EMERGING APPLICATIONS FOR IN SITU COSMOGENIC 14C IN GEOCHRONOLOGY

Although its short half-life restricts measurable exposure ages to the Holocene or latest Pleistocene, in situ cosmogenic ¹⁴C’s (in situ ¹⁴C, half-life 5.73 ky) utility can be realized best in concert with analyses of one or more longer-lived or stable cosmogenic nuclides (CNs), enabling the elucidation of complex exposure histories and rapid geomorphic processes. Recent advances in in situ ¹⁴C extraction and measurement have enabled development of applications that showcase its potentially transformative influence on Holocene and latest Pleistocene process-oriented geomorphic and Quaternary geologic research. I will focus this talk on the following examples, as well as other outstanding research that demonstrates the potential of in situ ¹⁴C.

¹⁰Be is clearly the workhorse of CNs for surface exposure dating, yet in certain situations its long half-life works against it. For example, in glaciated regions that have been covered by cold-based or polythermal ice, varying degrees of glacial erosion can yield bedrock and/or erratics with a significant ¹⁰Be signal inherited from prior exposures. Furthermore, ratios of long-lived CNs such as ²⁶Al/¹⁰Be are insensitive to burial related to the last glaciation. Combining these with measurements of in situ ¹⁴C, however, can resolve Holocene and latest Pleistocene burial/exposure histories, since in situ ¹⁴C will decay to near-background concentrations after cover by non-erosive ice for 20-30 kyr (e.g., Miller et al., 2006). New data from Baffin Island will illustrate this.

As another example, Goehring et al. (2011) developed a novel method of using this differential decay to elucidate Holocene erosion rates and changes in glacier length relative to the present. If a suite of samples with the same exposure/burial history is collected from proglacial bedrock, paired measurements of ¹⁰Be and ¹⁴C allow the erosion rate for each sample to be uniquely determined for each sample. Alternatively, if a suite of samples is collected with the same glacial erosion history, the exposure/burial history can be determined for all of the samples. If enough samples are collected with the same exposure/burial history, both the exposure history and erosion rate for each sample can be determined.

Goehring et al., 2011. Geology 39(7), p. 679; Miller et al., 2006, Quat. Geochron. 1(1), p. 74.