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:

1. 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.
2. 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).