Late Holocene (Little Ice Age Interval) Microvertebrates from Mackinac County, Michigan

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ABSTRACT

Microvertebrate remains from a dolomitic fissure in the Middle Silurian Engadine Dolomite, Mackinac County, Michigan, represent at least 9 species of Late Holocene (Little Ice Age Interval) vertebrates. These species include Bufo americanus and Pseudacris crucifer (a toad and a frog), Blarina brevicauda and Sorex cinereus (shrews), Myotis cf. Myotis septentrionalis (a bat), Peromyscus maniculatus or Peromyscus leucopus (mice), and Clethrionomys gapperi, Microtus pennsylvanicus, and Synaptomys cf. Synaptomys borealis (voles). All of these currently live in Mackinac County except for the Northern Vole (Synaptomys cf. S. borealis), which occurs in boreal habitats well north of Lake Superior. If the identification of the Northern Vole is correct, the temperature at the time of the accumulation of the bones was probably cooler than at present, and there was probably a greater percentage of coniferous trees in the area. Wetlands must have been present nearby to support the aquatic larvae of the toad and the frog. Wetlands currently lie within a few hundred meters of the site.

INTRODUCTION

Pleistocene vertebrate faunas have not been published from the Upper Peninsula of Michigan (Holman 2001) and, as far as we can ascertain, no natural assemblages of Holocene vertebrates have been described from the area. Thus the recovery of small, Late Holocene vertebrates from Mackinac County, Michigan, that represent the Little Ice Age Interval is of considerable interest. This locality is not an archaeological site, but represents a biota that accumulated under "natural" environmental conditions. This paper is a preliminary account of the Late Holocene microvertebrates from Mackinac County.

The Holocene Little Ice Age

The Holocene Epoch began about 10,000 years ago and was marked by the absence of many large terrestrial mammals that became extinct at the end of the Pleistocene (e.g., Holman 2001; Meltzer and Mead 1983). Holocene climates exhibited a warming trend in North America until about 5,000 years ago (Matsch 1976). Between about 7,000 and 5,000 years ago the warming trend peaked, a time span called the Hypsithermal Interval. At this time the world temperature is thought to have averaged about 2 to 3 degrees C warmer than it is at present, and the climate was relatively dry. Since the Hypsithermal Interval, the earth has generally become cooler and moister. In fact, this climatic trend has been called a Neogtaciation based on the growth of mountain glaciers in North America.

Detailed studies of glacial deposits postdating the Hypsithermal Interval along western North American glacial margins indicate that glacial expansion occurred during three intervals; the last of these three began in the fourteenth century and is called The Little Ice Age. In the European Alps, historical records show that the Little Ice Age itself was composed of several episodes of advances and retreats, some of which resulted in crop failures and famines. In fact, farms built near the glacial margin in Iceland were destroyed by an ice advance in the 1700s. In the middle 1800s ice began to retreat all over the world.

According to Kapp (1999), the most detailed, close-interval, paleo-ecological study that covers the last 1500 years in Michigan was done by Bernabo (1981) along a NW-SE transect from Charlevoix to Crawford Counties in northern Lower Michigan. In that statistical analysis, modern pollen and climatic data were compared with pollen data from sediment cores at four lakes. Bernabo suggested that a prolonged cooling period began 1200 years ago and that cooling reached a peak (1 degree C below a 1931-1960 mean) by the 1700s. He suggested that a warming of 0.5 degrees C occurred from AD 1750 to 1850. The Holocene vertebrate site from Mackinac County, Michigan, reported here has a radiocarbon date of AD 1732 [+ or -] 40 years before the present (YBP), thus it is of special interest. …