Search

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. [ABSTRACT FROM AUTHOR]

Carbonate lithologies host considerable quantities of the Earth's freshwater resources and partially supply a quarter of the global population with drinkable water. In addition, carbonates constitute substantial amounts of the global coastlines, yet it is not known if and how they can sustain freshened groundwater offshore. Here, we use controlled‐source electromagnetic, seismic reflection, and core sample data to derive a lithological model for the eastern margin of the Maltese Islands and identify four distinct resistivity anomalies within the Upper Coralline and Globigerina Limestone formations. The anomalies hosted in the former are likely associated with low porosities, whereas the anomaly within the latter is indicative of pore fluid freshening. Hydrogeological modeling suggests that freshened pore fluids, emplaced during sea‐level lowstands and preserved in low permeability units, are potentially still found within carbonate shelves. However, resource potential is low due to its relict nature and low permeability host environment. Plain Language Summary: Coastal regions with freshened groundwater beneath the seafloor are found worldwide. Here, we report on a geophysical study conducted off the eastern coastline of the Maltese Islands, where a number of resistivity anomalies are observed in limestones. The anomalies located offshore southeast Malta are likely associated with freshened groundwater. Hydrogeological modeling suggests that this groundwater was deposited during lower sea levels and preserved in fine‐grained units. Our study indicates that offshore freshened groundwater may be found offshore limestone coastline in dry climates, but its potential to be used as a source of freshwater is likely low. Key Points: Geophysical data and hydrogeological modeling are applied to detect offshore freshened groundwater in a semi‐arid carbonate settingGlobigerina Limestone and Blue Clay located offshore SE Malta likely host a disconnected offshore freshened groundwater bodyThe resistive anomalies within the Upper Coralline Limestone are interpreted as localized porosity variations [ABSTRACT FROM AUTHOR]