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6. Discussion and Synthesis of the Processes and Timing of Dune Formation and Archaeological Site Burial in Coastal Settings of Lake Michigan
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111 chapter six Discussion and Synthesis of the Processes and Timing of Dune Formation and Archaeological Site Burial in Coastal Settings of Lake Michigan Integrating the Natural Factors Chronology of Dune Formation Results from this study provide a comprehensive record of eolian sand mobilization and dune formation, particularly in the northeastern part of the Lake Michigan basin. In contrast to previous studies of dunes along the southeastern shoreline (e.g., Arbogast and Loope 1999; Arbogast et al. 2002; Hansen et al. 2003), which largely reconstructed dune histories through radiocarbon dating of charcoal in buried soils, this study systematically used OSL dating to establish chronological control to define intervals of eolian sediment deposition. Conversely, periods of dune stabilization were defined by the presence of paleosols using both 14C ages of organic material within them and by the age and duration of archaeological deposits where they are included. As presented in appendix A, OSL dating estimates the most recent time that sand grains were exposed to sunlight; in other words, it estimates the most recent time the sediment was being blown by the wind and deposited to form dunes. As a consequence, we are able to more accurately assess the timing of dune construction because OSL provides an age estimate of the specific moment of final sand deposition. Conversely, as the age of the samples increases, for any given number of measurements, the confidence interval of the sample (i.e., the standard deviation, or σ) also becomes larger. Unless one obtains increasingly larger numbers of measurements as sample ages increase, measurements of older time periods are less precise than those of more recent time periods. Chronological interpretations derived from 14C dating of buried soils, in contrast, may be erroneous due to uncertainties associated with the residence time of organic materials and record intervals of dune stability, not construction. Despite our intentional focus on sampling and dating middle Holocene dunes, the outcome of our research revealed that few such dunes actually exist. Moreover, 112 Locations of Important Archaeological Sites and Dune Sample Locales around Lake Michigan 50km L a k e M i c h i g a n Lake Huron Lake Superior “hinge line” Eastport, Torch Bay Scott Point EkdahlGoudreau Manistique sand quarry Winter Summer Is. Antrim Creek, Barnes Twp. North Manitou South Manitou Fisherman’s Island O’Neil, Solomon Seal Mt. McSauba Brevort, Moran, and Round Lake dune fields Ludington State Park Sleeping Bear Portage, Petoskey State Park Wilderness State Park Arcadia Camp Miniwanca Warren Dunes State Park Wycamp Creek Frankfort upli u p l i subsidence s u b s i d e n c e Porter Creek South 6-1. Locations of archaeological sites and dune-sampling locales around Lake Michigan . Dashed line indicates the approximate position of the Algonquin and Nipissing hinge line; after Leverett and Taylor (1915). Areas of presumed subsidence and uplift labeled on either side of “hinge line.” Dune Formation and Archaeological Site Burial 113 the few middle Holocene dunes identified are also not the largest dunes within any given dune series. The oldest dunes recognized in this study occur near the village of Eastport and date to circa 5,300 years ago. This pulse of dune formation correlatesverywellintimeandaltitudewithLakeNipissing(Lewis1969;Monaghan and Lovis 2005). Similar age dunes have also been generally recorded in nearby high-perched dunefields at Sleeping Bear Dunes National Lakeshore (Snyder 1985) and at the Arcadia dunes (Blumer 2008). A significant finding of this study is that construction of coastal dunes on topographically low lacustrine surfaces in the northeastern part of the basin during the middle Holocene was apparently rare. The dune at Eastport was the only location sampled during this research that provided an age associated with the Nipissing phase. Although a similar age of circa 4,800 years ago was acquired at Petoskey State Park (Cordoba-Lepczyk and Arbogast 2005), it is the only other known sample dating to this period in this part of the basin. This chronological pattern stands in stark contrast to the record of dune construction along the southeastern shore of Lake Michigan, where significant mid-Holocene dune deposits have been recognized near Muskegon (Arbogast and Loope 1999), Holland (Arbogast et al. 2002), and Van Buren State Park (Van Oort et al. 2001). In this context, backdunes dating to this period of time near Holland are built up to 10 m high. The first extensive period of dune construction recognized on topographically low lacustrine surfaces in the northeastern part of the Lake Michigan basin began circa 3...