Mark Schmidt

Offshore freshened groundwater (OFG) is a significant yet underexplored component of global water resources, occurring widely along many continental margins. However, its dynamics remain poorly constrained due to limited offshore observations and the absence of robust evaluation models for unambiguous emplacement interpretation. To address this knowledge gap, we integrate existing porewater data and geological constraints from the Canterbury Bight with paleo‐hydrogeochemical modeling to investigate the origin, evolution, and biogeochemical processes related to OFG driven by sea‐level fluctuations. Our simulation results indicate that modern recharge from onshore aquifers contributes only a limited fraction of the present OFG. The majority of OFG has mean groundwater ages of less than ∼300 kyr, suggesting that it mainly originates from local meteoric recharge during sea‐level lowstands since the late Pleistocene. OFG volume and paleo‐submarine groundwater discharge along the continental shelf have varied periodically in response to sea‐level fluctuations throughout glacial‐interglacial cycles. The Canterbury Bight OFG system is currently in a non‐steady state, undergoing gradual salinization by downward intrusion from overlying seawater at an estimated rate of 0.06 cm yr ⁻¹ . The long‐term evolution of the OFG system has increased the complexity of biogeochemical processes within shelf sediments through freshwater–seawater mixing. By integrating paleo‐hydrogeological reconstruction, geological archives, and reactive transport modeling, this study provides a comprehensive framework for understanding OFG formation and evolution, advancing beyond previous models focused primarily on salinity.

Sediment resuspension of blue carbon ecosystems (e.g., seagrass beds) and muddy sediments exposes buried particulate organic carbon to oxygenated waters and remineralization, potentially enhancing carbon dioxide fluxes. However, the kinetics of carbon degradation under oxic and anoxic conditions are poorly constrained. We report the results of incubation experiments with sediments from Kiel Bight to simulate sediment resuspension events induced by natural and anthropogenic resuspension in this area. A numerical model determined that oxic carbon remineralization rates were up to two-fold higher than those under anoxic conditions. A coupled sediment-water column model demonstrated that pyrite oxidation, rather than carbon oxidation, has the potential to induce large carbon dioxide emissions to the atmosphere following anthropogenic sediment disturbance by trawling. Upscaling to muddy areas of Kiel Bight suggests an annual emission of up to ~14 k tonnes of carbon dioxide per year. Pyrite oxidation may contribute to a weakening of the carbon shelf pump and a reduction of anthropogenic carbon dioxide uptake.

Recent studies have proposed calcite and dunite as possible alkaline materials for enhanced benthic weathering in shallow depocenters of the Baltic Sea as a marine carbon dioxide removal strategy. In this study, insights on calcite and dunite weathering from laboratory incubations and long-term benthocosm experiments are combined with a numerical box-model to assess the carbon dioxide uptake potential of mineral addition to organic-rich sediments in the southwest Baltic Sea. The results reveal that calcite has an up to 10-fold higher carbon dioxide uptake efficiency and is therefore the preferable material for enhanced benthic weathering as a marine carbon dioxide removal method, with costs per tonne of sequestered carbon dioxide ranging between 82 and 462 euro for calcite while reaching 558–1920 euro for dunite. These findings could be applicable to other areas in the Baltic Sea and also globally to sediments in the wider coastal shelf with similar geochemical properties.

Natural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (<50 m). Their formation has previously been linked to methane seepage from the seafloor. Here, we characterize over 50,000 pockmarks based on their morphology, geochemical signature, and the subsurface pre‐conditions using a new integrated geoscientific data set, combining geophysical and sedimentological data with geochemical porewater and oceanographic analysis. We test whether the methane seepage is indeed responsible for pockmark formation. However, our data suggest that neither the seepage of light hydrocarbons nor groundwater is driving pockmark formation. Because of this lack of evidence for fluid seepage, we favor the previously suggested biotic formation but also discuss positive feedback mechanisms in ocean bottom currents as a formation process. Based on a comparison of pockmarks to the central and southeastern North Sea, we find that local lithology significantly affects pockmark morphology. Muddy lithologies favor the formation of larger, long‐lived structures, while sandy lithologies lead to short‐lived, small‐scale structures that are large in area but with shallow incision depth. We conclude that pockmarks in sandy environments might have been overlooked globally due to their shallow incision depth and recommend reevaluating the role of hydrocarbon ebullition in pockmark formation.

The simulation of deep‐sea conditions in laboratories is technically challenging but necessary for experiments that aim at a deeper understanding of physiological mechanisms or host‐symbiont interactions of deep‐sea organisms. In a proof‐of‐concept study, we designed a recirculating system for long‐term culture (>2 yr) of deep‐sea mussels Gigantidas childressi (previously Bathymodiolus childressi). Mussels were automatically (and safely) supplied with a maximum stable level of ~60 μmol L⁻¹ methane in seawater using a novel methane–air mixing system. Experimental animals also received daily doses of live microalgae. Condition indices of cultured G. childressi remained high over the years, and low shell growth rates could be detected, too, which is indicative of positive energy budgets. Using stable isotope data, we demonstrate that G. childressi in our culture system gained energy, both, from the digestion of methane‐oxidizing endosymbionts and from digesting particulate food (microalgae). Limitations of the system, as well as opportunities for future experimental approaches involving deep‐sea mussels, are discussed.

Cruise AL575 is fully embedded in the project GEOSTOR, which is part of the DAM mission “Marine Carbon Sinks in Decarbonization Pathways”. The investigations contribute to the assessment of the mechanisms and potential risks of CO2 leakage along boreholes drilled through the sedimentary overburden above subseafloor storage formations in the North Sea. A total of 50 abandoned wells were investigated in the British sector of the North Sea with the ship's single- and multi-beam echosounder systems, and at 4 selected boreholes showing hydroacoustic gas release the gases, water column and sediments were sampled and gas fluxes were quantified during ROV dives. In addition, the 9 abandoned wells in the GEOSTOR study area in the central part of the German North Sea sector were surveyed hydroacoustically. Biogenic methane from shallow gas accumulations in the upper 1000 m of the sediment strata is emitted at the abandoned wells. It rises to the seafloor along a migration pathway introduced by the drilled borehole. The cruise allowed to enlarge the dataset on gas leakage at abandoned wells collected during previous North Sea cruises and increase our understanding of the underlying mechanism that serves as an analog for CO2 leakage from storage units perforated by old wells. (Alkor-Berichte AL575)