ESHS/HSS Annual Meeting

Discordant Measurements and Models of Concordance in Mid-Century Geochronology

• In Event: Ground Truth: From Local Data to Global Theories of the Earth

English Abstract

The twentieth century witnessed the development of radiometric dating as a set of methods to determine the age of geologic samples based on the radioactive isotopes they contained. These methods formed the basis for creating a quantitative version of the geologic timescale it is sometimes framed as. However, the quantification of geologic time was less straightforward than the simple application of nuclear science to geology. The jump from a limited selection of geologic samples to new measurement methods embedded in a conception of quantitative Earth history did not just involve the application of new instrumentation but also significant conceptual work as well.
In this paper, I want to examine the role played by models in connecting isotope measurements on diverse geologic samples to a vision of a singular quantified Earth history in the mid-20th century. I will focus on two cases in which significant models emerged based on measurement surveys: (A) the conception of primeval lead based on measurements performed by the physicist A. O. Nier in the 1940s, and (B) the construction of the Concordia curve by G. Wetherill in the 1950s. I trace how both models were developed as a way to structure and explain sample analyses that frequently diverged from expectations and from one another. I also show how these models form a link between work on locally specific, often difficult samples and abstracted conceptions of deep time.
While I mainly focus on a discussion about how and why these models were constructed and what they were intended to illuminate—universal Earth history, measurable geologic time—I also hope to highlight what these abstractions obscured. Understanding how and why certain forms of local specificity became invisible in the construction of global geochronological frameworks illuminates a central methodological dynamic of the physical Earth sciences: the production of planetary-scale concepts through selective work with stubbornly local materials.

Author

• Nanina Foehr, LMU Munich