1. The Deposition of Sediment: How it happens, deposition rates, and the difference between clay/silt deposited in the ocean and ash or dust that has settled out of the atmosphere. Clay/silt particles that enter the ocean via rivers or in the form of wind-blown aerosols accumulate very slowly, under normal circumstances, by settling through the water column. Clay accumulates in the deep sea at typically no more than 1 mm per 1000 years. (Carbonate accumulates to form limestone more rapidly than this, but does not accumulate on the deepest ocean floor.) Volcanic dust/ash and/or meteor-impact debris would accumulate much more rapidly, and would weather to clay as it descended through the ocean.

2. Stratigraphic Continuity: Many confused the change in lithology at the K-T boundary with a fault. No need for a fault, just an abrupt change in the type of material that accumulates. In some sedimentary "sequences," erosion occurs between two periods of deposition, so that there is an "unconformity," that is, an age gap in the sequence. A fault would probably "cross-cut" the sedimentary layers, shifting them relative to each other.

3. Scale of Geologic Events: Many commented in Exercise 2 that they doubted one meteorite impact, or any single event in general, could affect the global climate. Doubt is OK, as it is the driving force of scientific inquiry. We will address the K/T impact hypothesis in more detail as the class moves along. First we need to introduce My Favorite Mineral #9, gypsum. As fate would have it, two high-profile articles on contemporary meteor/comet impacts were published this week:
   • in the NYTimes Science section on Tuesday, and addresses a suspicious seismic event near the remote Australian "camp" of the Oum terrorist sect. Was it a nuclear test? This is not known for certain, but data collected by nearby seismometers bears the earmarks of a meteor strike, not a buried explosion. The researchers note that an iron meteorite 3 meters in diameter would generate an earthquake of magnitude 3.6, approximately the size of the mystery event.

   • In this week's New Yorker there is an article on comet strikes, and efforts to estimate their danger to human civilization. The article catalogues a number of meteor strikes and near-misses in the last few decades. Remember Tunguska!

4. Absolute terms: Many lines of evidence were presented using words that suggest "always true" or "always false." This is risky business in scientific writing. Mathematical statements like 2+2=4 are good candidates for "always true," and Newton's Laws and quantum mechanics will probably be held valid a 1000 years hence. However, complex systems like the Earth's climate display astonishing variability, and geologic evidence can be scarce, so we should not expect definitive answers to all earth-science controversies. The classroom we hold class in was under one kilometer of ice 20,000 years ago. So much for New England summers always being warm and humid!

back to global change home page