Organic carbon remineralization rate (OCRR) and the calcium carbonate production rate (CCPR) are influential variables on the efficiency of the biological carbon pump (BCP) but are not well understood in Red Sea. We used historical cruise data of carbonate chemistry, oxygen, and transient tracers from five locations along the north–south central axis of the Red Sea to estimate OCRR and CCPR from tracer‐based water mean ages (Γ), apparent oxygen utilization (AOU), and alkalinity utilization (AU). This resulted in the first basin‐wide and depth‐resolving (100–1,000 m) OCRR and CCPR estimates. Spatial distributions for Γ, AOU, and AU were strongly influenced by the large‐scale circulation and showed maxima intermediate depths (400–500 m). Conversely, OCRR and CCPR showed no statistically significant latitudinal differences and peaked (6.5 ± 4.3 and 11.9 ± 4.6 mmol C m −3 yr −1 , respectively) at 100‐m depth, which decreased to nearly constant values (3.8 ± 0.7 and 1.4 ± 0.3 mmol C m −3 yr −1 , respectively) at 300 m and deeper. By depth‐integrating CCPR, we estimated annual calcium carbonate production (CCP) of (0.8 ± 0.3) × 10 12 mol, or 0.6% of global ocean production, in the Red Sea, which has only 0.12% of the world ocean area. High correlation between AU and Γ indicated in situ alkalinity removal taking place also in subsurface and deep waters, probably due to chemical precipitation, which has been previously reported for the area. CCP‐induced AU affects the carbonate chemistry in the Red Sea water column, and we hypothesize that it also impacts that of the Gulf of Aden through the outflowing Red Sea Outflow Water.

Due to low calcium carbonate saturation states, and winter mixing that brings anthropogenic carbon to the deep ocean, the Nordic Seas and their cold-water corals are vulnerable to ocean acidification. Here, we present a detailed investigation of the changes in pH and aragonite saturation in the Nordic Seas from preindustrial times to 2100, by using in situ observations, gridded climatological data, and projections for three different future scenarios with the Norwegian Earth System Model (NorESM1-ME). During the period of regular ocean biogeochemistry observations from 1981–2019, the pH decreased with rates of 2–3 × 10−3 yr−1 in the upper 200 m of the Nordic Seas. In some regions, the pH decrease can be detected down to 2000 m depth. This resulted in a decrease in the aragonite saturation state, which is now close to undersaturation in the depth layer of 1000–2000 m. The model simulations suggest that the pH of the Nordic Seas will decrease at an overall faster rate than the global ocean from the preindustrial era to 2100, bringing the Nordic Seas' pH closer to the global average. In the esmRCP8.5 scenario, the whole water column is projected to be undersaturated with respect to aragonite at the end of the 21st century, thereby endangering all cold-water corals of the Nordic Seas. In the esmRCP4.5 scenario, the deepest cold-water coral reefs are projected to be exposed to undersaturation. Exposure of all cold-water corals to corrosive waters can only be avoided with marginal under the esmRCP2.6 scenario. Over all timescales, the main driver of the pH drop is the increase in dissolved inorganic carbon (CT) caused by the raising anthropogenic CO2, followed by the temperature increase. Thermodynamic salinity effects are of secondary importance. We find substantial changes in total alkalinity (AT) and CT as a result of the salinification, or decreased freshwater content, of the Atlantic water during all time periods, and as a result of an increased freshwater export in polar waters in past and future scenarios. However, the net impact of this decrease (increase) in freshwater content on pH is negligible, as the effects of a concentration (dilution) of CT and AT are canceling.

The Ocean Weather Station M (OWSM) is situated at a fixed position in the Norwegian Sea, one of the major basins of the Nordic Seas, which represents an important area for uptake of atmospheric CO2 as well as deep water formation. At OWSM, the inorganic carbon cycle has been regularly monitored since 2001, and significant interannual changes of the carbonate system have been determined. Data collected at this site since the 1990s have been included, and over the 28 last years the surface fugacity of CO2 (fCO(2)) has increased by 2.92 +/- 0.37 mu atm yr(-1), while surface pH and aragonite saturation (Omega(Ar)) have decreased by -0.0033 +/- 0.0005 yr(-1) and -0.018 +/- 0.003 yr(-1), respectively. This corresponds to a surface pH change of -0.092 over 28 years, which is comparable to the global mean pH decrease of -0.1 since the onset of the industrial revolution. Our estimates suggest that 80% of the surface pH trend at OWSM is driven by uptake of CO2 from the atmosphere. In the deepest layer, Omega(Ar) has decreased significantly (-0.006 +/- 0.001 yr(-1)) over the last 28 years, now occasionally reaching undersaturated values (Omega(Ar) < 1). As a rough estimate, the saturation horizon has shoaled by 7 m yr(-1) between 1994 and 2021. The increase in surface fCO(2) is confirmed by semi-continuous measurements of CO2 from the site (2.69 +/- 0.14 mu atm yr(-1)), and thus, the area has become less of a net sink for atmospheric CO2, taking into consideration an atmospheric CO2 increase at OWSM of 2.27 +/- 0.08 mu atm yr(-1).

Towards better understanding of carbon and oxygen biogeochemical rates in the Red Sea Salma Elageed1,3 , A M. Omar2, Emil Jeansson2, Elsheikh B. Ali1 , Ingunn Skjelvan2 , Knut Barthel3 , Truls Johannessen3, P.Zhai4 1Institute of Marine Research, Red Sea University, Port Sudan, Sudan 2 NORCE, Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway 3 Geophysical Institute, University of Bergen, Bergen, Norway 4 Geoscience Dept., Princeton University, USA Abstract The Red Sea is one of the warmest and saltiest seas in the world, with surface water temperatures of 26–30°C and salinities of 36–41. The sea gains heat in the south and loses heat in the north and this gives a large-scale thermohaline circulation pattern with a northward surface flow and a southward flow at sill depth. At smaller spatial scales, along-coastal currents and upwelling occur. Here we summarise the main results from two studies that are parts of a PhD-study. We demonstrate how multi-spatial scale circulation and biological processes influence rates of: air-sea flux of carbon dioxide (CO2), oxygen utilization (OU), and removal of total alkalinity by calcification and sedimentation, i.e., alkalinity utilization (AU). In the first study, based on cruise data collected in the Red Sea in 2011 and 1982 (Aegaeo and MEROU cruises, respectively), we combine depth profiles of tracer-based water mass ages, AU, and OU to derive the first-ever basin-wide, long time integrated utilization rates of alkalinity (AUR) and oxygen (OUR). Results reveal that the large-scale circulation impacts the water masse ages and OU while remineralization of organic matter and calcification also influences in depth variations of OU and AU. The highest rates for OUR and AUR occur in the surface water followed by a swift attenuation of the rates towards zero for AUR and ~5 µmol kg-1 for OUR at 500 m depth. In the second study, new carbon and hydrography data from the Sudanese coastal Red Sea were used to investigate seasonal dynamics of sea surface partial pressure of CO2 (pCO2) and air–sea CO2 exchange. The results show that seasonal pCO2 change was primarily driven by temperature changes while along-coast advection, upwelling of CO2-rich deep water, and uptake of atmospheric CO2 also contributed to changes in dissolved inorganic carbon and total alkalinity. Furthermore, based on a compilation of historical and our new data, the region seems to have transformed from being a source of CO2 to the atmosphere throughout the year to becoming a sink of CO2 during parts of the year.

The dynamics of sea surface pCO2 (pCO2w) and air–sea CO2 exchange of the Sudanese coastal Red Sea has for the first time been studied over a full annual cycle (October 2014–October​ 2015) based on semi-continuous measurements from moored autonomous sensors. pCO2w showed a seasonal amplitude of approximately 70 μatm, overlaid by a high frequency (3-4 days) signal of around 10 μatm. The highest values, of about 440 μatm occurred during summer and fall, while the lowest values of about 370 μatm occurred during winter. The monthly pCO2w change was primarily driven by temperature, i.e., heating and cooling of the water surface. Additionally, Dissolved Inorganic Carbon (DIC) and Total Alkalinity (AT) contributed significantly to the observed change in pCO2w as a consequence of along-coast advection and upwelling of CO2-rich deep water, and likely biological production, and uptake of atmospheric CO2. The area is a net annual source for atmospheric CO2 of 0.180 ± 0.009 mol CO2 m−2 y−1. Based on a compilation of historic and our new data, altogether covering the years 1977 to 2015, long term trends of pCO2w were determined for the seasons winter–spring (1.75 ± 0.72 μatm y−1) and summer -fall (180 ± 0.41 μatm y−1), both weaker than the atmospheric trend (1.96 ± 0.02 μatm y−1). We are suggesting that the study region has transformed from being a source of CO2 to the atmosphere throughout the year to becoming a sink of CO2 during parts of the year. The long term pCO2w trend was to a large degree driven by increasing DIC, but increasing AT and temperature also played a role.

The H2020 project Integrated Arctic Observation System (INTAROS) aspires to increase the temporal and geographic coverage of in situ observations and add new key geophysical and biogeochemical variables in selected regions of the Arctic. By using a combination of mature and new instruments and sensors in integration with existing observatories, INTAROS aims to fill selected gaps in the present-day system and build additional capacity of the Arctic monitoring networks for ocean and sea ice. Three reference sites have been selected as key locations for monitoring ongoing Arctic changes: Costal Greenland, paramount for freshwater output from the Greenland ice sheet; North of Svalbard (covering the region from shelf to deep basin) - the hot-spot for ocean-air-sea ice interactions, and heat and biological energy input to the European Arctic; and Fram Strait - the critical gateway for exchanges between the Arctic and the World oceans. The existing observatories in the reference sites have been extended with new moorings and novel autonomous instrumentation, in particular for biogeochemical measurements and sea ice observations. Bottom-mounted instruments have been also implemented for seismic observations. A distributed observatory for ocean and sea ice in the Arctic Ocean and sub-Arctic seas includes non-stationary components such as ice-tethered observing platforms, float, gliders, and ships of opportunities, collecting multidisciplinary observations, still missing from the Arctic regions. New sensors, integrated platforms and experimental set-ups are currently under implementation during a two-year long deployment phase (2018-2020) with an aim to evaluate their sustained use in a future iAOS. New observations will be used for integration of new data products, demonstration studies and stakeholder consultations, contributing also to ongoing and future long-term initiatives (e.g. SAON).

Research Infrastructures (RIs) are large-scale facilities encompassing instruments, resources, data and services used by the scientific community to conduct high-level research in their respective fields. The development and integration of marine environmental RIs as European Research Vessel Operators [ERVO] (2020) is the response of the European Commission (EC) to global marine challenges through research, technological development and innovation. These infrastructures (EMSO ERIC, Euro-Argo ERIC, ICOS-ERIC Marine, LifeWatch ERIC, and EMBRC-ERIC) include specialized vessels, fixed-point monitoring systems, Lagrangian floats, test facilities, genomics observatories, bio-sensing, and Virtual Research Environments (VREs), among others. Marine ecosystems are vital for life on Earth. Global climate change is progressing rapidly, and geo-hazards, such as earthquakes, volcanic eruptions, and tsunamis, cause large losses of human life and have massive worldwide socio-economic impacts. Enhancing our marine environmental monitoring and prediction capabilities will increase our ability to respond adequately to major challenges and efficiently. Collaboration among European marine RIs aligns with and has contributed to the OceanObs’19 Conference statement and the objectives of the UN Decade of Ocean Science for Sustainable Development (2021–2030). This collaboration actively participates and supports concrete actions to increase the quality and quantity of more integrated and sustained observations in the ocean worldwide. From an innovation perspective, the next decade will increasingly count on marine RIs to support the development of new technologies and their validation in the field, increasing market uptake and produce a shift in observing capabilities and strategies.

Abstract. Being windows to the deep ocean, the Nordic Seas play an important role in transferring anthropogenic carbon, and thus ocean acidification, to the abyss. Due to its location in high latitudes, it is further more sensitive to acidification compared with many other oceanic regions. Here we make a detailed investigation of the acidification of the Nordic Seas, and its drivers, since pre-Industrial to 2100 by using in situ measurements, gridded climatological data, and simulations from one Earth System Model (ESM). In the last 40 years, pH has decreased by 0.11 units in the Nordic Seas surface waters, a change that is twice as large as that between 1850–1980. We find that present trends are larger than expected from the increase in atmospheric CO2 alone, which is related to a faster increase in the seawater pCO2 compared with that of the atmosphere, i.e. a weakening of the pCO2 undersaturation of the Nordic Seas. The pH drop, mainly driven by an uptake of anthropogenic CO2, is significant all over the Nordic Seas, except for in the Barents Sea Opening, where it is counteracted by a significant increase in alkalinity. We also find that the acidification signal penetrates relatively deep, in some regions down to 2000 m. This has resulted in a significant decrease in the aragonite saturation state, which approaches undersaturation at 1000–2000 m in the modern ocean. Future scenarios suggest an additional drop of 0.1–0.4 units, depending on the emission scenario, in surface pH until 2100. In the worst case scenario, RCP8.5, the entire water column will be undersaturated with respect to aragonite by the end of the century, threatening Nordic Seas cold-water corals and their ecosystems. The model simulations suggest that aragonite undersaturation can be avoided at depths where the majority of the cold-water corals live in the RCP2.6 and RCP4.5 scenarios. As these results are based on one model only, we request additional observational and model studies to better quantify the transfer of anthropogenic CO2 to deep waters and its effect on future pH in the Nordic Seas.

1Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway 2Max Planck Institute for Meteorology, Hamburg, Germany 3NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway 4Institute of Marine Research, Fram Centre, Tromsø, Norway 5 Norwegian Polar Institute, Tromsø, Norway 6Marine and Freshwater Research Institute, Reykjavík, Iceland

ICOS-Oceans is the marine domain of the European Research Infrastructure Consortium “Integrated Carbon Observation System” (ICOS). It aims at delivering high quality greenhouse gas (GHG) observations and derived data products (e.g. regional GHG-flux maps) for constraining the GHG balance on a European level, on a sustained long-term basis. ICOS-Oceans currently consists of 11 Ship of Opportunity lines (SOOP – Ship of Opportunity Program) and 10 Fixed Observation Stations (FOS) spread across European waters, including the North Atlantic Ocean and the Barents, North, Baltic and Mediterranean Seas. The stations operate in a harmonised and standardised way based on community-proven protocols and methods for ocean GHG observations improving operational conformity as well as quality control and assurance of the data. This enables the network to focus on long term research into the marine carbon cycle and the anthropogenic carbon sink, while preparing the network to include other GHG fluxes. ICOS data are processed on a near real-time basis and will be published on the ICOS Carbon Portal, allowing monthly estimates of CO2 air-sea exchange to be quantified for European waters. ICOS establishes transparent operational data management routines following the FAIR (Findable, Accessible, Interoperable and Reusable) guiding principles allowing amongst others reproducibility, interoperability and traceability. The ICOS-Oceans network is actively integrating with the atmospheric (e.g. improved atmospheric measurements onboard SOOP lines) and ecosystem (e.g. oceanic direct gas flux measurements) domains of ICOS, and utilises techniques developed by the ICOS Central Facilities and the Carbon Portal. There is a strong interaction with the international ocean carbon cycle community to enhance interoperability and harmonise data flow. The future vision of ICOS-Oceans includes ship-based ocean survey sections to obtain a 3-dimensional understanding of marine carbon cycle processes and optimise the existing network design.

This study evaluates the trends in anthropogenic carbon (C-ant) in the different sub-basins of the Arctic Ocean between 1987 and 2015. Data were extracted from the GLODAPv2 data product as well as two GEOTRACERS cruises in the Arctic Ocean from 2015 and C-ant was evaluated using the transient time distribution (TTD) approach. In the Nansen and Amundsen sub-basins, the C-ant trend in the Atlantic Waters (AW, depths: 200-500 m) and dense AW (dAW, depths: 500-800 m) is about + 0.7 mu mol kg(-1) yr(-1). As we move into the Makarov and West Canadian sub-basins, the C-ant trend in the AW and dAW is smaller. The upper Polar Deep Water (uPDW, depths: 800-1600 m) has a C-ant trend of about +0.4-0.5 mu mol kg(-1) yr(-1) in the Nansen, Amundsen and West Canadian sub-basins. The trend is smallest in the South Canadian sub-basin, with a value of about + 0.2 mu mol kg(-1) yr(-1). Ventilation primarily governs C(ant )trends while the influence of the Revelle factor plays a secondary role. The increase in the C-ant column inventory is estimated to be 0.7-1.0 mol C m(-2) yr(-1) in the Nansen, Amundsen and Makarov sub-basins. By extrapolating the results from our defined sub-basins to the full Arctic Ocean, we estimate an C-ant accumulation of 25 Tg C yr(-1) in the Arctic Ocean and an inventory of about 3.6-3.9 Pg C between 2005 and 2015.

Shelf seas play an important role in the global carbon cycle, absorbing atmospheric carbon dioxide (CO2) and exporting carbon (C) to the open ocean and sediments. The magnitude of these processes is poorly constrained, because observations are typically interpolated over multiple years. Here, we used 298500 observations of CO2 fugacity (fCO2) from a single year (2015), to estimate the net influx of atmospheric CO2 as 26.2 ± 4.7 Tg C yr−1 over the open NW European shelf. CO2 influx from the atmosphere was dominated by influx during winter as a consequence of high winds, despite a smaller, thermally-driven, air-sea fCO2 gradient compared to the larger, biologically-driven summer gradient. In order to understand this climate regulation service, we constructed a carbon-budget supplemented by data from the literature, where the NW European shelf is treated as a box with carbon entering and leaving the box. This budget showed that net C-burial was a small sink of 1.3 ± 3.1 Tg C yr−1, while CO2 efflux from estuaries to the atmosphere, removed the majority of river C-inputs. In contrast, the input from the Baltic Sea likely contributes to net export via the continental shelf pump and advection (34.4 ± 6.0 Tg C yr−1).