Aeolian Processes of the “Dead Dunes”, Nagliai Nature Reserve, Lithuania: A Ground Penetrating Radar Investigation

Authors

Joseph Beck, Eric Drost, and Logan Bergevin

Abstract

The Nagliai Nature Reserve protects a large aeolian (wind-blown) dune field known as the “Dead” dunes which over time has buried four villages and two old cemetery sites. Dead (Mirusios), or Gray (Pilkosios) dunes are large sand hills (> 60 m) built by strong winds coming onshore from the Baltic Sea. The Reserve also protects habitats for rare plants. Human activity is limited to a single trail within the Reserve except for scientific observations. In collaboration with the Vilnius University and the Re-serve, multiple ground penetrating radar transects were collected to image the internal structure of the Dead dunes to better understand their formation and to test the ability of GPR to map paleosols in this environment.  GPR is a non-invasive imaging system which can give insight into the earth below the surface and is based on the propagation and reflection of pulsed electro-magnetic energy.  Data was collected using a pulse_EKKO 1000 GPR system with topographic data collected for each transect using a TopCon RL-H3CL laser level. Using radar stratigraphic principles to guide the interpretation of the processed transects resulted in observing inclined reflections (prograding) and subhorizontal reflections (paleosols).  The project provides an initial “look” inside these unique aeolian environments.

Introduction

The Curonian Spit, a narrow 0.4 to 4 km wide deposition bar stretching 98 km from the Sambian Peninsula to the Klaipeda Strait in the north located off the coast of Russia and Lithuania, separates the Curonian Lagoon from the Baltic Sea (Figure 1). A prominent landform feature of the Curonian Spit is the Dead (Mirusios) Dunes, also referred to as the Grey (Pilkosios) Dunes. These dunes can reach heights of 67 meters and have covered four villages and two cemeteries during a period of fast migration between 1675 and 1854 (Dobrotin et. al. 2013). During this time, the dunes were migrating at a rate between 0.5 and 15 meters annually as a result of deforestation (Atlas Obscura 2019). Currently, human activity is reduced to a single trail as this site is a UNESCO World Heritage Center.

Figure 1. The study area for this project is located on the Curonian Spit. The spit spans across both Lithuania and Russia. (Map created by August Guenthner and modified by Eric Drost)

The Curonian Spit was created by sediment deposition, both aeolian and alluvial, on glacial moraine, around 5,000 years ago (UNESCO 2019). A steady influx of sediments allowed the spit to remain above sea level. Aeolian dunes are formed by the accumulation of sand-sized particles transported by wind (Figure 2).This sand is either moved via saltation (bouncing along a bed) or creep (pushing) up the stoss (upflow) side which then slides down the lee (downflow) side (Merck 2017). The Dead Dunes were created by dry sand blowing and accumulating on the Curonian Spit, driven by strong winds originating over the Baltic Sea. These dunes have, over time, buried a former soil surface. These buried soils are called paleosols. Soils take time to form, and any discernible paleosols represent periods of stability on the Curonian Spit (Dobrotin et. al. 2013). The purpose of the research was to create imagery of the internal structures of the Dead Dunes to better understand their formation and to test the ability of ground penetrating radar (GPR) to discern and map paleosols (Figure 3) in this depositional environment.

Figure 2. Model of aeolian dune formation (University of Maryland Department of Geology)

Figure 3. Trench dug at the study site showing the existence of paleosols in the dune field. The paleosols are the darker soils found in the image. 

Methods

n the summer of 2017, we collected multiple Ground Penetrating Radar (GPR) lines using multiple antenna frequencies, 225 Mega Hertz (MHz), 450 MHz, 900 MHz. The lines were collected in the shape of star (Figure 4). The GPR system used for to collect the data was a pulse_EKKO 1000 GPR system from Sensors and Software. For the purposes of this project, lines collected using the 450 MHz antenna (Figure 5) were used to determine the location of the paleosols. A Topcon Laser Leveler was used to collect relative elevation data to topographically correct the GPR profiles (Figure 6).

Figure 4. Schematic showing the orientation of the GPR survey.

Figure 5. Sensors and Software pulse_EKKO 1000 GPR system used to collect the GPR data equipped with 450 MHz antenna.

Results

Figure 7. Line 22: Erosional truncation (interpreted as local scouring of the dune surface), downlap (interpreted as dune migration or surface reworking), trough (scour and fill), and high-angle facies (grainflow cross strata) are all present.

Figure 8. Line 16: Erosional truncation (interpreted as local scouring of the dune surface), downlap (interpreted as dune migration or surface reworking), and high-angle facies (grainflow cross strata) are all present.

Figure 9. Line 17: Erosional truncation (interpreted as local scouring of the dune surface), downlap (interpreted as dune migration or surface reworking), and high-angle facies (grainflow cross strata) are all present.

Discussion and Conclusion

The GPR survey provided results in the form of 2D profiles that enable both internal dune stratigraphy and paleosol detection in the substrate. Using Figure 7 as a guide, features such as erosional truncation (erosional unconformities interpreted as local scouring of the dune surface) and downlap (ter-mination of steeply dipping strata overlying more gently dipping strata interpreted as dune migration or surface reworking) can be derived from the survey images. Figures X, Y, and Z, show inclined reflections, highlighted by the yellow lines, that are interpreted as bedding surfaces/slip faces of a dune migrating to the east over time. The longer reflections that are present through the entirety of the profiles and highlighted in red in the preceding figures, are interpreted as paleosols and represent periods of stability on the Curonian Spit. Using a lower frequency GPR antenna would potentially image reflections to a greater depth, while higher frequency antenna would provide greater resolution at the cost of a lower depth of penetration. This study confirms that, through the use of GPR, the movement of dunes and the location of paleosols can be determined with minimal disturbance to this UNESCO protected site. Further research to aid in interpretation of these paleosols would include taking core samples to confirm the presence of the in-terpreted paleosols, OSL (optically stimulated luminescence) dating which would provide a temporal control, and particle size analysis, which would help in paleoclimatic reconstruction, i.e. the climate in which these dunes and paleosols formed in.

Figure 10. Radar facies used for stratigraphic interpretation (Hugenholz et. al, 2007).

Acknowledgements

We would like to thank the Office of Research and Sponsored Programs Student Blugold Commitment Differential Tuition funds through the University of Wisconsin-Eau Claire Student/Faculty Research Collab-oration Program. We thank the International Fellows Programs for Research, Service, and Creative Activity for funding the Travel to Lithuania to collect this data. We thank the Donatas Pupienis from Vilnius University for guiding us in the field and assisting in data collection.

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