MINERALOGICAL ANOMALIES ASSOCIATED WITH VERTEBRATE TRACKS IN SANDY SUBSTRATES: INSIGHTS FROM LOW-FIELD MAGNETIC SUSCEPTIBILITY EXPERIMENTS

Tracks exposed to mild winnowing, such as aeolian deflation of siliciclastic media, exhibit higher heavy-mineral concentrations (HMCs), especially along marginal ridges (MR). Such anomalies are potentially detectable using low-field bulk magnetic susceptibility (MS). In situ tracks from beach and dune sites along the U.S. Atlantic Coast contain marginal-ridge HMCs with MS values 3.7 to >10 times greater (enrichment factor, primarily due to magnetite content) than the background quartz-rich fraction. In laboratory experiments, indenter traces produced using hoof casts (equid, cervid) and bison hooves, yield enrichment factors of 1.7-9.5. Two modes of HMCs were produced in the laboratory sand box. In Mode 1, a 1-3 mm-thick HMC was imprinted with a bison hoof, followed by surface deflation. In Mode 2, bison hoof was indented through a thicker HMC and buried by quartz sand without deflation. These experiments demonstrate that traces impressed through thin (<1 mm) HMC and subsequently exposed to wind gusts of 5-10 m/s generate MS values on the scale of 10s of µSI, with subsequent enrichment that can exceed 1000 µSI. Even thin HMCs are sufficient for producing strong reflections in high-frequency (800-2300 MHz) georadar images. Our study has broader applications to detecting, mapping, and quantifying mineralogically-accentuated vertebrate tracks in siliciclastic sandy substrates.