SEDGE NEIL
(advisor: Amy Myrbo/Emi Ito)
Drill cores collected by the Minnesota Geological Survey (MGS) in a study of central Minnesota till stratigraphy were found to contain putative lake sediments between till horizons. This lacustrine material was of indeterminate age, but was suspected to date from a previous interglacial period. Alternately, the sediments could have been deposited during an interstadial (i.e., during a retreat of the ice sheet rather than a full interglacial). No pre-Holocene interglacial lake sediments have been described from Minnesota (H.E. Wright, pers. comm.), so these sediments were of some interest. Their possible utility in providing dating control on the bounding tills was also considered by MGS scientists.
Scientists at the Limnological Research Center (LRC) determined that the sediments were indeed lacustrine, based on lithology, mineralogy, and microfossils. Sedge Neil worked with MGS scientists Gary Meyer and Alan Knaeble to reconstruct core sections from field notes and then transferred the cores to split PVC for storage and high-resolution digital imaging. Later the sediment composition was characterized by (1) smear slide analysis for semi-quantitative grain size, mineralogy, diatom flora, and organic content; and (2) analyzing samples for total carbon (TC) and total inorganic (carbonate) carbon (TIC) by coulometric titration. The cores were found to be predominantly authigenic calcite (marl) with varying amounts of organic matter, diatoms, and clastic material, and are interpreted as a littoral or shallow-lake benthic facies, with fossil and lithological evidence of lake-level change. Samples were also taken for fossil pollen analysis by Dr. Ivanka Stefanova of the LRC, who found a floral succession very similar to that of the first several thousand years of the Holocene, i.e., a transition from spruce to pine forest and then to a mixed deciduous-grass savanna. She and Dr. Wright surmised from these data that the lake sediments were undoubtedly from an interglacial rather than an interstadial period, given the long period required for such a sequence of vegetation shifts.
Finally, samples from the most calcite-rich horizons were taken and treated, by dispersal in water, sieving at 40 m m, and freeze-drying, for exploratory uranium-series dating by Dr. R. Lawrence Edwards in the Minnesota Isotope Laboratory. The procedure was not completed until after the end of the intern session; Dr. Edwards determined that the sediments were >200,000 years old, although the exact ages could not be determined. The samples were smaller than ideal, and were somewhat "dirty," that is, with clastic material supplying too much initial Th to obtain dates with reasonably small error bars.
Based on these promising results, further work will likely be done on these cores.