Browsing by Subject "Sediment cores"

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    The geologic history of Lake of the Woods, Minnesota, reconstructed using seismic-reflection imaging and sediment core analysis
    (2014-07) Hougardy, Devin D.
    The history of glacial Lake Agassiz is complex and has intrigued researchers for over a century. Over the course of its ~5,000 year existence, the size, shape, and location of Lake Agassiz changed dramatically depending on the location of the southern margin of the Laurentide Ice Sheet (LIS), the location and elevation of outflow channels, and differential isostatic rebound. Some of the best-preserved sequences of Lake Agassiz sediments are found in remnant lake basins where erosional processes are less pronounced than in adjacent higher-elevation regions. Lake of the Woods (LOTW), Minnesota, is among the largest of the Lake Agassiz remnant lakes and is an ideal location for Lake Agassiz sediment accumulation.High-resolution seismic-reflection (CHIRP) data collected from the southern basin of LOTW reveal up to 28 m of stratified lacustrine sediment deposited on top of glacial diamicton and bedrock. Five seismic units (SU A-E) were identified and described based on their reflection character, reflection configuration, and external geometries. Three prominent erosional unconformities (UNCF 1-3) underlie the upper three seismic units and indicate that deposition at LOTW was interrupted by a series of relatively large fluctuations in lake level. The lowermost unconformity (UNCF-1) truncates uniformly draped reflections within SU-B at the margins of the basin, where as much as four meters of sediment were eroded. The drop in lake level is interpreted to be contemporaneous with the onset of the low-stand Moorhead phase of Lake Agassiz identified from subaerial deposits in the Red River Valley, Rainy River basin, and Lake Winnipeg. A rise in lake level, indicated by onlapping reflections within SU-C onto UNCF-1, shifted the wave base outwards and as much as 11 m of sediment were deposited (SU-C) in the middle of the basin before a second drop, and subsequent rise, in lake level resulted in the formation of UNCF-2. Reflections in the lower part of SU-D onlap onto UNCF-2 only near the margins of the basin, suggesting that water occupied much of the middle of the southern basin after lake level drawdown. The reflection character and configuration of SU-C and SU-D are genetically different indicating that the depositional environment had changed following the formation of UNCF-2. Piston-type sediment cores collected from the southern basin of LOTW at depths that correspond to the middle of SU-D contain high amounts of organic material and charcoal fragments and sediment that are probably not related to Lake Agassiz. Instead, they were likely deposited during a transitional phase between when Lake Agassiz left the LOTW basin (UNCF-2) and inundation of LOTW from the northern basin due to differential isostatic rebound (UNCF-3). All sediment cores collected from the southern basin of LOTW record the uppermost unconformity, analogous in depth to UNCF-3 in the seismic images, which separates modern sediments from mid to late-Holocene sediments. The lithology of sediments below this unconformity varies across the basin from gray clay to laminated silt and clay. Radiocarbon ages from two peat layers immediately below the unconformity indicate that subaerial conditions had existed prior to the formation of UNCF-1, at about 7.75 ka cal BP. The timing correlates well with other lakes in the upper Midwest that record a prolonged dry climate during the mid-Holocene. UNCF-3 is planar and erosional across the entire survey area but erosion is greatest in the northern part of the basin as the result of a southward transgressing wave base driven by differential isostatic rebound. Deposition in the southern basin probably resumed around 3.3 ka cal BP, though no radiocarbon dates were collected directly above UNCF-3. The lithology of sediment above UNCF-3 is highly uniform across the basin and represents modern sedimentation. Late-Holocene sedimentation rates were calculated at about 0.9 mm year-1 and are roughly double the sedimentation rates in the NW Angle basin, suggesting that erosion of the southern shoreline contributes significantly to deposition in the southern basin.
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    A postglacial record of climate change from East Crooked Lake and Tofte Lake, Minnesota.
    (2010-09) Wendt, Kelly Michael
    The postglacial Holocene climate history of Minnesota is characterized by a cool postglacial period beginning about 10 ka and lasting until about 8 ka when dryer conditions ushered in a Prairie Period which lasted until about 4 ka at which time moisture increased. From about 4 ka to the present climate conditions have remained relatively stable. This history of Minnesota climate has been observed by Dean and colleagues in numerous studies at Elk Lake, Minnesota. In these studies the researchers looked at pollen, geochemistry, diatoms, magnetic properties, and isotopes. This study uses sediment cores from nearby East Crooked Lake to see if scanning X-ray fluorescence results are comparable to classic inorganic geochemistry results from Dean at Elk Lake. A sediment core from Tofte Lake near Ely, MN is also compared to see if the results from Elk Lake and East Crooked Lake are regional in extent. The results confirm that scanning XRF is comparable to the classic, time consuming geochemistry methods used by Dean at Elk Lake and that the postglacial Holocene history recorded at Elk Lake was also recorded at East Crooked Lake. The timing of events at Elk Lake and East Crooked Lake are adjusted to correlate with the more accurate dating of events at Steel Lake, MN. The findings do not support that the climate events at Elk Lake and East Crooked Lake are regional in their extent to Tofte Lake. Tofte Lake did not experience a dryer Prairie Period.