Dosimetric and radiocarbon chronology of a pre-Wisconsinan mastodon fossil locality at East Milford, Nova Scotia, Canada

Abstract

We report the final results of a multidisciplinary geochronological study of two subfossil mastodon remains and the sediments associated with them, from the East Milford, N.S. mastodon locality discovered in 1991. The mastodons, which were found in a karstic sinkhole system, are the latest and the most complete of several mastodon remains discovered in Nova Scotia, New Brunswick and Maine over the last 170 years. The unconsolidated sediments containing the adult specimen were dated using optical dating (IRSL); the mastodon dental enamel was dated using electron spin resonance (ESR); and bone collagen from both individuals plus associated wood were dated using radiocarbon ( 14C) dating. Two samples of sediment adhering to the walls of the karstic cavity at the elevation corresponding to the location of the adult specimen yielded statistically indistinguishable IRSL ages of 127±13 (EMM1) and 143±16 ka (EMM2), with a weighted mean of 133±6 ka. This is consistent with pollen data which indicate an interglacial climate. On the basis of its stratigraphic position with respect to overlying till units, the locality is attributed to oxygen isotope stage 5. Three ESR dates on dental enamel (natural prompt, deproteinated prompt, and deproteinated delayed) yielded statistically indistinguishable ages whose weighted mean is 74.9±5.0 ka. This indicates consignment to the geologic record of the mastodon tooth during terminal oxygen isotope stage 5a, when the climatic cooling leading to the Wisconsinan glacial period had already begun. Fission-track analysis of the enamel and dentine cross sections showed minimal U uptake, resulting in no need for early, linear, or late uptake modelling. The direct ESR age indicates that the specimen either became mired in a cavity that was already at least partly infilled with older, probably waterlogged, sediments, or that the sediment's IRSL signal was not completely erased during deposition. The dosimetric ages are consistent with limiting radiocarbon ages on bone collagen (juvenile >45,760 years BP; adult ⩾37,040±1730 BP) and wood (near the adult mastodon >51,000 BP; organic-rich horizon above the karst surface >50,000 BP). These results provide the first direct age on a mastodon fossil from Atlantic Canada, and the first numeric estimate to demonstrate that mammoths were present in this region during pre-Wisconsinan times, specifically during late oxygen isotope stage 5a.