens B., Vorogushyn S.

In some catchments, the distribution of annual maximum streamflow shows heavy tail behavior, meaning the occurrence probability of extreme events is higher than if the upper tail decayed exponentially. Neglecting heavy tail behavior can lead to an underestimation of the likelihood of extreme floods and the associated risk. Partly contradictory results regarding the controls of heavy tail behavior exist in the literature and the knowledge is still very dispersed and limited. To better understand the drivers, we analyze the upper tail behavior and its controls for 480 catchments in Germany and Austria over a period of more than 50 years. The catchments span from quickly reacting mountain catchments to large lowland catchments, allowing for general conclusions. We compile a wide range of event and catchment characteristics and investigate their association with an indicator of the tail heaviness of flood distributions, namely the shape parameter of the GEV distribution. Following univariate analyses of these characteristics, along with an evaluation of different aggregations of event characteristics, multiple linear regression models as well as random forests are constructed. A novel slope indicator, which represents the relation between the return period of flood peaks and event characteristics, captures the controls of heavy tails best. Variables describing the catchment response are found to dominate the heavy tail behavior, followed by event precipitation, flood seasonality and catchment size. The pre-event moisture state in a catchment has no relevant impact on the tail heaviness even though it does influence flood magnitudes.

Michaud C.A., Huffard C.L., Smith K.L., Durkin C.A.

Large episodic pulses of particulate organic carbon (POC) at the deep-sea (~ 4000 m) time-series Sta. M in the Northeast Pacific Ocean have increased in frequency and magnitude over the past 32 years. We inferred the ecological drivers of these events by quantifying the phytoplankton and biomineral composition within particles collected by bottom-moored sediment traps immediately before, during, and after 14 high-flux events. Samples collected during high-flux events contained a significantly different phytoplankton community from other sampling periods. These particles contained relatively fewer intact phytoplankton cells and a sustained contribution from fragmented diatom frustules from species typical in coastal blooms. Biomineral fluxes did not appear to be driving high-flux events. We suggest that most of the observed high-flux events were generated by offshore transport of coastal diatom blooms, but that these particles were also highly transformed by deep-sea pelagic food webs before reaching bathypelagic depths.

Henson S.A., Laufkötter C., Leung S., Giering S.L., Palevsky H.I., Cavan E.L.

The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long-term storage of photosynthetically fixed carbon. This ‘biological carbon pump’ is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by ~200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including important fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, Earth system models disagree on the projected response of the global export flux to climate change, with estimates ranging from −41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritizes the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterization, and hence improved model parameterization, of export flux and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean. A synthesis of recent work on marine carbon export fluxes finds that many processes that are key to understanding the effects of a warming climate on ocean carbon cycling are missing from current climate models.

DeVries T.

The ocean is one of the most important sinks for anthropogenic CO2 emissions. Here, I use an ocean circulation inverse model (OCIM), ocean biogeochemical models, and pCO2 interpolation products to examine trends and variability in the oceanic CO2 sink. The OCIM quantifies the impacts of rising atmospheric CO2, changing sea surface temperatures, and gas transfer velocities on the oceanic CO2 sink. Together, these effects account for an oceanic CO2 uptake of 2.2 ± 0.1 PgC yr−1 from 1994 to 2007, and a net increase in the oceanic carbon inventory of 185 PgC from 1780 to 2020. However, these effects cannot account for the majority of the decadal variability shown in data-based reconstructions of the ocean CO2 sink over the past 30 years. This implies that decadal variability of the ocean CO2 sink is predominantly driven by changes in ocean circulation or biology that act to redistribute both natural and anthropogenic carbon in the ocean.

Nowicki M., DeVries T., Siegel D.A.

The ocean's biological carbon pump transfers carbon from the surface ocean to the deep ocean by several distinct pathways, including gravitational settling of organic particles, mixing and advection of suspended organic carbon, and active transport by vertically migrating metazoans. Carbon exported by these pathways can be sequestered as respired CO2 in the deep ocean for years to centuries. However, the contribution of each pathway to carbon export and sequestration remains highly uncertain. Here, satellite and in situ ocean biogeochemical observations are assimilated in an ensemble numerical model of the biological pump to quantify global and regional carbon export and sequestration. The ensemble mean global carbon export is 10.2 Pg C yr−1 and the total amount of carbon sequestered via the biological pump is 1,300 Pg C. The gravitational pump is responsible for 70% of the total global carbon export, 85% of which is zooplankton fecal pellets and 15% is sinking phytoplankton aggregates, while migrating zooplankton account for 10% of total export and physical mixing is responsible for the remaining 20%. These pathways have different sequestration times, with an average of 140 years for the gravitational pump, 150 years for the migrant pump, and only 50 years for the mixing pump. Regionally, the largest sequestration inventories and longest sequestration times are found in the northern high latitudes, while the shortest sequestration times are found in the subtropical gyres. These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.

Pedrosa‐Pamies R., Conte M.H., Weber J.C., Andersson A.J.

AbstractTropical cyclones erode and remobilize coastal sediments but their impact on the deep ocean remains unclear. Hurricane‐driven transport of carbonates and associated materials from reef carbonate platforms to the deep ocean has important implications for carbon storage, deep ecosystems and ocean chemistry as carbonate platform reef‐sourced aragonite and high‐Mg calcite (HMC) may dissolve and contribute to deep water total alkalinity. Here we describe two hurricane‐driven resuspension events where deep sediment plumes from the Bermuda Pedestal (NW Atlantic) were advected to deep waters surrounding the Oceanic Flux Program (OFP) mooring site, ∼75 km southeast of Bermuda. Hurricanes Fabian (Cat. 3, 2003) and Igor (Cat. 1, 2010) generated large near‐inertial waves propagating to >750 m depths, leading to widespread sediment resuspension from the Pedestal. Following Fabian, carbonate fluxes at the OFP site increased 15‐fold, 32‐fold, and 6‐fold at 500, 1,500 and 3,200 m, respectively, with the 1,500 m flux equivalent to the total annual carbonate flux. OFP traps similarly captured a large detrital carbonate plume following Igor; here, the plume was shallower and persisted longer. Microscopy, geochemistry, and mineralogy confirmed that both plumes consisted of fine‐grained shallow‐water detrital carbonates alongside other materials accumulated on the Pedestal including phosphorus, lithogenic, authigenic, and pollutant elements. Clay‐sized particles (<4 μm) in both plumes exhibited high contents of lithogenic and authigenic elements, and Zn, Cd, and V, facilitating their transport over long distances. Grain‐size, elemental, and lipid composition indicated that plumes intercepted at different depths originated from different source areas on the Pedestal.