Testing Lake Cores for Magnetic Signals to Determine mid-Holocene
Warming in Southern Minnesota
The primary focus of my work during the internship with Professor Subir Banerjee at the Institute for Rock Magnetism this summer was to determine whether or not local lake cores contain magnetic signals which signify the onset of the southern Minnesota warm (prairie period), which occurred in the mid-Holocene. My first efforts were spent analyzing a photograph of a core taken from Steel Lake, in southern Minnesota. The photograph, provided by Herb Wright of the University of Minnesota, pictured a series of core drives which reached a depth of approximately 900 meters and contained dark and light layers (laminations). It is known that these laminations are created annually, due to the seasonal changes and theoretically represent the time preceeding the warm period, as well as the warm period itself. I had originally hoped that by counting each of the varves and by recording the core's color variations, I would be able to determine an interesting pattern of sedimentation and deposition. I started by counting the exact number of varves per centimeter and simplified the results by averaging the varve count over a 1 O-cm section, in order to create a more distinctive trend (Fig 1).
Focus was then turned to Kimble Pond, also located in southern Minnesota. I began by plotting ARM/IRM data given to me by Professor Philip Camill (of Carleton College). Within the time period which I plotted (6ka to 1 Oka) there was a discernable fluctuation showing transitions from warm to cold periods between 8ka and 9.7ka; thus, I decided to concentrate my measurements on the pond cores representing this time period. I used the Lakeshore susceptometer to measure magnetic properties of more than 35 freeze-dried pond core samplesówhich were provided by professors Phillip Camill and Charles Umbanhower (of St.Olaf College). From the Lakeshore measurements, I derived each of the sample's frequency dependence (fd) of magnetic susceptibility (K), or Kfd, a proxy for bacteria-produced particles created in the colder, forest period.
Additionally, Professor Camill provided me data which correlated sample
number to sedimentation depth below the ice, which I plotted with the ARM/IRM
100 data and the derived Kfd values. ARM/IRM is a proxy for small grain
sized particles and it was predicted that possible correlated variations
with Kfd would signify gradual warming and cooling. Overall, there was
no distinguishable difference between the Kfd values for the pre-warming
and warming period (Figs. 2 and 3). Nor did there seem to be a very tight
ARM/IRM correlation with Kfd (Fig. 4).
