Preservation and Recognition of Ungulate Tracks in Sand: Neoichnology of Bison

Abstract

Bison produce trails, wallows, and trample grounds, visible in satellite imagery disturbing ~27,500 m2 at Yellowstone National Park (YNP; in USA) and ~10,700 m2 in Białowieża National Forest (BNF; in Poland and Belarus), and, without anthropogenic land change, these mega-traces persist in sand-dominated substrates for 6-26 years. The average wallow size ranges from ~17-40 m2, whereas the average trample ground varies in size from ~140-300 m2. Trail segments typically extend for ~260-380 m, but the longest trails at YNP traverse >3 km. Estimates of track volume indicate for a standard herd of 200 animals, over a daily distance of 10 km ~4000 m3 is pediturbated. Low sinuosity values of 1.16-1.10 characterize trails, and wallows display high aspect ratios >0.7, helping distinguish bison traces on the landscape. During the Holocene, as many as 40 million bison inhabited North America, so this study provides a qualitative baseline for considering the geomorphic ability of large ungulates.Caliper measurements indicate the surface expression of simulated bison tracks varies depending on the moisture content of the medium. The slope of the marginal ridges (MR) in dry (0% moisture by volume), moist (~10% moisture by volume) and wet (saturated) sand differ around the track perimeter, although the minimum slope of the marginal ridge increases with moisture content (dry sand ~10 cm, moist sand ~40 cm, wet sand ~20 cm). The maximum MR slope (~80°) occurs in a moist substrate. The aspect ratio of prints in wet sand is 0.60, reflecting the most elliptical hoofprint, whereas moist sand displayed the most circular track with an aspect ratio of 0.76. The interdigital angle decreased by ~5° with increasing moisture (dry = 56°, wet ≈ 51°). Photos document in dry sand, deformation fronts 2-3 cm in height are present, whereas in moist sand, transverse and radial cracks are present. In wet sand, debris flows form. Ground-penetrating radar (GPR) imaging reveals subsurface anomalies interpreted as undertracks and normal micro-faults. In dry sand, two poorly-defined sets of undertracks with 1 cm relief are visible 3-4 cm beneath the tracking surface. Normal faulting is absent. In moist and wet sand, 4-5 sets of detailed undertracks showing 2-3 cm of relief deform sediments 7-8 cm in depth. Several normal faults are present in moist and wet hoofprints. Combined surface and subsurface observations may indicate the moisture content of paleo-tracking surfaces, particularly if the substrate is saturated. When hoofprints are formed in an unfrozen substrate, freezing increases preservation potential. Partially thawed tracks are resistant to deflation (wind erosion), maintaining outlines of digits and the medial pocket until late stages of deflation. Billions of ungulate traces formed in aeolian periglacial settings may be preserved. Tracks exposed to aeolian action exhibit higher heavy-mineral concentrations (HMC) along marginal ridges (MR), which are detectable using low-field bulk magnetic susceptibility (MS). In situ tracks from Delaware and Virginia (USA) display marginal HMCs 3.7-10x greater than background MS, whereas in laboratory, simulated hoofprints show marginal HMCs 1.7x above background MS. HMCs readily occur in nature, so MS measurements of tracking surfaces may quantitatively indicate the length or intensity of aeolian processes. This experiment demonstrates hoofprints indented through a <1 mm thick HMC and subsequently exposed to 1 min wind gusts of 5-10 m/s form HMCs on the scale of 10’s of µSI. Billions of ungulate tracks displaying marginal HMCs are probably preserved, potentially providing a detailed regional paleo-wind record.