Because of the presence of glaciers, our knowledge of lake deposits in Minnesota only goes back about 13,000 years. (Glaciers have a lot to answer for. On the other hand, the glaciers make it easy to picture the prevailing conditions at, say, 20,000 years ago. Just get yourself a block of ice and imagine it's a mile thick.) One of the most thoroughly studied records of lake sedimentation in Minnesota comes from a place called Kirchner Marsh, west of Highway 3 a couple of miles north of Rosemount. Kirchner Marsh is what used to be a lake that formed in an ice-block depression, the result of a large mass of ice buried in a moraine missing the last call for the return trip to Canada. The ice was left to melt where it was, creating a lake in the glacial sediment. Over the next few millennia, an assortment of organic-rich crud more or less filled in the lake. This is a pretty typical history for a lake in Minnesota, lakes after all generally being ephemeral landforms in terms of geologic time.
The study of the prehistory of Kirchner Marsh and similar sites in Minnesota involves collecting sediment cores, from which you can pull fossils, study chemistry, get radiometric dates, and so on. In the case of Kirchner Marsh, researchers have taken cores about 39 ft (12 m) in length, covering the Holocene through to the latest Pleistocene. So, what can you find in about 39 ft (12 m) of Dakota County marsh sediments?
The major fossils of interest have been pollen from conifers and flowering plants (Winter 1962a; Wright et al. 1963), and diatoms (Florin 1970, Florin and Wright 1969; Brugam 1980), which are algae that live in jewel boxes of silica. Other types of fossils reported from the marsh include:
- Cysts of golden algae (Brugam 1980)
- Plant crud (peat, gyttja: Wright et al. 1963; charcoal: Watts and Winter 1966)
- More identifiable plant fragments, such as leaf fragments, conifer remains (needles, seeds, and cones), and angiosperm seeds and fruits (Watts and Winter 1966)
- The green alga Pediastrum (Wright et al. 1963)
- Various plant spores, representing mosses, clubmosses, quillworts, spikemosses, horsetails, and ferns (Wright et al. 1963; Watts and Winter 1966)
- Pollen of Ephedra (Watts and Winter 1966)
- Fungal spores (Florin and Wright 1969)
- Sponge spicules (Brugam 1980)
- Fish scales (Watts and Winter 1966)
- And, although not quite fossils in the way we usually think of them, concentrations of fossil plant pigments (Sanger and Gorman 1972)
Once you've got the core and the fossils, what do you do with them? The classic type of study involves sampling the core at regular intervals for your fossil of choice, and then preparing a chart showing abundance versus position in the core. In some cases, such as fire studies, you are interested primarily in the changing abundance of charcoal over time. In other cases, such as pollen studies, you will have divided the pollen by plant species, so your chart may have a few dozen columns showing various species declining or increasing, with local departures and arrivals. From this information, you can interpret the site over time and compare the assemblages to modern settings: say for example at the bottom of the core is pollen from a spruce forest, then it transitions to a prairie, then a broadleaf forest, and back to a prairie. The presence or absence of certain plants and the composition of assemblages tell you about the local temperature and precipitation. With a few well-placed radiocarbon dates you can look at the timing for these changes as well.
In the case of Kirchner Marsh, a number of publications since the 1960s have built up a picture of the lake going back to about 13,270 radiocarbon years before present (radiocarbon years being similar but not quite the same as calendar years, and requiring conversion). The deposits are divided into a number of pollen zones with their own typical assemblages. In ascending order, these are K, A-a, A-b, B, C-c, C-b, and C-a. At the onset of deposition, the area was a cold late glacial spruce parkland (K). Over the next 5,000 years, the parkland became a spruce forest (A-a and A-b), then a more diverse conifer and hardwood forest dominated by pine, birch, ash, alder, and fir (B), and finally an elm-oak deciduous forest (C-a). These reflect rising temperatures. The deciduous forest was replaced by a prairie and oak savanna setting (C-b), which existed for about 2,000 years during the middle Holocene. This is a local expression of a widespread warm and dry episode. Over the most recent ~5,000 years or so, the area around Kirchner Marsh has been occupied by mixed oak forests, with moderated temperatures and precipitation.
Partial Kirchner Marsh bibliography:
Amundson, D. C., and H. E. Wright, Jr. 1979. Forest changes in Minnesota at the end of the Pleistocene. Ecological Monographs 49(1):1–16.
Baker, R. G., and E. A. Bettis, III. 1996. Midwestern paleoenvironments from stream cutbank exposures: A preliminary report. Abstracts with Programs––Geological Society of America 28(7):A-470.
Brugam, R. B. 1978. Comparison between post-glacial diatom, macrofossil and pollen stratigraphy at Kirchner Marsh, Minnesota. Program and Abstracts––American Quaternary Association Conference 5:191.
Brugam, R. B. 1980. Postglacial diatom stratigraphy of Kirchner Marsh, Minnesota. Quaternary Research 13(1):133–146.
Florin, M.-B. 1970. Late-glacial diatoms of Kirchner Marsh, southeastern Minnesota. Nova Hedwigia 31:667–729.
Florin, M.-B., and H. E. Wright, Jr. 1969. Diatom evidence for the persistence of stagnant glacial ice in Minnesota. Geological Society of America Bulletin 80(4):695–704.
Geiss, C. E., and S. K. Banerjee. 1998. Climate variability as seen in two interglacial records from the Midwestern U.S.A. Annales Geophysicae 16(supplement to 1):213.
Geiss, C. E., C. E. Umbanhowar, P. Camill, and S. K. Banerjee. 2003. Sediment magnetic properties reveal Holocene climate change along the Minnesota prairie-forest ecotone. Journal of Paleolimnology 30(2):151–166.
Overpeck, J. T., T. Webb, III, and I. C. Prentice. 1985. Quantitative interpretation of fossil pollen spectra: Dissimilarity coefficients and the method of modern analogs. Quaternary Research 23(1):87–108.
Sanger, J. E., and E. Gorham. 1972. Stratigraphy of fossil pigments as a guide to the postglacial history of Kirchner Marsh, Minnesota. Limnology and Oceanography 17(6):840–854.
Watts, W. A., and T. C. Winter. 1966. Plant macrofossils from Kirchner Marsh, Minnesota: A paleoecological study. Geological Society of America Bulletin 77(12):1339–1359.
Webb, T., III. 1980. The reconstruction of climatic sequences from botanical data. Journal of Interdisciplinary History 10(4):749–772.
Webb, T., III and D. R. Clark. 1977. Calibrating micropaleontological data in climatic terms: A critical review. Annals of the New York Academy of Sciences 288:93–118.
Winter, T. C. 1961. A pollen analysis of Kirchner Marsh, Dakota County, Minnesota. Thesis. University of Minnesota, Minneapolis, Minnesota.
Winter, T. C. 1962a. Pollen sequence at Kirchner Marsh, Minnesota. Science 138(3539):526–528.
Winter, T. C. 1962b. Late- and post-glacial pollen diagrams for Kirchner Marsh, southeastern Minnesota. Pollen et Spores 4(2):388.
Winter, T. C., and H. E. Wright, Jr. 1977. Paleohydrologic phenomena recorded by lake sediments. Eos 58(4):188–196.
Wright, H. E., Jr., T. C. Winter, and H. L. Patten. 1963. Two pollen diagrams from southeastern Minnesota: problems in the regional late-glacial and postglacial vegetational history. Geological Society of America Bulletin 74(11):1371–1395.