Teaching Time Essay
Susan Zimmerman, Center for AMS, Lawrence Livermore National Lab
I discovered that I was a geologist as a freshman Russian major at Dickinson College in Carlisle PA, and learned that I loved teaching while working as a T.A. for the physical and historical geology labs for three years. In repeating all the same labs three times, I started to understand how the labs were designed, the points of the discussions, and the place of each exercise in the overall course. The year after my graduation, I was hired to stay as an instructor of two lab sections, and had to present the material and lead the discussions myself. One of my favorite labs to teach was the geologic time and relative/absolute dating lab, for which I developed my own introductory exercises.
I began my Ph.D. thinking my project was going to be primarily reconstruction of lake level at Mono Lake over the last ice age, but quickly discovered that the chronology was going to be the most challenging part. Over the course of several years, I used radiocarbon on shells and argon dating of ashes to constrain a correlation of a new record of the paleointensity of the magnetic field to global records (U/Th dating was an early bust). In measuring each set of dates, I had to think about the system I was using, and what weaknesses it had, as well as what information it did provide to help me figure out the age of the sediments. Radiocarbon is very susceptible to addition of modern carbon, for example, while Ar/Ar dating of single crystals from the ashes showed that some were crystals included from older eruptions. I correlated my paleointensity record to a stacked record from marine cores, introducing the question of how we knew the age of the cores – and how well, at any level in the stratigraphy. The conclusions I came to (Zimmerman et al., 2006) are still being challenged by others, using varying approaches and with biases different than mine.
I continued my pursuit of the question "How, and how well, do we know that age?" as a post-doc in the radiocarbon dating group at CAMS, working in detail on what material to date, how to prepare it, where unwanted carbon might be introduced, how the AMS measurement produces a number, and how to interpret the dates at the end. Depending on the precision and accuracy required, I've looked at how the age changes if I measure different materials from the same sample, or if I date the same material several times, and the complications introduced by the conversion from 14C years to calendar years through the calibration curve.
The same kinds of questions apply in my new work at CAMS, running the surface exposure dating activities, but the variables can be quite different. Here I'm starting fresh, because I haven't used beryllium, aluminum, and chlorine before, and the sample types, contamination/disruption considerations, and steps to calculating a date are quite different from carbon. The general view is the same, though; the dates that we use to describe earth's history are each an estimation of the age of a particular sample using a particular system. Where many different estimations give the same result, we have greater confidence in the age, but when they don't, we have to figure out what sources of error or uncertainty are causing the difference. In teaching students to think as scientists, it's important to introduce this dynamism in our understanding of earth history, so that they learn to ask how we know the ages that we teach them, and how well.
References:
Zimmerman, S.H., S.R. Hemming, D.V. Kent, and S.Y. Searle, 2006. Revised chronology for late Pleistocene Mono Lake sediments based on paleointensity correlation to the global reference curve. Earth & Planetary Science Letters, 252; 94-106.