Ungulate Tracks in Coastal Sands: Recognition and Sedimentological Significance

Buynevich, I.V., darrow, JS., Grimes, Z.T.A., Seminack, C.T., and Griffis, N., 2011. Ungulate tracks in coastal sands: recognition and sedimentological significance. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), 334 - 338. Szczecin, Poland, ISSN 0749-0208. Coastal sands serve as a substrate for numerous vertebrate and invertebrate traces. However, the dynamic nature and coarse texture of unconsolidated sand make the preservation and recognition of biogenic structures difficult. Our study addresses morphological, mineralogical, and geophysical aspects of ungulate (hoofed animal) tracks using field data from Assateague Island (Maryland, USA) and Curonian Spit (Lithuania), as well as laboratory experiments. The new findings suggest that textural and compositional anomalies related to these structures produce distinct surficial and subsurface signatures that can be analyzed and quantified using geophysical techniques, such as low-field magnetic susceptibility (MS) and high-frequency ground-penetrating radar (GPR). Many coastal and aeolian settings exhibit lithological anomalies, such as heavy-mineral concentrations (HMCs), which represent density lag formation during episodes of increased wave or wind action. There are two ways in which HMCs accentuate vertebrate tracks: 1) as a substrate for track-making activity, and 2) through a concentration of heavy minerals due to surficial expression of the biogenic structures. Following high-energy events, such as storms or increased wind activity, even thin (1-3 mm) concentrations of dense minerals accentuate biogenic structures in plan view and cross-section. Along Assateague Island, feral horses and two species of deer leave numerous tracks in a variety of coastal settings, including beach, dune, and overwash channels and fans. Depending on substrate consistency (saturated, dry, or organic-rich sand) the resulting equid (horse) and cervid (deer) tracks are I to 5 cm deep, often fringed by marginal ridges up to 3 cm in height. These ridges result in an increase in near-surface flow velocity, which causes the removal of fine-grained and light sediment fraction. This process was observed along a backshore area over a period of 24 hours and resulted in a formation of a deflation lag with coarse/heavy particles (localized HMC) that mimics the track morphology. Because many HMCs are enriched in ferrimagnetic minerals (magnetite), bulk MS measurements can be used for in situ analysis of track and undertrack laminae in the field and in the laboratory. An MS survey of a cervid track accentuated by a garnet-rich HMC and buried by 2-3 mm of diamagnetic (quartz) sand, revealed both the location and morphology of the structure. Where textural contrast, moisture content, or HMC thickness are comparable to the resolution of the electromagnetic GPR signal, they produce sharp subsurface reflections. In laboratory, equid tracks made in saturated quartz sand and buried by >1 cm of dry sand were resolved using a 2.3 GHz antenna. When accentuated by a thin HMC, buried ungulate tracks are recognized even at lower resolution (0.8 GHz). Our observations and experiments demonstrate that the challenges of recognizing and examining medium-to-large vertebrate tracks can be partially overcome by integrating sedimentological and geophysical approaches. The information on the distribution of modern and past animal traces can shed light on changing ecological patterns and environmental conditions in coastal settings.