The North Atlantic is a major hotspot for the uptake, accumulation, and storage of anthropogenic carbon (Canth), processes that are closely linked to the Atlantic Meridional Overturning Circulation (AMOC). However, the role of ocean transport in driving this accumulation remains poorly constrained, leading to uncertainty in future carbon uptake and circulation changes under climate forcing. CMIP6 models reasonably reproduce observed Canth transport at RAPID-MOCHA (26.5°N in the North Atlantic). Our results show that northward Canth transport increases under SSP1-2.6, SSP2-4.5, and SSP5-8.5, despite a concomitant weakening of AMOC-related volume transport. This strengthening occurs because increases in Canth concentration outweigh reductions in circulation strength. The western boundary current serves as the primary pathway for northward Canth transport and is particularly sensitive to future change. A time-of-emergence analysis indicates that trends in Canth transport become detectable within approximately 25–35 years, with earlier emergence under higher-emission scenarios. The RAPID observational record, that began in 2004, is approaching the detectability threshold for Canth transport trends, but it remains too short to robustly resolve long-term changes in AMOC strength, whose emergence timescale is substantially longer.

The Southern Ocean (SO) is a critical component of the global carbon cycle, acting as a significant sink for atmospheric carbon dioxide (CO2). Understanding the intricate processes governing CO2 uptake in the SO is paramount for comprehending the global carbon budget and predicting future climate scenarios. Recent observations suggest that changes in SO water masses, driven by climate-induced alterations in temperature and circulation patterns, can significantly impact CO2 uptake. Understanding these feedbacks is crucial for predicting the SO's future role as a carbon sink and its broader implications for climate mitigation efforts. In this work, we determine changes in the water mass composition and their characteristics, including their CO2 content, along the CUSTARD transect (54ºS-59ºS 90ºW) in Subantarctic Pacific waters. The CUSTARD transect crosses a region of formation of mode and intermediate waters. We use an extended Optimum Multiparameter (eOMP) analysis and data from three repeats of the CUSTARD transect in 1993 (expocode 316N19930222; data from GLODAPv2.2023), 2005-2006 (316N20050821 and 316N20060130; from GLODAPv2.2023), and 2019-2020 (74EQ20191202; the CUSTARD cruise). We observe isopycnal heaving in the southern part of the transect from 1993 to 2020. In the upper ocean (neutral density (γn) < 27.2 kg m-3), isopycnal heaving is linked to a temperature decrease of up to -2ºC and a salinity decrease of up to -0.15 between 1993 and 2005, extending to γn < 27.5 kg m-3 in 2019-2020. The physicochemical changes in the upper ocean are linked to changes in the water mass composition, including an increase in the volume of Antarctic Surface Water and Antarctic Intermediate Water and a decrease in the volume of SubAntarctic Mode Water over the 18-year study period. These water mass changes are accompanied by decreases in concentrations of oxygen, dissolved nutrients, and total alkalinity, along with an increase in total dissolved inorganic carbon of up to 40 µmol kg-3 for γn < 27.5 kg m-3 from 1993 to 2019-2020. For 27.5 kg m-3 < γn 28.2 kg m-3), Ross Sea Bottom Water replaces Adélie Bottom Water from 1993 to 2019-2020. The changes in water mass composition observed along the CUSTARD transect indicate circulation variations linked to the Southern Annular Mode (SAM), with a prevalent positive phase since 1995. Positive SAM pahses increase upwelling south of the Antarctic Polar Front and downwelling in the Subantarctic Zone. Due to these circulation changes, the SO’s uptake of atmospheric CO2 decreases during positive SAM phases, which are predicted to intensify with climate change.

The export of elements (particularly carbon, nitrogen and phosphorus) from the Poles critically supports global marine biodiversity and major fisheries as well as the sequestration of atmospheric carbon to the deep ocean. Ecosystem processes regulate this export, but major uncertainties remain in terms of how and by how much. Progress on understanding key ecosystem interactions is hindered by lack of data and their representation in Earth system models is poor. The two polar regions share similarities in environmental extremes which make them sensitive to the impacts of climate change. They both receive nutrients from multiple and diverse sources and the delivery of these nutrients to other oceans is regulated by similar ecosystem processes. However, the extent to which these ecosystem processes will be modified by climate change is unclear and urgently needs to be determined. BIOPOLE will determine how polar ecosystems regulate the balance of carbon and nutrients in the world’s oceans and, through it, their effect on global fish stocks and carbon storage. It will address this challenge by integrating ambitious fieldwork campaigns and innovative modelling in a multidisciplinary and highly coordinated approach. BIOPOLE will capitalise on world-leading capabilities and infrastructure in ocean and high-latitude research, including cutting-edge land-based facilities, state-of-the-art polar research vessels and innovative autonomous instrumentation. Collaboration with national and international partners will further strengthen BIOPOLE’s multidisciplinary approach and efficient use of infrastructure. BIOPOLE’s legacy will be the first assessment of the global impact of polar ecosystems on biogeochemical cycling and fish stocks; technologically-novel approaches and strong partnerships between leading international science groups.

Abstract The ocean biological carbon pump (BCP) plays a pivotal role in the global carbon cycle. The BCP magnitude is determined by the fraction of nutrients utilised in biological production and remineralised at depth, with the remainder being subducted into the interior unused as ‘preformed’ nutrients. This fraction is currently around 50% and subject to the interaction of biological processes and global scale circulation. Consequently, changes in circulation can potentially impact biological carbon storage. Here we provide observational evidence that the reduction in the Atlantic Meridional Overturning Circulation (AMOC) that occurred over the 2004-2018 period has been accompanied by substantial changes in nutrient transports and associated carbon storage. Persistent southward net nutrient transport across 26.5°N exceeded nutrient sources, except by the end of the period when the system approached balance. This transient net loss of nutrients from the North Atlantic was accompanied by increases in the ratio of remineralized to preformed nutrients, indicating an increasing BCP efficiency (and carbon storage). Our results thus demonstrate observable transient changes in large scale nutrient transports linked to AMOC changes over interannual - decadal timescales, with implications for future ocean carbon storage.

The eastern subpolar North Atlantic is a source of nutrients to the Northwest European Shelf and Arctic; however, biogeochemical transports in this important region are unknown. We examine variability in nutrients and carbon at the eastern boundary of the subpolar North Atlantic between 2017 and 2020, and calculate their transport by a branch of the North Atlantic Current and the European Slope Current. By combining observations from moorings and ship-based surveys, we derive novel biogeochemical property transports at high temporal resolution. Data from 63 m provide new evidence of a strong seasonal signal with silicate declining between April and May (-2.3 mu mol kg-1) and a concurrent increase in pH (0.04) and oxygen saturation (3.5%). Additionally, pH and oxygen saturation show a secondary peak in October during the autumn bloom. Biogeochemical transports are northwards and highly variable with volume transport dominating the variability over a multi-annual timescale. However, historical data suggests that nitrate and phosphate transports were 15% and 19% lower respectively in the late 2000s when the subpolar gyre circulation was weaker and lower nutrient source waters were dominant. These changes may have been amplified by concurrent reductions in volume transport. Changes in carbon and nutrient transports in the eastern subpolar North Atlantic may propagate downstream with potential effects on the Northwest European Shelf and Eurasian Arctic. Water flowing northward in the eastern subpolar North Atlantic transports nutrients onto the Northwest European Shelf and to the Arctic Ocean. However, biogeochemical transports through the region remain unknown. We examine variability in nutrients and carbon at the eastern boundary of the subpolar North Atlantic and calculate their transport by two important northward flowing currents. We use observations from ship-based surveys and instruments that recorded conditions in the ocean over 3 years to derive biogeochemical transports every 12 hr between May 2017 and October 2020. Transports of nutrients and carbon in the eastern subpolar North Atlantic are northwards although the transport is highly variable. Historical data suggests that when circulation around the subpolar North Atlantic is weaker, nutrient transports are around 15%-20% lower due to changes in the water nutrient concentrations. These changes may be amplified by concurrent reductions in current strength and volume transport. Changes in carbon and nutrient transports in the eastern subpolar North Atlantic may propagate downstream with potential effects on the Northwest European Shelf and Arctic Ocean. Silicate decreases and pH and oxygen increase from April to May with oxygen and pH showing a secondary peak in October Nutrient and carbon transports through the eastern boundary of the subpolar North Atlantic are northward and highly variable Biogeochemical transport magnitude may vary on multi-annual timescales and have downstream impacts

Some of the highest rates of primary production across the Southern Ocean occur in the seasonal ice zone (SIZ), making this a prominent area of importance for both local ecosystems and the global carbon cycle. There, the annual advance and retreat of ice impacts light and nutrient availability, as well as the circulation and stratification, thereby imposing a dominant control on phytoplankton growth. In this study, the drivers of variability in phytoplankton growth between 2002–2020 in the Weddell Gyre SIZ were assessed using satellite net primary production (NPP) products alongside chlorophyll-a and particulate organic carbon (POC) data from autonomous biogeochemical floats. Although the highest daily rates of NPP are consistently observed in the continental shelf region (water depths shallower than 2000 m), the open-ocean region's larger size and longer ice-free season mean that it dominates biological carbon uptake within the Weddell Gyre, accounting for 93 %–96 % of the basin's total annual NPP. Variability in the summer maximum ice-free area is the strongest predictor of inter-annual variability in total NPP across the Weddell Gyre, with greater ice-free area resulting in greater annual NPP, explaining nearly half of the variance (R2=42 %). In the shelf region, the return of sea ice cover controls the end of the productive season. In the open ocean, however, both satellite NPP and float data show that a decline in NPP occurs before the end of the ice-free season (∼ 80 to 130 d after sea ice retreat). Evidence of concurrent increases in float-observed chlorophyll-a and POC suggest that later in the summer season additional factors such as micro-nutrient availability or top-down controls (e.g. grazing) could be limiting NPP. These results indicate that in a warmer and more ice-free Weddell Gyre, notwithstanding compensating changes in nutrient supply, NPP is likely to be enhanced only up to a certain limit of ice-free days.

Coral reefs are subject to degradation by multiple environmental stressors which are predicted to intensify. Stress can alter ecosystem composition, with shifts from hard coral to macroalgae dominated reefs often accompanied by an increase in soft corals and sponges. Such changes may alter net ecosystem metabolism and biogeochemistry by shifting the balance between photosynthesis, respiration, calcification and dissolution. We deployed high temporal resolution pH and dissolved oxygen (DO) sensors at four Caribbean reef sites with varying covers of hard and soft corals, sponges and macroalgae. The resultant data indicated that the strength of the “metabolic pulse”, specifically the co‐variation in daily pH and DO oscillations, was driven by the net balance of light ‐dependent and ‐independent metabolism. pH and DO were positively correlated over the diel cycle at coral dominated sites, suggesting that photosynthesis and respiration were the major controlling processes, and further indicated by agreement with a simple production:respiration model. Whereas, at a site with high macroalgal cover, pH and DO decoupling was observed during daylight hours. This indicates that an unidentified light‐driven process altered the expected pH:DO relationship. We hypothesize that this could be mediated by the higher levels of macroalgae, which either stimulated bacterial‐mediated carbonate dissolution via the production and release of allelopathic compounds or retained oxygen, evolved during photosynthesis, in the gaseous form in seawater (ebullition). Our work demonstrates that high resolution monitoring of pH and DO provides insight into coral reef biogeochemical functioning and can be key for understanding long‐term changes in coral reef metabolism.

Abstract. Abrupt changes in ocean biogeochemical variables occur as a result of human-induced climate forcing as well as those which are more gradual and occur over longer timescales. These abrupt changes have not yet been identified and quantified to the same extent as the more gradual ones. We review and synthesise abrupt changes in ocean biogeochemistry under human-induced climatic forcing. We specifically address the ocean carbon and oxygen cycles because the related processes of acidification and deoxygenation provide important ecosystem hazards. Since biogeochemical cycles depend also on the physical environment, we also describe the relevant changes in warming, circulation, and sea ice. We include an overview of the reversibility or irreversibility of abrupt marine biogeochemical changes. Important implications of abrupt biogeochemical changes for ecosystems are also discussed. We conclude that there is evidence for increasing occurrence and extent of abrupt changes in ocean biogeochemistry as a consequence of rising greenhouse gas emissions.

Abstract. Some of the highest rates of primary production across the Southern Ocean occur in the seasonal ice zone (SIZ), making this area of prominent importance for both local ecosystems and the global carbon cycle. There, the annual advance and retreat of ice impact light and nutrient availability, as well as the circulation and stratification, imposing a dominant control on phytoplankton growth. In this study, the drivers and variability of net primary production (NPP) in the Weddell Gyre SIZ were assessed using satellite and autonomous biogeochemical float data. Although the highest daily rates of NPP are consistently observed in the continental shelf region (water depths shallower than 2000 m), the open ocean region's larger size and longer ice-free season mean that it dominates biological carbon uptake within the Gyre, accounting for 95 % of the Gyre's total annual NPP. Variability in the summer maximum ice-free area is the strongest predictor of inter-annual variability in total NPP across the Weddell Gyre (R2 = 62 %), with greater ice-free area resulting in greater annual NPP. In the shelf region, the return of sea-ice cover controls the end of the productive season. In the open ocean, however, a decline in NPP occurs before the end of the ice-free season (∼80 to 120 days after sea-ice retreat), suggesting that, later in the summer season, additional factors such as nutrient availability limit NPP. These results indicate that in a warmer and more ice-free Weddell Gyre, notwithstanding compensating changes in nutrient supply, NPP is likely to be enhanced only up to a certain limit of ice-free days.

In the Subantarctic Zone (SAZ) of the southeast Pacific, the densest, coolest, and freshest Subantarctic Mode Water (SAMW) is formed. There, water masses reset their physicochemical characteristics interchanging properties with the atmosphere, and play a critical role in global climate through their impact on the overturning circulation and oceanic heat and carbon uptake. We estimate the magnitude, variability and uncertainty of the air-sea flux of oxygen from five years of hourly observations around the Observatories Initiative (OOI) Southern Ocean mooring. The magnitude of oxygen fluxes depends greatly on the parameterization used, particularly for high wind events. Hence, there is a need for validation of the high wind speed regime at high latitudes. Surface waters remain undersaturated from autumn to mid-spring, when most of the annual oxygen uptake occurs. We calculate a total annual flux into the ocean of -12.6 ± 3.4 mol m-2 yr-1, with a thermal component of -10.3 ± 2.6 mol m-2 yr-1 and a non-thermal component of -1.0 ± 0.3 mol m-2 yr-1. These results provide the first estimate of oxygen fluxes for the region from high-frequency observations, surpassing previous estimates for the entire SAZ by one order of magnitude.

The Mesoamerican Barrier Reef, the second largest barrier reef in the world, is vitally important to the ecology and economy of Belize and neighbouring countries. Coral reefs are inherently vulnerable to ocean acidification and those exposed to significant riverine input may be under enhanced threat. In tropical rivers pCO2 levels may be linked to land use in their catchment, with conversion of pristine forest to agricultural land potentially enhancing carbon flux to the coastal ocean. We investigated the effect the Belize River may have on the carbonate chemistry of surrounding coastal ocean, applying a multidisciplinary methodology. Water samples were collected and measured for: total alkalinity; dissolved inorganic carbon (DIC); and δ13CDIC; and were complemented by pH and pCO2 data acquired from sensors mounted on an autonomous surface vehicle. Samples were collected from the source of the Belize river to the mouth and out past the barrier reef. pCO2 measuring >1000-µatm at the mouth of the Belize River suggests local high levels of respiration and low pH water being discharged into the coastal ocean. δ13CDIC samples were taken to identify terrestrial DIC signatures and used in combination with sensor data to identify potential controls on coastal pH. There was a distinct difference in δ13CDIC along the river with a range of 27 ‰ to - 13‰, while coastal δ13CDIC was heavier with a range of -11.5‰ to 1.5 ‰. These results demonstrate the complexity of processes that control coastal ocean acidification, which has implications for coastal economies that are heavily dependent on healthy coral reefs as a resource.

Biological processes in the subpolar Southern Ocean play a crucial role in the global carbon cycle, mediating CO2 exchange between the atmosphere and the densest waters of the global ocean. While historical perspectives have centred the importance of shelf-sea regions, recent reframing emphasises the role of the open ocean, and the cyclonic gyres. Here, we investigate the operation of the biological carbon pump (BCP) in the Weddell Gyre using satellite ocean colour and bio-Argo floats. We find first that a significant proportion (>54 %) of the inter-annual variability in NPP was explained by the area of open (ice-free) water. Spatial patterns suggest that peak productivity is associated with the ice edge. The seasonal decline in NPP occurs before ice cover returns, suggesting that other controls are limiting annual NPP (e.g. the exhaustion of iron). Comparing the shelf region to the open ocean, the shelf was seen to have higher rates of productivity, but NPP in the relatively less productive open ocean region accounted for ~95% of total carbon uptake each year. The total NPP in the Weddell Gyre (97-197 Tg C yr-1) is sufficient to supply the BCP-derived carbon that was previously observed to be exported from the region in Circumpolar Deep Water (~80 Tg C yr-1). NPP in the open ocean Weddell Gyre could thus provide the major source of carbon exported from the Weddell Gyre to the deep ocean via the horizontal circulation.

Whilst anthropogenic activities are significantly altering the climate, both warming the atmosphere and increasing CO2, the ocean is significantly ameliorating both effects. This effect is so important that the transient climate response to carbon emissions (TCRE), can be formulated primarily in terms of the ocean. We show that in direct analogy to the TCRE, Anthropogenic Carbon (Canth) and temperature increases in the ocean are linearly related, both globally and integrated over a range of scales. These ocean responses are typically of order 0.02K/mumol/kg, (equivalently ~80MJ/mol). This linear relation allows for direct translation between temperature and carbon inventory increases. Furthermore, we are far better able to decompose DIC changes into Canth increases and that of other carbon pools, than we are decomposing heat inventory changes into added and redistributed heat. By separating total DIC change into Canth and that of other carbon pools, we can therefore remove the effect of the transient response relationship between heat and carbon. This allows the production of estimates of added and redistributed heat in the ocean from remaining DIC changes. Our results suggest that the variability of the transient response is predominately set by heat uptake, not carbon, and that this variability may be traced to individual water masses. Therefore, it may be necessary to separate this transient response regionally in order to obtain accurate estimates of added and redistributed heat at a global scale using this technique. The Eulerian transient response is set predominantly by isotherm heave. The part of the transient response set by climate sensitivity, analogous to a semi-Lagrangian approach, is set largely by patterns of regional heat uptake.

The interface between land and sea is a key environment for biogeochemical carbon cycling, yet these dynamic environments are traditionally under sampled. Logistical limitations have historically precluded a comprehensive understanding of coastal zone processes, including ocean acidification. Using sensors on autonomous platforms is a promising approach to enhance data collection in these environments. Here, we evaluate the use of an autonomous surface vehicle (ASV), the C-Worker 4 (CW4), equipped with pH and pCO2 sensors and with the capacity to mount additional sensors for up to 10 other parameters, for the collection of high-resolution data in shallow coastal environments. We deployed the CW4 on two occasions in Belizean coastal waters for 2.5 and 4 days, demonstrating its capability for high-resolution spatial mapping of surface coastal biogeochemistry. This enabled the characterisation of small-scale variability and the identification of sources of low pH/high pCO2 waters as well as identifying potential controls on coastal pH. We demonstrated the capabilities of the CW4 in both pre-planned “autonomous” mission mode and remote “manually” operated mode. After documenting platform behaviour, we provide recommendations for further usage, such as the ideal mode of operation for better quality pH data, e.g., using constant speed. The CW4 has a high power supply capacity, which permits the deployment of multiple sensors sampling concurrently, a shallow draught, and is highly controllable and manoeuvrable. This makes it a highly suitable tool for observing and characterising the carbonate system alongside identifying potential drivers and controls in shallow coastal regions.

Wide-angle seismic data from the Eastern Black Sea have been used to determine the geological structure of the sediments, the entire crust, and upper mantle. Data were acquired using a combination of ocean-bottom seismometers (OBS), land seismometers, and a marine air-gun source, providing refracted and reflected energy recorded to offsets in excess of 100 km.

We present new data that explores the link between pore pressure and seismic velocity to estimate the magnitude of the overpressure within the deep sediments of the Eastern Black Sea basin. New wide-angle seismic data, combined with coincident reflection data, have been modelled simultaneously using the seismic tomography code, Jive3D, to provide a well-constrained seismic velocity model of the sediments. Our models reveal a widespread low-velocity zone at the depth of 5.5–8.5 km, which is characterized by a velocity decrease from 3.5 to ∼2.5 km s−1. Using two separate methods that relate changes in seismic velocity to changes in effective stress, we estimate pore pressures of at least 160 MPa within the low-velocity zone. These pore pressures give λ★ values of 0.8–0.9 within the centre of the basin and above the Mid-Black Sea High. The low-velocity zone occurs within the Maikop formation, an organic-rich mud layer identified as the source of mud volcanism in the Black Sea and South Caspian Sea.

The amount of magmatism that accompanies the extension and rupture of the continental lithosphere varies dramatically at rifts and margins around the world. Based on widely spaced geophysical transects, some margins are known to preserve a transition from magmatically robust to magmatically starved rifting along strike, but the nature of the transition is unknown. Wide-angle seismic data from the Black Sea provide the first direct observations of such a transition and show that it is abrupt, occurring over only similar to 20-30 km, and coincides with a transform fault. This abrupt transition cannot be explained solely by gradual along-margin variations in mantle properties, since these would be expected to result in a smooth transition from magma-poor to magma-rich rifting over hundreds of kilometers. We suggest that the abruptness of the transition results from the development of three-dimensional (3-D) melt migration due to along-strike variations in extension and thus the thickness of the lithosphere at the time of rifting. Localized magmatic addition attributed to melt focusing has been observed in modern mid-ocean ridges and active rift environments, but here we show that such processes can also produce abrupt along-strike changes from magma-poor to magma-rich rifting.