Thomas C. Winter
Harvey L. Patten
H.E. Wright Jr.
1963
<p><span>Kirchner Marsh and Lake Carlson are located 3 miles apart </span><span class="ScopusTermHighlight">in</span><span> Dakota County about 15 miles south of Minneapolis </span><span class="ScopusTermHighlight">in</span><span> the St. Croix moraine, which was formed by the Superior lobe during the Gary phase of the Wisconsin glaciation. During the Mankato phase that followed, the Des Moines lobe advanced to within a few miles of the sites. The region today is </span><span class="ScopusTermHighlight">in</span><span> a mixed-oak forest, with a maplebasswood forest 15 miles to the west and a re-entrant of the prairie on the sand plain south of the moraine. The general limit of coniferous trees is about 50 miles northeast of the sites, although outliers, especially of Pinus strobus, may be found along the Mississippi Valley a few miles to the east. One sediment core 12-13 m long from each site was analyzed for </span><span class="ScopusTermHighlight">pollen</span><span> content at 5-25-cm intervals. </span><span class="ScopusTermHighlight">Diagrams</span><span> based on percentage of total </span><span class="ScopusTermHighlight">pollen</span><span> (trees, shrubs, wind-pollinated herbs) show essentially identical sequences at the </span><span class="ScopusTermHighlight">two</span><span> sites, starting with the late-glacial phase of ice retreat. The </span><span class="ScopusTermHighlight">diagrams</span><span> have been subdivided into </span><span class="ScopusTermHighlight">pollen</span><span> zones according to the A-B-C sequence introduced by Deevey for New England. The late-glacial </span><span class="ScopusTermHighlight">pollen</span><span> record starts at Kirchner Marsh with a short Picea-Cyperaceae-Gramineae phase (Zone K), believed to represent a spruce parkland. Its C-14 date of 13,270 BP and the stratigraphy indicate a pre-</span><span class="ScopusTermHighlight">Two</span><span> Creeks and post- Gary correlation. Apparently the Kirchner site did not become established as a lake until this time owing to persistence of dead ice </span><span class="ScopusTermHighlight">in</span><span> the moraine. The absence of </span><span class="ScopusTermHighlight">pollen</span><span> of specific tundra indicators and the presence of </span><span class="ScopusTermHighlight">pollen</span><span> of such thermophilous plants as Fraxinus, Quercus, Corylus, Ambrosia, Humulus, and Typha latifolia imply that the climate was cool rather than cold. Zone A-a, which follows, correlates with the </span><span class="ScopusTermHighlight">Two</span><span> Creeks interstade. It is marked by the dominance of Picea, with appreciable percentages of Fraxinus and Ambrosia and with minor amounts of other thermophilous plants and the normal boreal associates of spruce like Betula, Larix, and Salix. Zone A-b, starting 12,050 C-14 years ago, correlates with the Valders ice advance. It is represented at both Kirchner and Carlson and shows the withdrawal of Fraxinus and Ambrosia and the slight rise of Artemisia. Except for the absence of pine </span><span class="ScopusTermHighlight">in</span><span> the late-glacial assemblage the vegetation implied by these three zones seems to have its closest modern counterpart </span><span class="ScopusTermHighlight">in</span><span> the southern fringe of the Boreal Forest of the Riding Mountain region of southwest Manitoba. It is concluded that pine did not migrate southward with the spruce during the Wisconsin glaciation, at least </span><span class="ScopusTermHighlight">in</span><span> the western Great Lakes region, and was thus eliminated from this region. During the </span><span class="ScopusTermHighlight">lateglacial</span><span> phases of ice retreat, herbs and spruce pioneered on the deglaciated terrain; pine did not follow until the destruction of the spruce forest at the end of the late-glacial phase. Zone B introduces </span><span class="ScopusTermHighlight">postglacial</span><span> time. It represents the time of rapid </span><span class="ScopusTermHighlight">Vegetational</span><span> succession following the deterioration of the spruce forest. Simultaneous maxima of Betula, Alnus, Fraxinus, and Abies occurred 10,230 years ago at Kirchner Marsh. These were followed rapidly by a Pinus maximum and then a rise of Ulmus, Quercus, and other deciduous types, dated as 9300 years ago at the correlative site of Madeha. This succession may represent differential rates of migration from refuges south and east of </span><span class="ScopusTermHighlight">Minnesota</span><span>. Deciduous trees dominate the C Zones. Zone C-a shows Ulmus and Ostrya /Carpinus followed by Quercus; it probably represents principally a mesic maple-basswood forest changing to oak. Zone C-b represents the advance of prairie into the region at the expense of the oak woodland or savanna. The large and abrupt fluctuations </span><span class="ScopusTermHighlight">in</span><span> the curves for Ambrosia-type and Chenopodiineae, especially at the Carlson site, may record encroachment of annual weeds onto intermittently dried lake bottoms. C-14 dates place Zone C-b between 7100 and 5100 years ago. </span><span class="ScopusTermHighlight">In</span><span> Zone C-c the Quercus again dominates until the abrupt increase </span><span class="ScopusTermHighlight">in</span><span> Ambrosiatype and Chenopodiineae that marks the time of forest clearance and land settlement 50-75 years ago. </span></p>
application/pdf
10.1130/0016-7606(1963)74[1371:TPDFSM]2.0.CO;2
en
Geological Society of America
Two pollen diagrams from southeastern Minnesota: Problems in the regional late-glacial and postglacial vegetational history
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