Teaching about Time
Maya Elrick, Earth and Planetary Sciences, University of New Mexico
My research focusses on millennial-, orbital- and My-scale paleoclimate cycles in marine sedimentary rocks, so dealing with trying to determine absolute time and rates in sedimentary successions is essential for me. Because of this, I stress time in sedimentary successions in my classes (Earth History, Sedimentology-Stratigraphy, Carbonate Sedimentology).
Two the main challenges I find in teaching about time in sedimentary geology are:
- The wide range of time spans sedimentary geologist work in from many tens to hundreds of millions of years to accumulate the km-thick successions observed in the geologic record to the few minutes/hours to deposit a single turbidite or storm bed. How numeric or absolute time is determined in sedimentary successions to ultimately evaluate rates of processes, duration of events, and for correlation.
- In my Sedimentology-Stratigraphy class, I focus more on the #2 with exercises and discussion and only cover #1 in passing during lectures. Interestingly, it seems like students are more interested in estimating time spans for #1 and do not appreciate how essential #2 is to geology.
In teaching about how to determine time in sedimentary successions, I start with relative time based on biostratigraphy and sequence stratigraphy, then make the point of not knowing absolute time from these methods. Then, I discuss isotopic age dating in the context of dating mainly igneous and metamorphic rocks, then use these numeric dates tie into the sedimentary record through magneostratigraphy, Sr-isotope curves, and the use of interbedded ash beds and tie points to extrapolate stage boundary ages. Recently, I added discussions and exercises on the use of astrochronology to providing age control and refinement of the geologic time scale. The astrochronology exercise utilizes a range of techniques and concepts covered previously in class (magnetostratigraphy, biostratigraphy, cyclostratigraphy, paleoclimate change recorded in sedimentary rocks, marine sedimentology).