Puck Bay Pockmarks and Submarine Groundwater Discharge

• First report on the existence of pockmarks within Puck Bay.
• Pockmarks were found during a hydroacoustic survey during 2020.
• Survey area is located in the southwestern part of Puck Bay at a depth of 25–27 m.
• Depleted chloride concentrations were found in the sediment pore water inside the pockmark depressions.
• Pockmark creation is likely rooted to groundwater moving through the Miocene-Pleistocene aquifers into the bay.
• 1D modeling of vertical Cl- concentration profiles showed freshened groundwater flowing upwards inside the pockmarks.
• Submarine groundwater discharge (SGD) magnitude ranges from 1.53-10-2 to 18-10-2 L⋅m-2⋅h-1.
• Groundwater seepage causes a salinity decrease of 0.12 PSU 3 cm above the seafloor within the pockmarks, compared to reference sites.
• Water advection allows for SGD detection at several meters above the seafloor as decreased salinity in the thermocline layer.
• Study uses chemical, physical, and geophysical aspects of oceanography in the bay area.
• Bathymetric measurements were taken using a multibeam echosounder, ~37 km² seabed area.
• Profiles included chloride concentration in sediment pore water and a water column CTD profile.
• Focus of the work was on discovering active SGD sites in the seabed of the deep part of Puck Bay.
• The influence of freshwater SGD on thermohaline structure in the water column was reported.
• Outflow of water with low chloride indicates an exchange of land-based/sea waters.
• Groundwater drainage is an additional source of freshwater in seas, following river runoff.

Introduction: Submarine Groundwater Discharge (SGD)

• SGD is all water flow through continental/insular margins from the seabed to the coastal ocean.
• SGD includes fresh and saline components, occurring from nearshore to offshore seepage.
• Inland seas, Baltic Sea, SGD enhances anthropogenic impact on offshore waters.
• SGD significantly affects marine organism function, i.e. microbial communities and meiofauna.
• Puck Bay is a drainage area for underground aquifers.

Aquifers & Groundwater Flow

• Four aquifers in Puck Bay extend from land in an easterly direction below the seafloor.
• Lower-level Oligocene and Upper Cretaceous aquifers are covered by impermeable sediments.
• The Pleistocene aquifer is fed by water from the morainic upland, penetrating the Miocene aquifer.
• Fresh water flows towards the sea through Miocene aquifer sands.
• The result of the seepage is a reduction in chlorides along with oxygen and hydrogen isotopes in the bottom sediments.
• Along the coast is coastal drainage of shallow groundwater at the Holocene level.
• Western part of the Puck Bay bottom is groundwater discharge impacted, almost half the bottom area.
• Mean fresh groundwater flux from the bottom is 10 L⋅m-2⋅h-1.
• Freshwater seepage through the seabed is 12% of rivers and streams discharge into Puck Bay.
• Vertical changes in chloride at offshore sampling sites didn't indicate freshened groundwater near the sediment-seawater interface.
• Pore water chemistry data show massive groundwater inflow into the Puck Bay waters.

Study Area

• A single, distinct pockmark structure (Böttcher et al., 2024) has been found in the area
• Hydroacoustic surveys had not been conducted for pockmarks in Puck Bay prior to 2020.
• Examinations of offshore SGD was performed using an automatic CTD probe.

Materials and Methods

• Fieldwork conducted during cruises of the vessel r/v Oceanograf (Institute of Oceanography, University of Gdańsk) in 2020.
• Bathymetric data collection based on Teledyne Reson SeaBat 7125 multibeam echosounder (MBES).
• Operation in RTK (Real Time Kinematic) mode using a Trimble SPS855 receiver with a Trimble Zephyr Model 2 Rugged GNSS antenna.
• Internet based position corrections transmitted from EUPOS/SAPOS systems.
• The RTK receiver is coupled to the vessel's Dynamic Positioning System (DPS) via integrated navigation system software.
• Scanned an approximately 37 km² seabed area between the Hel Peninsula and the western coast of the bay.
• Data captured up to 7 knots of vessel speed.
• The SeaBat 7125 echosounder transducer operating at 400 kHz facilitated high-resolution bottom shape data.

Seafloor Morphology

• Pockmarks at 25 to 27 m on a smooth seafloor that slopes gently to deeper depths in eastern Puck Bay.
• They form seabed depressions, characterized by different sizes and depths that can be caused by gas/gas liquid flow.
• Regularly spaced sandbars are located west.
• Five pockmarks presented in Table 1.

Sediment and water information

• Sites designated D were located closest to or in the center of pockmarks with reference sites (R) located outside
• Collected intact core, bottom water and pore water samples
• Analysis to determine chloride concentration
• Sediment cores up to 40 cm long was collected by gravity corer on Aug 30 and December 5, 2020
• Sediment was analyzed with pipette method for sedimentation analysis (Jackson & Saeger, 1935),
• Classification with Shepard classification (1954).
• Porosity 0.655 located near sampling area (Bolalek, 1992a/1992b)

Seawater and Sediment components/estimation

• Chlorinity (‰), and salinity (S‰) computed from seawater and sediment samples • Magnitude of A, w/D, B parameters were obtained using optimal fit of function from Eq. (2) to the experimental data (least square method) • Effective molecular diffusion coefficient for chloride ions in sediment, D=Do/θ2, where θ is tortuosity associated with porosity (Boudreau, 1997) • Finally the specific discharge (w) was computed.

Results: Chloride profiles

• Three porewater Cl- profiles were distinguished:
• Constant or slight decrease in [Cl] with from 115 to 95 mmol·L-¹ (characteristic of R = reference sites).
• Decrease in [Cl-] from ca. 100 mmol·L-¹ to 60.0 mmol·L-¹ (D13 Aug.2020; D12 Aug.2020; D8 Aug.2020; D10 Aug.2020; D8a Dec.2020).
• Pronounced decrease in [Cl-] from ca. 90 to almost 0.6 mmol·L-¹ (D9 Dec.2020; D8a Aug.2020; D9 Aug.2020; Fig. 4a).

Submarine Activity

• The B parameters suggest that groundwater had some amount chloride ions before discharging.
• Strong Cl- decrease was observed when Cl-'s dependence on depth in upper sediments was linear
• Pore water and diffusion control the transport of chloride
• Signs of sediment freshening associated with poor diffusion were observed at some reference sites with [Cl-] vertical linear changes.
• The values of specific discharges obtained can cause temporal variation in groundwater movement with standstill at certain periods of time
• Specific discharge varied depending on pockmarks where pore water did not move and low content chlorides were found

Other Factors

• Gas flow can factor in the formation of pockmarks
• Upward flow in muddy sediments decreases strength of sediments which causes erosion by bottom currents
• Spatial distribution of seawater temperature/salinity may come from the greater depths Gulf Of Gdansk which carries colder + saline water
• The coarse sediment is thought to be less susceptible caused by water and movements so pockmarks lift in fine sediment
• It is possible that Pockmarks were made from Miocene–Pleistocene intensive SGD from Puck Bay geological structure
• Pockmarks are in area where two aquifers merge 5-6 km shore

Seawater factors

• D sites at 0.12 PSU is lower than reference and probe sites with low Salinity
• Thermocline is where temperature decreases abruptly, and may cause vertical water movement

Conclusions

• Pockmarks are clear ground water flow relations and studies needs to see ground water origins from these aquifers
• They may also be an SGD, with systematic studies to assess SGD importance