Relative Dating

1. Mammoths? Mrs. Awad, Mr. Baldwin & Mr. Bihn Peat Clay Layer Glacial Till Dolomitic Limestone If Our School Was Here Over 10,000 Years Ago, What Would Our Mascot Be? Abstract / Background Information Bones of mammoths with odd marks on them have been found in northern Illinois and southern Wisconsin. Illinois is not the typical habitat of large elephant-like animals, so why do we find mammoth bones in Illinois and what are these strange lacerations on them? In 1936, work crews channelizing the outflow creek from Mud Lake encountered roughly 20 leg and foot bones of a Woolly Mammoth. These bones represent what scientists believe to be butchered remains of Woolly Mammoths that place Paleoindians in the area 1,000 years before originally thought (FIA, 2004). Relative dating have placed these bones in a sequence of events that shows these animals lived relative recently in relation to soil and rock layers beneath them. Absolute dating techniques have been used to put an actual date on them as well. Geologic practices have helped tell the story of why mammoths are found in northern Illinois. Geologic Time Discussion • The mammoths found in northern Illinois were dated found to be 14,470 years old (FIA, 2004). Carbon-14 dating was used to determine this age. The half-life of C14 is 5,730 years (Tarbuck & Lutgens, 2005). Using the graph in Figure 3, we can determine that after 14,470 years, C14 would decay, through beta decay, roughly 2.53 half-lives. At the time the bones were excavated (Figure 5), there was: • - 34.75% of the original C14 • 65.25% changed to N14 through beta decay • the graph in Figure 3, we can determine • that after 14,470 years, C14 would decay, • through beta decay, roughly 2.53 • half-lives. At the time the bones were • excavated (Figure 5), there was: • - 65.25% changed to (Tarbuck & Lutgens, 2005). Figure 4. Living organisms, such as mammoths, constantly cycle C14 until they die (Belew, 2009) Absolute Dating Radioactivity – Decay of parent isotopes into daughter isotopes Half-life is the number of years it takes for ½ of the original number of parent isotope atoms to decay to a particular daughter isotope, as can be seen in the decay of U238 to Pb206 as depicted in (Figure 1) (Tarbuck & Lutgens, 2005). Knowing the ratio of parent to daughter isotopes can be used to determine the age of the sample. The leg bone seen in (Figure 5) to the right contained both radioactive Carbon -14 and normal Carbon -12. The mammoths found in northern Illinois were radiocarbon dated and found to be 14,470 years old (FIA, 2004). This was done by first determining that 34.75% of the original C14(Parent Isotope) had changed to N14 through beta decay. Then using (Figure 3) it was determined that 2.53 half lives had elapsed since the mammoth died. Since the half-life of C14 is 5,730 years, a little multiplication yields the age of the mammoth 2.53 x 5,730 = 14,470 years old (Tarbuck & Lutgens, 2005). Figure 5. Samples were taken from this lower leg bone from a mammoth found in Kenosha, WI for C14 dating (FIA, 2004). Figure 6. Overhead picture of the Mud Lake excavation site where leg bones were removed during a construction project (FIA, 2004). Geologists were able to decide how deep to dig based on layers found at other sites (Figure 6). Depositional Environment Figure 1. Pathway of unstable parent isotope U238 to daughter isotope Pb206 following steps of alpha and beta emissions (Tarbuck & Lutgens, 2005). -Swampy Environment -Terminal glacial deposits with low energy water present -Melting glacier deposits -Shallow ocean with CaCO3 precipitation Conclusions To answer the question, “Why are mammoth bones found in northern Illinois?” one simply needs to look at the geologic cross sections in the area. Relative dating shows that a sequence of environments were present in the area before the grasslands and wetlands we are used to today. The cross section of the Mud Lake site displays what once was a swampy environment that came after a site that was near the terminus of a retreating glacier. This shows climate in the area was once much different than what we are used to today. Putting an actual date on these bones found places this environment roughly 14,000 years ago. Knowing the average decay rate of C14 helped geologists place a numerical date on these mammoth bones. It was previously thought that human ancestors did not occupy the area until 13,000 years ago. Relative dating and absolute dating practices have now moved this date back an extra 1,000 years. Markings on the bones shows these animals were butchered by recent human ancestors. This helps place a landmark with a date for the evolution and migration of Homo sapians living in North America during the Pleistocene. Had Maine East been located in the area nearly 10,000 years ago, we may have changed from Demon Pride to Mammoth Pride. Figure 3. Parent Daughter isotope curves plotted over several half lives. The percent parent plus the daughter always equals 100. Relative Dating Relative Dating is a process by which rocks, fossils, etc. are placed in their proper sequence of formation (i.e. which formed 1st, 2nd, 3rd, etc. -Scientists do not try to assign a specific numerical age to the rocks, fossils, etc., they are simply placed in an ordered sequence by which were deposited or created. Several laws or principles are applied to assign the relative date (Tarbuck & Lutgens, 2005). Mammoth fossils found in the area were from the Pliestocene Epoch of the Quaternary Period The Law of Superposition states that the Sedimentary rocks on top are younger than the undisturbed Sedimentary rocks below them (Tarbuck & Lutgens, 2005). The mammoth bones were found in a clay layer, on top of a layer of glacial till, which was located on top of a layer of Dolomitic Limestone and directly below a layer of peat. According to the Law of Superposition, the layer of peat is the youngest while the layer of Dolomitic Limestone is the oldest because the peat horizon is on the top (Figure 7). References Belew, B (2009). Evolution 101: What are the problems with Carbon-14 Dating? Retrieved March 10, 2010, from http://www.examiner.com/x-4865-Christian-Worldview-Examiner~y2009m8d14-Evolution-101-What-are-the-problems-with-Carbon14-dating. Friends of the Ice Age. (2004) Butterbroad, J. woollymammoth.org. Retrieved February 21, 2010, from http://www.woollymammoth.org/. Tarbuck, E. J. & Lutgens, F. K. (2005). Earth: an introduction to physical geology, 8th ed. Pearson Education, Inc., Upper Saddle River, New Jersey. Wisconsin Geological and History Survey. (2009). Bedrock Geology of Wisconsin [Map]. Retrieved February 23, 2010, from http://www.uwex.edu/wgnhs/bdrk.htm Woolly Mammoth (2009). Illinois State Geologic Survey. Retrieved February 22, 2010, from http://www.isgs.illinois.edu/education/ice-age-res/mammoth.shtml. *Unless noted, all pictures from class handouts or were created by one of the authors Figure 7. Mock cross section of the layers found in the Mud Lake excavation site in Kenosha, WI (drawing not to scale).