Two Creeks Interstade Dated through Dendrochronology and AMS

Dendrochronological analysis of fossil wood from Two Creeks, Wisconsin, reveals that the Two Creekan Intetstade lasted at least 252 yr. The sites crossdated by tree rings cover an area of about 970 km2. AMS determinations from the beginning and end of the chronology open a 14 C time window for the episode from 12,050 to 11,750 yr B.P. The interval is contemporaneous with the Older Dryas in northern Europe. The development of a forest covering at least 970 km2 on the western shore of Lake Michigan indicates a water level about as low as in modern times. Glacier retreat must have opened drainage channels either through the Straits of Mackinac or via the Indian River Plateau into the eastern lakes. The beginning of the tree-ring chronology coincides with the peak of meltwater pulse 1A at 12,000 yr B.P. Increased amounts of meltwater seem to have disturbed the heat exchange between the waters and the atmosphere in the North Atlantic off the Gulf of St. Lawrence or affected the δ18O-ratio of the evaporation, causing the climatic or isotopic reversal of the Older Dryas in Greenland and northern Europe.

Evidence for the Two Creeks interstade in the Lake Huron basin

New radiocarbon age dates for the Cheboygan bryophyte bed in northern lower Michigan indicate that the bed was not deposited during the Mackinaw interstade, as was previously proposed, but is correlative to the Two Creeks forest bed deposited during the Two Creeks interstade approximately 11 850 BP. Furthermore, the till overlying the bryophyte bed does not represent continuous deposition by ice throughout the Two Creeks interstade, as proposed by others, but represents deposition during the Greatlakean stade. A major implication resulting from the reassignment of the age of the Cheboygan bryophyte bed is that the Straits of Mackinac could have been ice free during the Two Creeks interstade and that during that time the Kirkfield phase of glacial Lake Algonquin may have extended into the Lake Michigan basin.

Evidence for the intra-Glenwood (Mackinaw) low-water phase of glacial Lake Chicago

A 14C age estimate of 13 470 ± 130 BP (ISGS-1378) from organic material at the base of transgressive lake deposits exposed in a southern Lake Michigan shore bluff near Riverside, Michigan, confirms that an intra-Glenwood low-water phase occurred in the Lake Michigan basin during the Mackinaw (Cary – Port Huron) Interstade. The altitude of organic material at Riverside suggests that the water plane was at or below modern lake level during the intra-Glenwood low-water phase. This observation indicates that drainage from the Lake Michigan basin was eastward, probably through the Straits of Mackinac into the Lake Huron basin. Such a drainage pattern implies that the correlative lake occupying the Lake Huron basin (Arkona low-water phase) must have had a level either congruent with or below that of the Lake Michigan basin. This lake system ultimately drained from the Lake Huron basin through a low eastern outlet (probably the Trent River lowland). The existence of low-level lakes at this time also indicates that the ice margin of the Lake Michigan lobe must have retreated at least as far north as the Straits of Mackinac region during the Mackinaw Interstade.

Lakes Michigan and Huron limit gene flow between the subspecies of the butterfly Limenitis arthemis

Near the Straits of Mackinac, the Limenitis arthemis population on Michigan's Upper Peninsula consists mostly of the disruptively banded L. a. arthemis, while the population on its Lower Peninsula consists mostly of the unbanded, mimetic L. a. astyanax and arthemis–astyanax intergrades. Except at the straits, the Upper and Lower peninsulas are broadly separated by lakes Michigan and Huron. On the Lower Peninsula, arthemis-like forms are most common on the shore close to the Upper Peninsula but are much less frequent only 20 km south, probably because of the northward flow of astyanax genes. Neither population is in Hardy–Weinberg equilibrium because of a deficiency of "hétérozygotes," possibly because of cross-lake emigration at the straits.