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Ferromanganese concretions commonly occur in shallow‐water coastal regions worldwide. In the Baltic Sea, they can record information about past and present underwater environments and could be a potential source for critical raw materials. We report on their microstructural characteristics and magnetic properties and link them to their formation mechanisms and environmental significance. Microstructural investigations from nano‐ and micro‐computed tomography, electron microscopy, and micro‐X‐ray fluorescence elemental mapping reveal diverse growth patterns within concretions of different morphologies. Alternating Fe‐ and Mn‐rich growth bands indicate fluctuating redox conditions during formation. Bullet‐shaped magnetofossils, produced by magnetotactic bacteria, are present, which suggests the influence of bacterial activity on concretion formation. Spheroidal concretions, which occur in deeper and more tranquil environments, have enhanced microbial biomineralization and magnetofossil preservation. Conversely, crusts and discoidal concretions from shallower and more energetic environments contain fewer magnetofossils and have a greater detrital content. Our results provide insights into concretion formation mechanisms and highlight the importance of diagenetic processes, oxygen availability, and bacterial activity in the Baltic Sea. Key Points: Magnetic minerals within shallow water Fe‐Mn concretions can provide valuable environmental information about their formationMagnetic properties are linked to specific growth patterns in Baltic Sea Fe‐Mn concretionsSpheroidal and crust/discoidal concretions are dominated by biogenic and pedogenic magnetic phases, respectively [ABSTRACT FROM AUTHOR]

On the Northwest European Shelf rivers provide freshwater to the coastal seas. This coastal freshwater can be misrepresented in ocean models without effective coastal resolution. This leaves an unanswered question; is freshwater retained around Scotland and what affects its variability? Here, we deploy and run an unstructured model with enhanced coastal resolution to simulate the Northwest European Shelf from 1993 to 2019, the Scottish Shelf Water‐Reanalysis Service (SSW‐RS) long‐time run. The unstructured nature of the model grid means it more accurately captures a "bubble" of Coastal Water than a 7 km structured grid model (the Atlantic Margin Model 7 km). Surface salinity in the SSW‐RS shows salinity fronts within 80 km of the coast around west and north Scotland that disintegrates east of Orkney. There are periods characterized by high coastal salinity when freshwater is more actively advected away from the coast. Empirical orthogonal function statistical analysis shows the first two modes in surface salinity account for 66% of the variance. The first mode correlates with North Atlantic Oscillation and the salinity driven velocity variability which change the salinity through advection and diffusion. The second mode correlates with Ekman transport variability where the north of Scotland acts as a wedge causing bipolar dynamics either side. Freshwater is trapped in the west, while saline water from the north reduces the freshwater pathway to the North Sea. This is important for salinity distribution, stratification in the North Sea, marine habitats and frontal transport. Plain Language Summary: The outflow of riverwater into the sea around Scotland is important for marine habitats, nutrient concentrations and the transport of pollution or planktonic larvae. The response of the sea to the sum of all river outflow is often missed in simulations of the ocean and leaves questions about how much water is retained near the coast and when it can escape. In our ocean simulation the Scottish Shelf Water‐Reanalysis Service long run, we tackle this problem with high coastal resolution and an extensive river data set. Our simulation shows an improvement in salinity over a coarser resolution model of the same region around Scotland. Generally fresh, river‐sourced water is retained by a front within 80 km of the coast around Scotland to the west of Orkney but this pattern is disrupted to the east of Orkney. Statistics show the majority of the variability in the freshwater around Scotland goes through stages at the time period of years when is it captured close to the coast and stages when it is more able to move away from the coast. This is found to correlate with density driven currents and the wind speed which uses the north of Scotland as a wedge for the water, causing bipolar dynamics either side. Key Points: Our simulation accurately resolves coastal freshwater due to its variable resolutionA coastal salinity front is found west of Orkney with periodic release of its freshwater east of OrkneyThe movement of freshwater into and away from the coastal region is influenced by salinity driven currents and wind [ABSTRACT FROM AUTHOR]

Over the satellite record, the Barents Sea winter maximum in sea ice extent has declined and was increasingly limited to areas north of the Polar Front after 2005 by warming Atlantic Water (AW) and Barents Sea Water (BSW). Sea ice extent here continues to garner interest, not least because it is associated with extreme winter weather in Europe and Asia. Previous model studies suggest there is a possibility that natural variability will cause southward re‐expansion of the lost sea ice cover but reducing uncertainties requires a better understanding of the processes driving BSW variability. To address questions about BSW variability, we used a high‐resolution model validated with observations over 1985–2014 to calculate the watermass transport, heat, and freshwater budgets within the central Barents Sea, south of the Polar Front. The model shows BSW volume minima events in years centering at 1990 and 2004, meaning a reduction in the Barents Sea's volume reservoir (also termed "memory") of water that is consistent with historical BSW properties. Both events were preceded by extensive winter sea ice and substantial summer net sea ice melt. The event in 2004 was more extreme and led to warming AW occupying a greater volume in the Barents Sea after 2005. The reduced "memory" of BSW volume could impede a return to the more extensive winter sea ice regime and make further reduction in winter sea ice possible. Plain Language Summary: Winter sea ice in the Barents Sea, in the eastern Arctic Ocean, has been in decline, particularly since 2000. The sea ice extent in this region is associated with atmospheric weather conditions during winter in Europe and Asia. An important factor in understanding the sea ice retreat is understanding the variability in Barents Sea temperature and salinity. Using a high‐resolution simulation, the source of variability in Barents Sea Water (BSW) properties has been quantified. This shows two events, 1990 and 2004, when BSW volume reduced, giving less "memory" of previous conditions. The event in 2004 was larger and followed a large net sea ice melt. The volume of BSW present in the Barents Sea did not fully recover after 2005. This could make it more difficult for winter sea ice to return to the region. Key Points: We identify events of minimum dense water volume in the Barents Sea, with the 2002–2005 event being unique with large sea ice importDuring this event, a freshwater anomaly from sea ice melt enhanced the salinity gradient, reducing dense water formation and exportAfter the event in 2006–2014, the proportion of dense water present in the southern Barents Sea remained smaller than during 1985–2002 [ABSTRACT FROM AUTHOR]

Ocean deoxygenation threatens ocean productivity, carbon cycling and marine ecosystems. Shelf seas are highly dynamic regions, which contributes to their high productivity and also makes monitoring and constraining their oxygen status a challenge. Here, using the temperate Celtic shelf sea (April and July 2015) as a case study, we present high‐resolution ocean glider observations of turbulence and biogeochemical parameters, demonstrating the potential of these autonomous platforms for environmental monitoring. We estimate vertical turbulent oxygen fluxes be 25% higher in summer than in spring, due to the presence of subsurface chlorophyll and associated oxygen maxima at the seasonal thermocline. We demonstrate that glider‐based estimates were able to constrain similar bottom layer respiration rates as those derived from traditional ship‐based measurements. We suggest ocean gliders are useful monitoring tools that can aid sustainable management of shelf sea ecosystems. Plain Language Summary: Oxygen levels in the ocean are decreasing. Oxygen is needed by almost all life in the oceans, thus low oxygen levels can result in dramatic changes to marine ecosystems. The decrease in oxygen levels is particularly alarming in the coastal ocean or "shelf sea" (the region between the land and the deep open ocean), which supports the majority of global fisheries (over 90%). Therefore, there is both an urgent societal and an environmental need to better understand processes influencing oxygen levels in the coastal ocean, such as physical water circulation and mixing, and biological oxygen production and consumption. Here we present turbulent mixing data collected over 40 days in a typical shelf sea using an unmanned, autonomous underwater vehicle (AUV) called an ocean glider. We use this data combined with oxygen data to calculate the contribution of physical oxygen fluxes to the observed change in oxygen, and from this deduce how much of the change was driven by biology. We prove that AUVs may be used as an effective method for monitoring oxygen dynamics and that this can aid responsible marine management in shelf seas. Key Points: Glider observations are able to integrate physical and biogeochemical processes influencing oxygen depletion in shelf seasMixing and advection replenish respired oxygen in the bottom mixed layerThe presence of a subsurface oxygen maxima results in larger oxygen fluxes to the bottom mixed layer in summer compared to spring [ABSTRACT FROM AUTHOR]