During Spring 2007 the Geochemistry of Natural waters class conducted four projects. The abstracts, papers and posters of these four are given below. The first two abstracts were presented at the GSA meeting in Denver in the fall of 2007.
1: Geochemisry of sericite (white mica) from a saprolite of the precambrian morton gneiss in the minnesota river valley of southwestern minnesota
Jack Gibbons, Department of Geology, Carleton College, Northfield, MN 5505, gibbonsj@carleton.edu, Zack McGuire, Department of Geology, Carleton College, Northfield, MN 5505, mcguirez@carleton.edu, Michael Barret, Department of Geology, Carleton College, Northfield, MN 5505, barrettm@carleton.edu, William Mitchell, Department of Geology, Carleton College, Northfield, MN 5505, mitchelb@carleton.edu and Bereket Haielab, Department of Geology, Carleton College, Northfield, MN 5505, bhaileab@carleton.edu
The saprolite developed on the Precambrian Morton Gneiss in the Minnesota River valley of southwestern Minnesota is ideal for studying the rates of chemical and physical weathering. In addition to the few remaining original minerals of the parent rock, the saprolite contains secondary minerals such as kaolinite and sericite (white mica). Sericite is a by-product of weathering of K-feldspar, and makes up ~ 1-2 % of the saprolite. The sericite grains range in size from 0.3 to 0.5 mm in diameter. For this study, sericites were separated from the bulk sample and analyzed for their chemistry using X-ray diffractometer (XRD), scanning electron microscopy (SEM), scanning electron microprobe (SEMP), and Fourier-transform infrared spectroscopy (FTIR). Bulk sample was analyzed for major, minor and trace elements using X-ray fluorescence. In the field, in the hand sample, under the SEM, and under the petrographic microscope, the sericite appears to be true white mica. However, analysis of the sericite using SEM, SEMP and XRF indicate that the sericite within the saprolite are highly potassium (K) depleted, with range of 0.04 - 0.11 wt. % K compared to the published range of ~ 8-11 wt. %. We propose that the K-depletion of the sericite resulted from an additional weathering event that took place since the formation of the sericite and the sericite appears to be transitioning chemically towards kaolinite.
Keywords: Morton Gneiss, Weathering, Saprolite, Sericite, White mica
2: Carbon and oxygen isotope analysis of modern freshwater mollusk shells: applications for climate reconstruction
Lauren C. Andrews, Department of Geology, Carleton College, Northfield, MN 5505, andrewsl@carleton.edu, Sarah C. Bergman, Department of Geology, Carleton College, Northfield, MN 5505 bergmans@carleton.edu, Gloria Jimenez, Department of Geology, Carleton College, Northfield, MN 5505, jimenezg@carleton.edu, Yuichiro Takeshita, Department of Geology, Carleton College, Northfield, MN 5505 takeshiy@carleton.edu, Carl Andrew Ulberg, Department of Geology, Carleton College, Northfield, MN 55057, ulbergc@carleton.edu and Bereket Haileab, Department of Geology, Carleton College, Northfield, MN 5505, bhaileab@carletonl.edu
Stable carbon and oxygen isotope profiles from modern bivalve shells can be used to reconstruct hydrologic changes in freshwater bodies. Potamilns alatus and Lampsilis sp. from the Cannon River, southeastern Minnestoa, display clear seasonal growth banding profiles that were analyzed for d 18O and d 13C content. The d 18O recorded in the Potamilns alatus shell is mainly controlled by seasonal fluctuations in the hydrologic conditions of the Cannon River as demonstrated by the persistent variation between seasonally deposited growth bands. d 13C signatures from the Potamilns alatus demonstrate less coherent variations and are more highly dependent on individualized metabolic processes, masking possible environmental influences. Using d 18O signatures of seasonal growth bands, shells of the freshwater mussel Potamilns alatus offer a high-resolution record of temperature variation on seasonal to multi-year time scales. In contrast, the Lampsilis sp. shell studied shows little variation on any time scale and may be less useful as a paleoclimate indicator. Changes in growth band d 18O in Potamilns alatus shells reflect variations in ambient water temperature and geochemistry of the Cannon River, so that the mollusk may serve as a high-resolution paleoclimate proxy.
Key words: Oxygen, Carbon, Isotope, Freshwater, Mussels
3: Oxygen isotope analysis of sericite from a saprolite near Redwood Falls, Minnesota
Jake Gold, Hannah Kinzie, Ellen Root, and Ellen Valkevich
Stable oxygen and hydrogen isotope ratios can be used to infer paleoclimatic and paleotemperature conditions for periods throughout Earth’s history. Secondary minerals formed in igneous and metamorphic rocks during periods of weathering record isotope ratios of fluids at the time of weathering. Analyses of sericite grains collected from weathered Morton Gneiss saprolite near Redwood Falls, Minnesota yielded δ18O values of 40-50 o/oo, suggesting water-rock interaction with enriched 18O waters. Recorded values are higher than those expected from natural processes, suggesting the need for further refinement of analysis methods in order to obtain accurate oxygen isotope ratios for micaceous minerals.
Key words: saprolite, sericite, the Morton Gneiss, Redwood Falls, Minnesota, oxygen isotope
4: The Role of Iron-oxidizing Bacteria in Aquifers of Southeastern Minnesota: An Analysis of Springs in the Cannon River Wilderness Park and Goliath's Cave
Megan Rohrssen, Susan Schnur and Chris Ward
Iron-oxidizing bacteria have long been associated with the formation of iron-oxide-coated mats at springs in southeastern Minnesota. However, no study has yet attempted to characterize the complex ecosystems involved in the production of these mats and the impacts of iron-oxidation on stream chemistry. The presence of iron-oxide-bearing mats at springs and in cave systems reflects groundwater redox processes that have yet to be fully documented. Iron-oxidizing bacteria from two circumneutral groundwater systems in SE Minnesota were cultured in FeS-O gradient tubes. Bacteria from different sites grew at different levels in the tubes, indicating differing optimum iron and oxygen concentrations. A discussion of potential redox cycles and possible future interdisciplinary research topics is presented.