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CEES Supported Undergraduate Research

Guest blog submitted by undergraduate student, Samantha Brickles (sbrickle@umail.iu.edu)

In January of 2014 I embarked upon a research project with my faculty mentor, Dr. Kathy Licht. As an undergraduate Geology major at IUPUI, an independent research project has been an exciting and fulfilling opportunity. The project has been a rewarding learning experience and, like much scientific progress before me, a little puzzling at times. The project is titled “Zircon Geochronology of Indiana Till as an Indicator of Provenance”.  Zircons are naturally occurring minerals within the Earth’s crust that form in igneous rocks. They are not susceptible to weathering processes and therefore can also be found in sedimentary deposits eroded from the igneous rocks in which they originally formed.  Zircons act as geochronometers, tracking the passage of time as radioactive Uranium (U) within the crystal decays to Lead (Pb). The ratio of parent isotope (U) to daughter isotope (Pb) measured within the zircon defines its age, which in turn defines the formation age of the rock containing it. In this study, we look for a distinctive pattern or signature from the ages of zircon crystals in order to differentiate sources of glacial  deposits. 

Approximately 23,000 yrs. ago, during the Wisconsin glacial advance, three ice lobes from the Laurentide ice sheet originating in Canada flowed into Indiana (Fig. 1).  The Lake Michigan lobe entered from the north, the Erie lobe from the northeast, and the Saginaw lobe in between. The distribution of glacial deposits (till) from the lobes is easily discernable in some locations where the land has been molded into curved ridges called moraines, but in other locations, the ice sheet history is uncertain. This research investigates whether the ages of sand-sized zircon minerals in the glacial deposits can be used as a reliable tracer of glacial ice lobe movement when the direction of ice flow is ambiguous. I collected glacial till samples from Lake Michigan, Saginaw, and Erie lobe tills and one sample from a till of unknown origin using a shovel and Ziplock bags (Fig. 2). I then wet sieved the samples to isolate the sand fraction where zircons are found (Fig. 3), and finally, I analyzed the samples at Washington State University.

                                     

Figure 1: Map depicts glacial maxima in Indiana and simplified location of sampling sites. Squares represent samples which are located downstream from their source ice and star represents unknown-sourced sample (Indiana Geological Survey, 2013).

Figure 2: Samantha Brickles measures depth and collects till with shovel. Photo credit: Kathy Licht    

Figure 2: Samantha Brickles measures                    Figure 3Example of wet sieving. 
depth and collects till with shovel.
Photo credit: Kathy Licht

Because the upstream geology has distinct ages indicated by the zircons, it was hypothesized that the samples would reflect that by containing distinctive zircon populations. For instance, upstream of Lake Michigan the crust is 2.65-3.20 Ga (billion years) and upstream of Lake Erie, the crust is 1.0-1.3 Ga and therefore I hypothesized that the Lake Michigan  till would have an abundance of 2.65-3.20 Ga zircons. A total of 450 grains were analyzed via MC-LAICPMS and zircon ages range from ~4.05-3.11 Ga. surprisingly all 4 samples have broadly similar distributions with dominant populations 1-1.3 Ga, and smaller peaks 2.55-2.8 Ga. Because there is a strong similarity in the distribution of ages between the four samples and a simple explanation cannot be reached connecting the unknown sample to one of the known ice lobes, more work is needed to assess whether this methodology could be successfully used in the Midwest as an indicator of till provenance.  Other factors such as eroded sedimentary rocks in the Michigan basin and reworking of older tills may need to be taken into consideration.

Perhaps the most important benefit to understanding our glacial history is groundwater resource management. Indiana groundwater flows in layers of sand and gravel several tens of feet thick, deposited between extensive layers of till. When we have an understanding of where this till comes from, its thickness, and its composition, we can improve mapping of aquifers and better address groundwater contamination.

We issue great appreciation to CEES and the Indiana Geological Survey for making this pilot research project possible.

 

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