Fun with rocks, more fun with mud

Happy Friday to all again! I don’t know where the weeks go—they just fly by. I am still feeling pretty worn out from all the travelling we’ve been doing this summer, but I am feeling better than last week. Thankfully, I am taking a much-needed vacation at the beginning of September.

In my last post, I described our chief scientist training cruise on Lake Superior. Anthony and I collected a number of sediment cores, and one feature of the last set of cores we collected near Isle Royale intrigued me: sand-sized black spots. Now, I think up to this point I have neglected to mention why we are even interested in mud from the bottom of lakes. Sediments (mud) record the chemical, physical, and biological history of oceans and lakes. As geochemists, Anthony and I are most interested in discovering the current and past chemistry of natural waters. However, natural systems are unlike nice, neat laboratory experiments in that they are complex, and none of these processes (chemical, physical, and biological) is unaffected by the others. My approach to my science is to consider these systems as a whole as much as possible. Within sediments, there are mainly lithogenic, biogenic, and authigenic components. Lithogenic material is derived from rocks; biogenic material comes from living things; and authigenic material precipitates from the water. My suspicion was that the black spots were lithogenic material weathered from the surrounding land, but I didn’t know much about the local rocks. I was aware that the Upper Peninsula of Michigan and northern Minnesota are somewhat geologically special in the United States in that there are some very old rocks exposed there. Here is what I found out.

I will start with a disclaimer that traditional geology is not my forte, but I have been trying to learn as much as I can since I started graduate school. So be kind to me, and I will do my best to summarize the information. I looked into both the geology of the Upper Peninsula and northern Minnesota, ignoring the most recent rocks. About 1.1 billion years ago, the Midcontinent Rift formed. This rift is similar to what is currently happening in the Afar region of Africa, where a new ocean is forming from the Red Sea through East Africa. As the continental crust opened, basalt lava flows filled the Midcontinent Rift. However, rifting ceased after a few million years, and a new ocean failed to form. The rift was filled with sediments and compressed as the supercontinent Rodinia formed. Younger sediments later covered these rocks but they were exposed as the Pleistocene glaciers scoured the landscape. Along the western shore of Lake Superior, some of these rocks are known as the Duluth Complex. During the period of compression, hydrothermal fluids moved through the faults created during rifting, depositing native copper and other metal ores. Iron and copper mining have been sources of major economic activity in the Upper Peninsula. I found a much more detailed geologic history of the region in this fantastic field trip guide for those who are interested.

The result of all this is that I think my suspicions that the black grains in my core are igneous rock weathered from the area are correct. The other hypothesis about the grains was that they were bugs, but I don’t think so. Now that I know something about the local geology, I know what kind of contribution the local rocks can be making to the trace metal concentrations that I will measure. Hurrah for geology! See you next week.