Neoichnology of vertebrate traces along the western barrier coast of Ukraine: preservation potential and subsurface visualization

A diverse suite of vertebrate traces covers beach, aeolian, and bay-side (deflation flats) surfaces along the NW Black Sea coast of Ukraine. These include avian, ungulate, and canid footprints (length >5 cm; depth ~2 cm), as well as mammal burrows. The preservation of biogenic structures is enhanced by rapid burial (low-energy sedimentation or event deposition), algal mat formation, and salt encrustation. Continuous high-frequency (800 MHz) ground-penetrating radar (GPR) imaging aided in visualizing subsurface sections of an active burrow complex within a beach-dune ridge. Images near an active fox burrow captured distinct subsurface anomalies (point-source hyperbolic diffractions) in the upper aeolian section above the water table. Unfilled tunnel sections are easily distinguished from buried roots and other targets based on signal velocity and polarity reversals relative to air-to-sediment response at the ground surface. The diffraction geometry (angle) is related to signal velocity, providing valuable information about relative saturation of the overlying substrate. Decimeter-scale deformation of shallow reflections may be attributed to tracking surfaces, with similar examples found immediately below modern surfaces affected by anthropogenic trampling. It is likely that muddy lagoonal tracking surfaces may be preserved under layers of sand (overwash or aeolian deposition) and, following saltwater expulsion, may be recognized in geophysical images as clear deformed paleo-surfaces. Heavy-mineral concentrations (e.g. magnetite-rich sand) are common for beach and dune horizons that have undergone rewor king and such anomalies often accentuate physical and biogenic deforma tion structures. Due to moderate-to-high fraction of fer- ri-and para mag netic minerals, these anomalies are also well-expressed in GPR images due to its electromagnetic signal response. A conceptual framework of trace preservation potential (taphonomy) and geophysical recognition (GPR) suitability is proposed for this coas tal region, with implications to paleo-environmental reconstruction.


Introduction
Most modern depositional settings associated with aquatic and marginal environments at one time contain many billions of traces produced by a variety of vertebrate [Vialov 1966;Fornós et al. 2002;Hasiotis et al. 2007;Milàn et al. 2007;Buynevich 2015] and invertebrate [Frey & Pemberton 1986;Zonneveld 2016] organisms.However, preservation (taphonomy) and detection of these structures, particularly in unconsolidated sandy and mixed sand-mud substrates (= media) have been long considered to be one of the most challenging aspects in both neo-and paleoichnological research [Loope 1986;Allen 1997;Fanelli et al. 2007;Buynevich 2020].Many larger or persistent traces rework the substrate enough to produce zoogeomorphic-scale impact [Laporte & Behrensmeyer, 1980;Butler, 1995;Scott et al. 2008].In recent decades, novel and refined applications of high-resolution geophysical techniques, such as ground-penetrating radar (GPR or georadar) showed success in effective, rapid, continuous imaging of shallow tracks and large burrows [Stott 1996;Buynevich 2010Buynevich , 2011;;Urban et al. 2019].A combination of field observations and geophysical imaging must be the first integral step in assessing the appearance, preservation potential and recognition of each trace in a particular environment and substrate type.
Coastal accumulation forms (barriers) and back-barrier settings (limans) along the western coast of Ukraine (Bessarabian Liman Coast; Fig. 1 a), offers a unique opportunity to investigate a diverse suite of vertebrate traces across rapidly changing sedimentological, hydrological, and ecological gradients.The aims of this study are: 1) description of vertebrate traces in coastal settings; 2) assessment of georadar imaging as a tool for subsurface visualization of vertebrate biogenic structures, and 3) establishment of a conceptual scheme of trace taphonomy and recognition potential.

Materials and Methods
Field research during the summer of 2012 included ground-based photography, measurements, and high-resolution geophysical imaging.Geolocation was provided using a hand-held GPS.Subsurface imaging was conducted using a MALÅ 800 MHz ground-penetrating radar (GPR) system with a monostatic antenna (Fig. 1 b).No topographic correction was applied for short segments, with signal velocities of 14 and 6 cm/ns for unsaturated and saturated sands, respectively (based on empirical data and hyperbola fitting; [Chalaib et al. 2014]).Radargrams (2D or B-scans) were post-processed in RadExplorer v. 1.41 software package using standard algorithms (for a detailed methodology of neoichnological applications of GPR, see [Buynevich et al. 2014]).

Results
Coastal accumulation forms along the Black Sea margin (barriers and back-barrier flats; Fig. 1 a) contain a diverse suite of vertebrate and invertebrate traces, with the former being the focus of this study (Fig. 2).Along single-ridge (retrograding) or multiple beach-dune ridge strandplain (prograding) sections, traces in sand include both footprints and bioturbation structures (burrows).Along the Albatross strandplain, a live fox observed near the burrow with a fresh spoil pile at the entrance confirmed its current use (Fig. 3 a).A geophysical (GPR) survey, part of a larger geological dataset, was collected over the ridge near the burrow.The image revealed a series of hyperbolic diffractions (T in Fig. 3 b) within a background of sub-horizontal bounding surfaces (C in Fig. 3 b).

Discussion
The results of this study show a promising potential of neoichnological experiments to in situ analysis of track expression in substrates with different geotechnical properties (texture, moisture content; see Fig. 2).Whereas the high salinity prevents immediate georadar imaging, it allows assessing the role of salt encrustation and algal mat formation in improving trace preservation potential.
Along sand-dominated beach-dune ridge complexes, GPR images near an active fox burrow (Fig. 3 a) captured a number of distinct subsurface anomalies in the upper (aeolian) section above the water table (Fig. 3 b).Unfilled tunnel sections can be easily distinguished from buried roots based on signal velocity and polarity structure: blue-red-blue (-/+/-) vs. red-blue-red (+/-/+) of the air-tosediment response at the ground surface [Buynevich et al. 2014;Chlaib et al. 2014].It may be even possible to identify tracking surfaces by focusing on deformation along bedding surfaces (circled area in Fig. 3 b [Milàn et al. 2007;Buynevich et al. 2015]).
Heavy-mineral concentrations, common for beach and dune horizons that experienced winnowing and deflation, work well at accentuating biogenic deformation structures [van der Lingen & Andrews 1969; Lewis & Titheridge 1978].Furthermore, thin horizons with moderate-to-high fraction of ferri-and paramagnetic minerals would aid in accentuating sedimentary structures in GPR images р-ISSN 2617-6157 е-ISSN 2617-6165 GEO&BIO • 2023 • том 24 due to its electromagnetic signal response [Buynevich et al. 2014;Buynevich 2020].Along beach/ dune ridge complexes, such as this study, such mineralogical anomalies are typical for dune-base and beach sections, in contrast to a relatively quartz-rich dune substrate presented in Fig. 3.A conceptual framework of relative trace preservation (taphonomic) and recognition (GPR-based visualization) potential is presented in Fig. 4. Preservation increases with rapid burial in low-energy settings (bay sedimentation, aeolian aggradation) or storm wave and wind related event burial [Milàn et al. 2007;Buynevich et al. 2011].Within bay (liman) bottoms (e.g.fish traces), along their margins (Fig. 2), and in hypersaline lagoons and swales, salt encrustation and algal mat growth greatly enhances their preservation potential [Frey and Pemberton 1986;Marty et al. 2009;Urban et al. 2019].Some of the Tertiary paleo-lagoon settings described by Vialov [1966] in western Ukraine may have had similar paleoenvironmental context.Coastal cliffs composed of loess materials may potentially contain a vast archive of footprints and burrows, with geophysical imaging emerging as a vital technique to locate tracking surfaces.Within sand-dominated systems, especially aeolian lithosomes (dunes, loess), not only burrows, but also potential tracking surface deformation (circle in Fig. 3 b) can be resolved [Loope 1986;Milàn & Bromley 2006;Milàn et al. 2007;Buynevich 2011;Buynevich et al. 2014;Urban et al. 2019].GPR imaging is not possible in brackish or saline settings due to signal attenuation; however, exposed upper sections, especially following rainfall events, may potentially increase its suitability (e.g., low beach ridges, deflation flats; Fig. 4).
This study demonstrates a wide suite of vertebrate traces, both surface and subsurface, which have moderate to relatively high preservation potential.Georadar imaging has immediate implications to geological, zoological, ecological, archaeological, and conservation research that relies on non-invasive techniques [Hasiotis et al. 2007;Zonneveld 2016;Buynevich 2020].Future research will combine neoichnological experiments and observations with detailed 3D georadar imaging to refine the approach presented here into the paleoichnological record.

Conclusions
Subaerially exposed coastal accumulation forms (barriers and back-barrier flats) along the Black Sea coast of Ukraine exhibit a diverse suite of vertebrate and invertebrate traces.
High preservation potential is predicted for traces that undergo rapid burial (wave deposition, aeolian aggradation, back-barrier sedimentation), consolidation due to algal mats and salt incrustation (liman flats), and those with subsurface components (burrows).
High-resolution geophysical imaging, such as georadar (frequency > 400 MHz), has a high potential in locating and mapping relict tracking surfaces and buried biogenic structures (large footprints and burrows).
Numerous traces (many billions) in Tertiary and Quaternary sequences can be imaged prior to excavation to uncover relict zoogenic structures, which provide valuable insight into past behavior, substrate properties during trackmaking, and paleoenvironmental context.