Late Pleistocene arctic ground squirrel middens as palaeoecological archives of east Beringia

Abstract

This thesis explores Pleistocene-aged palaeoecological records from Yukon Territory in the unglaciated landscape of east Beringia. This region was home to the mammoth steppe, a loess-fed cold and arid grassland-forb ecosystem that supported diverse megafauna. Although a focus has remained on Beringia’s megafauna, this thesis focuses on the palaeoecological importance of arctic ground squirrel (Sciuridae: Urocitellus parryii Richardson) middens, and the role they continue to play in understanding the steppe-tundra ecosystems of Beringia. This thesis uses plant, invertebrate, and vertebrate remains from arctic ground squirrel middens, pore-ice water isotopes, and sedimentary ancient DNA (sedaDNA) from late Pleistocene sites in easternmost Beringia to investigate records of small mammals, the occurrence of previously undocumented invertebrate taxa, and the persistence of steppe-tundra in a landscape experiencing the expansion of shrubs. The late Pleistocene record of small mammals from Yukon Territory has predominantly been reported from sites in the Old Crow basin that are limited by complex stratigraphy and mixed faunal assemblages. Chapter 2 presents a chronologically constrained record of the late Pleistocene small mammal community preserved in arctic ground squirrel middens from the Klondike goldfields. This chapter recovers the remains of arctic ground squirrels (U. parryii), lemmings (Dicrostonyx and Lemmus) and several species of vole (Microtus) that provide insight into the fossorial behaviour of small mammals during the late Pleistocene. By the continued analysis of these Pleistocene middens this thesis further emphasises that they are unique and valuable archives for the preservation of invertebrate life. Previous work has shown the remains of beetles (Coleoptera) and non-biting midges (Chironomidae) from Pleistocene deposits across Beringia, however, the remains of fleas (Siphonaptera), mites (Astigmata and Mesostigmata), thrips (Thysanoptera) and grasshoppers (Orthoptera) are either rare (fleas and mites) or not reported (thrips and grasshoppers). Chapter 3 explores these underrepresented taxa showing an 80,000-year long association between the flea Oropsylla alaskensis and its host, the arctic ground squirrel, and the first Quaternary records of the glycyphagid mite Fusacarus sp., thrips, and three individuals of gomphocerine grasshoppers. In the case of the glycyphagid mite, Fuscacarus sp. we document the first record of this genus from both the fossil and present-day record in Yukon Territory. The preservation of these invertebrate taxa provides data on the biogeographic and phylogenetic histories of Beringian invertebrates, particularly when considering this region as a refugium during the Last Glacial Maximum. Lastly, this thesis explores several latest Pleistocene sites to track the persistence of steppe-tundra in easternmost Beringia. The regional expansion of shrubs around 14,000 cal yr BP is recorded from sites in interior Alaska, but local records of steppe persistence is limited to only a few sites in Yukon Territory. Plant and invertebrate assemblages from latest Pleistocene arctic ground squirrel middens record the persistence of steppe-tundra until at least 13,680 cal yr BP by the presence of taxa including the plants Penstemon cf. gormanii and Silene cf. involucrate subsp. tenella, and the weevil Connatichela artemisiae. These species remain components of grasslands and south-facing azonal steppe communities in present-day Yukon. This thesis further explores steppe persistence by analysing sedaDNA and pore-ice isotopes from Mint Gulch, a site spanning from ~16,000 to 13,180 cal yr BP. The Mint Gulch site indicates a co-occurrence of steppe and shrub-associated taxa by ~13,910 cal yr BP. The persistence of steppe-tundra, despite a shifting regional hydroclimate, is further supported by macrofossil records that include steppe-tundra taxa like Artemisia sp. (likely A. frigida), Potentilla sp., and Penstemon cf. gormanii, the dry-tundra weevil Lepidophorus lineaticollis, and the steppe-tundra weevil C. artemisiae. A multiproxy approach to the steppe-tundra to shrub-tundra transition captures the co-occurrence of steppe tundra taxa like mammoth and horse in addition to the arrival of browsers and shrub dependent taxa like moose, elk, and willow ptarmigan. Collectively, this thesis contributes to an understanding of the biodiversity, resilience and ultimately the collapse of Beringia’s steppe-tundra ecosystem at the end of the Pleistocene