Submarine groundwater discharge at Forsmark, Gulf of Bothnia, provided by Ra isotopes

Abstract

Submarine groundwater discharge (SGD) to the Gulf of Bothnia, Baltic Sea, has been believed to be insignificant from hydrological models, yet geochemical investigations of SGD in this basin are limited. In this study, 223 Ra, 224 Ra, 226 Ra, and 228 Ra have been complemented by stable δ 18 O and δ 2 Η isotopes to characterize and quantify SGD rates from the coast of Forsmark, Sweden to Öregrundsgrepen Strait (Gulf of Bothnia). In shallow bays, SGD has been traced using relations between Ra, δ 2 Η and δ 18 Ο isotopes and salinity. Zonal SGD from deformation zones, expressed as hydraulically conductive fractures, overlain by the Börstilåsen esker, a strip of conductive gravel sediment, have been traced through offshore 224 Ra xs and 226 Ra trends. On the basis of a 224 Ra xs mass balance, the SGD flow rate ranges from (5.5 ± 3.0) • 10 3 m 3 d-1 to (950 ± 520) • 10 3 m 3 d-1. These rates are up to two orders of magnitude higher than those determined from local hydrological models, which consider only the fresh component of SGD. From the divergence between the hydrological and 224 Ra xs models, it is inferred that the site is influenced by a component of recirculated seawater.

Submarine groundwater discharge site in the First Salpausselkä ice-marginal formation, south Finland

“…A cosmogenic 10 Be age of 12 500 ± 700 for the ice-margin retreat supports the varve chronology (Rinterknecht et al, 2004). The First Salpausselkä was deposited as a narrow ridge of contiguous meltwater fans and local feeding eskers that were formed along the ice-margin grounding line (Virkkala, 1963;Glückert, 1986;Fyfe, 1990;Kujansuu et al, 1993) in an icecontact lake that was more than 100 m deep in Hanko (Fyfe, 1990). After the ice-margin retreat, the till and subaqueous ice-contact fan deposits were successively covered by glaciolacustrine rhythmically alternating (varved) silt and clay, and postglacial lacustrine poorly bedded clay (Virtasalo et al, 2007(Virtasalo et al, , 2014.…”

“…The top of the ice-marginal formation was exposed to waves and eventually to wind as it gradually rose from the sea. The original ridge morphology became truncated and flattened from the top, the glaciolacustrine and postglacial lacustrine silts and clays were removed, and the underlying subaqueous icecontact fan deposits were reworked by wind waves and currents (Virkkala, 1963;Glückert, 1986;Fyfe, 1990;Kujansuu et al, 1993). Fine sand was redeposited as beach ridges on the south side of the peninsula and partially reworked into aeolian dunes (Fyfe, 1990).…”

“…The stratigraphical position immediately above the till, as well as the proximity and increasing unit thickness towards the First Salpausselkä ice-marginal formation, indicates meltwater origin for the sandy deposit. SU3 is interpreted to comprise a distal part of a subaqueous ice-contact fan that is part of the First Salpausselkä formation (Virkkala, 1963;Glückert, 1986;Fyfe, 1990;Kujansuu et al, 1993). The poorly stratified fine-sand beds were deposited probably by density flows and occasional debris flows from the upper fan slope (Winsemann et al, 2009;Lang and Winsemann, 2013;Lang et al, 2017).…”

“…Jankowska et al, 1994;Szymczycha et al, 2012Szymczycha et al, , 2016. In addition, seafloor morphological features such as terraces and pockmarks, interpreted to be produced by SGD, have been documented from the Stockholm Archipelago (Söderberg and Flodén, 1997;Jakobsson et al, 2016) and SGD rates have been modelled through a 224 Ra mass balance offshore Forsmark in Sweden (Krall et al, 2017). For the Puck Bay, Szymczycha et al (2012) calculated that SGD contributes ca.…”

“…The key sub-seafloor aquifers in the Eckernförde Bay and Forsmark are Late Pleistocene glacigenic sand deposits (Jensen et al, 2002;Krall et al, 2017). In the Stockholm Archipelago, silty couplet layers in glacial varved clays act as groundwater conduits (Söderberg and Flodén, 1997).…”