Laboratory 2: Geologic Time

Assignment:

In the lab manual (5th Edition of manual by Busch) read laboratory 7
Do problems

3, 7, 8, 9, 10, 11

Do the Ice Core problem, given below

Ice Core problem:

Given the increased concern about global warming, accurate understanding of the history of climate change becomes all the more critical. The study of drill cores from ice sheets in Greenland and Antarctica has yielded important results through the analysis of stable isotope ratios such as O¹⁸/O¹⁶ in the H₂O contained in the ice at varying depths in the drillhole. One problem inherent in this technique is the determination of the absolute age of the ice at any particular level. While individual annual layers may be countable at shallow levels, deeper in the ice these layers become thinner and eventually indistinguishable. Scientists must make numerous assumptions if they wish to be able to compare data from one core to another or to that from other paleoclimatological studies (e.g. tree rings, pollen studies, etc.).

PROBLEM:

We have cored a portion of the Greenland ice cap to a depth of 1 kilometer. At the surface an annual layer of snow is 1 meter thick. In the lower portions of the core, annual layers are indistinct. We have information, however, that in another Greenland core annual layers at 1000 m average 1 cm thick. Assuming that our core is similar and that the rate of change of the ratio of age to depth is constant, we want to derive a formula which expresses the age of the ice at any given depth and determine the age of the oldest ice in the core. A similar formula expresses the distance traveled at a constant acceleration ( distance = 1/2 at² ). In our case, age = 1/2 r (depth)², where r is the rate at which the number of annual layers/ meter changes with depth. Our assumptions suggest that r = (100 yr/m - 1 yr/m)/1000 m = .099 yr/m². Thus, age = 1/2 (.1) depth², and our oldest ice would be 1/2 (.1) 1000² = 50,000 years.

Now assume that at a depth of 300 m we encounter a leaf fragment whose C¹⁴ P/D ratio is .067 . How might we change our age formula, and what would be the age of our oldest ice?