Stable isotope study of submarine groundwater discharge at the Hanko Peninsula, south Finland
Abstract
A submarine groundwater discharge (SGD) site was recently documented at the Hanko Peninsula in south Finland, northern Baltic Sea. The local stratigraphy and aquifer geometry were studied using a multifaceted dataset of offshore seismic sub-bottom profiles, multibeam and sidescan sonar images of the seafloor, radon measurements of seawater and groundwater, and onshore ground-penetrating radar and refraction seismic profiles. The aquifer is hosted in the distal sand-dominated part of a subaqueous ice-contact fan that belongs to the First Salpausselkä ice-marginal formation. The SGD takes place predominantly through pockmarks on the seafloor. The pockmarks are present on the edge and slope of a sandy shore platform, at the water depths of ca. 11 meters. Elevated Rn-222 activity concentrations in near-bottom water at the pockmark locations strongly indicate groundwater influence, and permit calculating a rough estimate of 1 cm/d for the discharge rate. Porewater profiles of elements of marine affinity such as Cl show strong vertical gradients in sediments below the pockmarks. The isotopic ratios δ2H and δ18O measured for local groundwater, pockmark porewater and local seawater demonstrate that waters discharged from the pockmarks are mixtures of the groundwater and seawater end-members. The share of fresh groundwater in the pockmark porewaters is ca. 83%, whereas it is ca. 9% in the water column above the pockmarks. This work resulted from the BONUS SEAMOUNT project supported by BONUS (Art 185), funded jointly by the EU, the Academy of Finland (Grant No. 311983), and the Federal Ministry of Education and Research, Germany (Grant No. 03F0771B). Related publication: Virtasalo, J.J., Schröder, J.F., Luoma, S., Majaniemi, J., Mursu, J. & Scholten, J.: Submarine groundwater discharge site in the First Salpausselkä ice-marginal formation, south Finland. Solid Earth Discuss., https://doi.org/10.5194/se-2018-131.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2019
- Bibcode:
- 2019EGUGA..21.5157V