Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea

Abstract

Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well-known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high-frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high-quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic-geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high-frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped pockmarks. The small pockmarks, which we refer to as "intrapockmarks," have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of "eyed" pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic-rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.Plain Language Summary Groundwater that seeps out of the ocean floor is a common global phenomenon. Marine ecosystems are highly dependent on nutrient supply from land, and recent studies have suggested that groundwater seeping out of the seafloor supplies even more nutrients to the world's oceans than rivers do. Also, nearshore freshwater springs have been used as a source of drinking water for decades and large offshore groundwater reserves in continental shelves can potentially prevent future freshwater shortages. We use echo sounding methods to search for indications for submarine groundwater. The methods enable us to carry out detailed investigations of intriguing seafloor depressions (i.e., "pockmarks"). Our accurate measurements give new insights into the morphology and characterization of the Eckernförde Bay pockmarks and reveal a new type of pockmark that is related to seeping groundwater. In the muddy sediment of Eckernförde Bay, methane gas forms in the sediments due to microbial decomposition of biomass. In areas of enhanced groundwater advection, this methane gas is brought closer to the seafloor, where pockmarks form and where we can detect the gas with our sonar system. Given the abundant global distribution of muddy gaseous sediments, our findings have important implications for the future detect...

A State-Of-The-Art Perspective on the Characterization of Subterranean Estuaries at the Regional Scale

“…Seafloor morphological SGD proxies, such as pockmarks (e.g., Hoffmann et al, 2020), submarine terraces (Jakobsson et al, 2020), and depressions in carbonate rocks (Oehler et al, 2019a), are visible in the bathymetry. There are several techniques available to map the seafloor.…”

Morphology, Formation, and Activity of Three Different Pockmark Systems in Peri-Alpine Lake Thun, Switzerland

“…In sediments hosting high organic carbon concentration, CH 4 levels can increase to exceed CH 4 solubility, which allows the formation of gas bubbles. Once generated, CH 4 migrates toward the atmosphere either through CH 4 gas-bubble release (ebullition), diffusive transport when the overlying water is anoxic (Schmid et al, 2017), or advective transport by groundwater discharge (Schlüter et al, 2004;Hoffmann et al, 2020). The CH 4 isotopic signature in the cored sediments at DPS (Figure 5) suggests a bacterial origin, with a δ 13 CH 4 that is similar to the CH 4 isotopic composition measured in several active pockmarks in Lake Constance, Germany (∼-69 to −80 ‰; Bussmann et al, 2011).…”

“…Unit pockmarks activity likely triggered by groundwater discharge-Unit pockmarks have already been observed in marine systems but, due to their small size, they remained rather under-investigated, as their contribution to fluid seepage was thought to be negligible compared to larger pockmarks (Hovland et al, 2010). However, more recent studies challenged this assumption, demonstrating that unit pockmarks can significantly contribute to fluid release (Bussmann and Suess, 1998;Hovland et al, 2010;Hoffmann et al, 2020). In particular, Hovland et al (2010) highlighted that the unit pockmarks present on the Norwegian continental shelf and slope were responsible for intense continuous local porewater seepage, while larger pockmarks seemed to be associated with periodic or intermittent gas ebullition.…”

“…Pockmarks are crater-like depressions that naturally form on the seabed and the bottom of lakes by upward fluid migration of gas and/or liquid through the unconsolidated sediment column (King and MacLean, 1970;Hovland and Judd, 1988). When active, pockmarks can induce significant fluid seepage to the hydrosphere, which may strongly affect the surrounding ecosystem (Bussmann et al, 2011;Virtasalo et al, 2019;Hoffmann et al, 2020). It has previously been shown, for instance, that submarine groundwater discharge via pockmarks can constitute an important source of nutrients, or of harmful substances, to the ocean (Moore, 2010).…”

Sedimentary fluxes of trace metals, radioisotopes and greenhouse gases in the southwestern Baltic Sea Cruise No. AL543, 23.08.2020 – 28.08.2020, Kiel – Kiel - SEDITRACE

“…According to these authors, submarine groundwater discharge occurs where the clayey seal is thinned and the Holocene mud coverage is weak enough to be penetrated by artesian groundwater. Moreover, free gas produced by methanogenesis in the organic-rich Holocene mud is uplifted by the local ascent of groundwater forming the characteristic intrapockmark structures at Mittelgrund (Hoffmann et al, 2020).…”