1. You are a graduate student in paleontology, studying a fossil site below an a
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1. You are a graduate student in paleontology, studying a fossil site below an ancient coastal lagoon. A simplified and exaggerated profile of the rocks and sediments at your site is shown here. The profile is not drawn to scale Formation 1 is composed of fine-grained sediments in which mammal fossils are abundant, but no dinosaurs are found (assume you would have found some dinosaurs had there been any present on Earth when this layer was formed) Formation 2 is a layer of red clay in which several theropod and mammal fossils are found. Formation 3 is composed of gravelly sediments Formation 4 Chemical analysis of this layer and a few calculations reveal that of the 10,000 atoms you sample X atoms (see below) of the 4°K that was originally present in the sample (at the time the lava cooled) have decayed into the two daughter isotopes, 40Ar and 40Ca Formation 5 is rich in reef deposits where fossil fish are abundant. Formation 6 is another surface-deposited layer derived from a volcanic lava flow. Chemical analysis of a feldspar sample from this layer reveals that Y of the 40K originally in the sample at its formation have decayed into 40Ar and 40Ca Formation 7 is composed of fine-grained sediment Formation 8 is an igneous (granite) intrusion. Your chemical analyses of the granite crystals demonstrate that for every 100,000 atoms of 28U in the sample, there are Z (see below) atoms of 206Pb present 4 A) Based on stratigraphic analysis of the profile, list the order in which the formations (1-8) were formed, from most recent to most ancient. (1 point) Estimate the probably age (in terms of millions of years from the present) for each formation. For formations that can't be dated with radioisotopes, give a maximum and minimum age estimate, if both can be estimated from the data at hand. These estimates can be used to estimate the ages of fossils found in each formation. Based on your calculations (carry out all calculations to 5 significant digits), construct a table showing minimum and maximum age estimates for each formation. You will have to use outside information on known fossil ages and geological history to fill in a few of these maxima and minima! (10 points) Would it be surprising to also find fossils of dinosaurs in formation 5? Briefly justify your answer. (2 points) B) C)Explanation / Answer
A) Most recent to Most ancient: 1-2-8-3-5-7-6-4
B) From radiodating equation, we get
T= [t1/2 * ln(N0/N)]/ln2 where N0=initial, N=end number of parents atoms, t1/2 = half life
For 40K, t1/2 = 1.248 Ga
238U = 4.5 Ga
For formation 4, t = 3.19 Ga
For formation 6, t = 2.89 Ga
For formation 8, t=255 ma = Permian
Formation 3 is older than formation 8 and younger than 4,5,6. Thus, it is carboniferous-early permian.
Formation 5 has fish fossil which is of Ordovician (550 Ma) to Devonian (360 Ma).
Formation 2 has therapod which is of early Jurassic.
Formation 1 has mammal fossil without dinosaurs, thus it is after than 65 Ma (K-T boundary).
Formation 7 is younger than formation 6, thus is of Cambrian. It can be interpreted by maintaining the younging direction.
C) It would be surprising to find dinosaurs in formation 5 which is paleozoic. Dinosaurs are Mesozoic reptile. It will be true in case of bio-disturbance or reversal.
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