Different pockmark systems and their potential importance for the hydrological and biogeochemical balance of a perialpine lake Adeline N.Y. Cojean*, Maciej Bartosiewicz**, Jeremy Zimmermann*, Moritz F. Lehmann**, Katrina Kremer*** and Stefanie B. Wirth* * Centre for Hydrogeology and Geothermics, University of Neuchatel, Rue Emile-Argand 11, CH-2000 Neuchâtel ([email protected]) ** Department of Environmental Sciences, University of Basel, Bernoullistrasse 30, CH-4056 Basel ***Swiss Seismological Service (SED), ETH Zürich, Sonneggstrasse 5, CH-8006 Zürich Lacustrine pockmarks Ø Much less investigated than marine pockmark systems Ø Fluid-flow formation Ø CH4 gas ebullition => Lake Constance (Wessel 2010; Bussmann, 2011) Ø Groundwater discharge => Lake Neuchâtel (Reusch 2015; Wirth et al., in prep.) Pockmarks in Lake Thun, Switzerland Thun Lake Thun Tannmoos Fault gypsum carying bedrock Einigen Fault Zone Spiez Fabbri et al., 2017 Beatenberg Interlaken Research questions Thun Ø Are there more pockmarks in Lake Lake Thun Thun? Ø If yes, where are they? Ø What is their mechanism of formation? Spikes in electrical Taanmoos conductivity Ø What is their influence on the lake hydrological and biogeochemical Einigen budget? Fault Zone Beatenberg karst system Spiez Beatenberg Beaten Connected to Daerligen karst system Interlaken Intensive CH4 bubbling Different pockmarks systems in Lake Thun Thun Lake Thun Connection to karst system leads to groundwater discharge? Tannmoos Einigen Fault Zone Beatenberg karst system Spiez Beatenberg Beaten Daerligen Interlaken Results – Beaten (connected to karst system) Pockmark Sediment overflowSediment overflow Reference Ø More intense groundwater discharge to the lake in the past Ø Episodically active Pockmark Reference Different pockmarks systems in Lake Thun Thun Lake Thun Tannmoos Einigen Fault Zone Beatenberg karst system Spiez Beatenberg Beaten Daerligen Interlaken Results – Därligen (CH4 bubbling) Gas bubbling Pockmark Different pockmarks systems in Lake Thun Thun Lake Thun Tannmoos Einigen Fault Zone Beatenberg karst system Spiez Beatenberg Beaten Spikes in electrical conductivity and more liquefied sediments in Daerligen the pockmark suggest Interlaken groundwater discharge? Results – Tannmoos (groundwater discharge) Results – Tannmoos (groundwater discharge) Pockmark Pockmark Reference Reference Water depth (m) Water Water depth (m) Water Sediment (cm) depth Sediment (cm) depth -1 + -1 Total dissolved Fe concentration (µmol L ) Total dissolved NH4 concentration (µmol L ) Ø NH + concentration is also lower inside the pockmark, particularly in deeper sediment layers 4 + Ø Total dissolved Fe and NH4 display similar vertical profiles Take-home messages Lake Thun Ø Three pockmark systems were found in Lake Thun Ø Variable formation mechanisms; via gas-flow at Därligen and groundwater discharge at Tannmoos and Beatenberg Tannmoos Ø More liquified sediments inside the pockmark than at a reference site + Ø Lower total dissolved Fe and NH4 concentrations inside the pockmark than at a reference site => episodic O2 penetration? Active groundwater discharge? Ø Further experimental work and sampling campaigns will help to better assess the influence of this pockmark system on the lake biogeochemistry .