Although offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. Here we integrate controlled-source electromagnetic, seismic reflection and borehole data with hydrological modelling to quantitatively characterise a previously unknown OFG system near Canterbury, New Zealand. The OFG system consists of one main, and two smaller, low salinity groundwater bodies. The main body extends up to 60 km from the coast and a seawater depth of 110 m. We attribute along-shelf variability in salinity to permeability heterogeneity due to permeable conduits and normal faults, and to recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, modelling results suggest that the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka. Global volumetric estimates of OFG will be significantly revised if active margins, with steep coastal topographies like the Canterbury margin, are considered.

Shallow seabed depressions attributed to focused fluid seepage, known as pockmarks, have been documented in all continental margins. In this study, we demonstrate how pockmark formation can be the result of a combination of multiple factors—fluid type, overpressures, seafloor sediment type, stratigraphy and bottom currents. We integrate multibeam echosounder and seismic reflection data, sediment cores and pore water samples, with numerical models of groundwater and gas hydrates, from the Canterbury Margin (off New Zealand). More than 6800 surface pockmarks, reaching densities of 100 per km², and an undefined number of buried pockmarks, are identified in the middle to outer shelf and lower continental slope. Fluid conduits across the shelf and slope include shallow to deep chimneys/pipes. Methane with a biogenic and/or thermogenic origin is the main fluid forming flow and escape features, although saline and freshened groundwaters may also be seeping across the slope. The main drivers of fluid flow and seepage are overpressure across the slope generated by sediment loading and thin sediment overburden above the overpressured interval in the outer shelf. Other processes (e.g. methane generation and flow, a reduction in hydrostatic pressure due to sea‐level lowering) may also account for fluid flow and seepage features, particularly across the shelf. Pockmark occurrence coincides with muddy sediments at the seafloor, whereas their planform is elongated by bottom currents.

Mobile coastal sediments, such as sand and gravel, build up and protect wave-dominated coastlines. In sediment-starved coastal environments, knowledge about the natural sources and transport pathways of those sediments is of utmost importance for the understanding and management of coastlines. Along the Baltic Sea coast of Schleswig-Holstein (Germany), the retreat of active cliffs—made of cohesive Pleistocene deposits—supplies a wide size range of sediments to the coastal system. The material is reworked and sorted by hydrodynamic forcing: the less mobile stones and boulders remain close to the source area; the finest sediments, mostly clay and silt, are transported offshore into areas of low energy; and the fractions of sand and fine gravels mostly remain in the nearshore zone, where they make up the littoral sediment budget. They contribute to the morphodynamic development of sandy coastlines and nearshore bar systems. Exemplarily for this coastal stretch and based on an extensive review of local studies we quantify the volume of the potential littoral sediment budget from cliff retreat. At an average retreat rate of 0.24 m yr⁻¹ (<0.1–0.73 m yr⁻¹), the assessment indicates a weighted average sediment volume of 1.5 m³ yr⁻¹ m⁻¹ (<0.1–9.5 m³ yr⁻¹ m⁻¹) per meter active cliff. For the whole area, this results in an absolute sediment budget V_s,total of 39,000–161,000 m³ yr⁻¹. The accuracy of the results is limited by system understanding and data quality and coverage. The study discusses uncertainties in the calculation of littoral sediment budgets from cliff retreat and provides the first area-wide budget assessment along the sediment-starved Baltic Sea coastline of Schleswig-Holstein.

The Schleswig-Holstein (SH) Baltic Sea coast consists of different morphological features (e.g., bars, beaches, dunes, cliffs), which are continuously shaped by various along- and cross-shore sediment transport patterns driven by local currents generated from tides, seiches, winds and waves (Soomere et al., 2012). Understanding local scale sediment transport is of utmost importance to manage the scarce coastal sediments and to identify suitable mitigation strategies against extreme forcing scenarios (Hofstede and Hamann, 2022). This investigation was carried out at the exemplary cliff coast at Stohl, at which the glacial till cliff (maximum height about 20 m) is the source of nearshore sediments (Averes et al., 2021). The main objective of this study is to investigate the relative importance of different forcing conditions on local sediment transport pathways.

Offshore freshened groundwater (OFG) is an important component of the global water cycle. Bodies of fresh and moderately brackish groundwater have been documented up to 100 km from modern shorelines. Their global volumetric estimates are on the order of 500,000 cubic kilometres, which is two orders of magnitude greater than the volume of groundwater extracted globally from continental aquifers since 1900. The potential use of OFG systems as a source of potable water is a main driving force for their improved understanding. OFG systems also play a fundamental role in biogeochemical fluxes to the ocean and in benthic and sub-seafloor ecology.The majority of OFG systems were emplaced by groundwater migration across topographic gradients via permeable connections between offshore and onshore aquifers. The characteristics and dynamics of the offshore aquifers remain poorly constrained, however. There are many first order questions waiting to be addressed, mainly related to the geometry, distribution and dimensions of these aquifers, as well as their flow and emplacement dynamics.In this study we integrate hydrological modelling with borehole data and offshore geophysical observations from the Canterbury margin (New Zealand) to quantitatively characterise an onshore-offshore groundwater system. Onshore, the main aquifers are hosted in gravels down to at least 150 m depth, with unconnected sand and silt/clay layers forming aquitards. The regional flow of groundwater in the Canterbury aquifers is from the foothills of the Southern Alps towards the sea. Offshore, the groundwater system consists of one main, and two smaller, low salinity groundwater bodies. The main OFG body extends up to a distance of 60 km from the coast to a water depth of 110 m, has a maximum thickness of at least 250 m, and an estimated volume that ranges between 56 and 213 cubic kilometres. It exhibits along-shelf variability in salinity, which we attribute to permeability heterogeneity due to permeable conduits and normal faults, and recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka.This study demonstrates that the integration of hydrological modelling, borehole and geophysical data is a powerful approach to quantitatively characterise groundwater systems across continental margins. Applying this approach globally is likely to result in a significant revision of global volumetric estimates of OFG.

Sea level rise along with the changing climate leads to severe enhancement of hydrodynamic impact to coastlines worldwide. Along the Baltic Sea coast of Schleswig-Holstein (Germany), this leads to the erosion of exposed glacial cliffs (up to 30 % of the coastline) and abrasion platforms (unknown extend). Irreversible land loss and seafloor deepening are the consequences, causing socio-economic and environmental concerns in affected areas. However, the adjacent coastal sections benefit from the development as the mobilized material constitutes the main sediment source to the nearshore bar and beach systems. Here, temporal built up of nearshore bars and the deposition at sandspits and beaches functions as natural shore protection.The heterogenous and dynamic morphology, exposition and geology of the cliff sections and their offshore continuation complicates system understanding and management of the Schleswig-Holstein coastline. The availability of coarse-grained sediments (sand, gravel, stones) from the poorly sorted glacial till, forming the cliffs, is comparatively low. This lack of obtained material suitable to build up a coastal morphology attributes a central role to the source areas and the quantification of the sediment budget regarding coastal preservation.On this account we attempt to develop a strategy towards a classified coastal sediment budget, which is based on a comprehensive field and literature data base, addressing the highly variable character of the observed coastline described in morphological, morphodynamic, geological, sedimentological, hydrodynamic and anthropogenic parameters.The coastline of Schleswig-Holstein is structured into 58 active cliff sections for individual description via categorized cliff profiles. Furthermore, 22 abrasion platforms are defined in the offshore region and characterized by descriptive summaries. The data summary reveals well investigated zones (e.g. Schönhagen, Stohl, Heiligenhafen, Brodten), serving as potential pilot areas for complementary studies, but also identifies study areas which require further research.The literature values for past cliff retreat and eroded sediment volumes bear high uncertainties. This is due to the fact that former studies are based on unequal spatial extend of cliff sections, variable time intervals and differing methods. Further, computation of eroded material volumes is lacking important input parameters, e.g. the degree of compaction and the grain size distribution. This is considered for budget calculations and their confidence for individual coastal units in template form.The current study compiles and visualizes the heterogenous data for further scientific applications. The project aims to support future studies on the sediment availability and transport in the near-shore system using hydrodynamic modelling and thus creates a sound scientific base for system understanding and new governmental regulations concerning coastal protection measures at the Schleswig-Holstein Baltic Sea.