THE IMPORTANCE OF SAMPLING DEPTH AND SPATIAL VARIABILITY ON TERRESTRIAL δ13CORG VALUES ACROSS THE K-PG BOUNDARY

Organic carbon isotope (δ¹³C_org) analysis of organic material has the potential to produce precise correlations in terrestrial environments. A negative δ¹³C_org excursion recognized worldwide has been also been observed at the Cretaceous-Paleogene (K‑Pg) boundary in the intensely studied Hell Creek area of Montana. The K-Pg boundary in this region coincides very closely with a lithological contact between two formations, and thus provides an ideal location at which to test the use of δ¹³C_org in high-resolution stratigraphy. Previous studies have produced stratigraphic records of δ¹³C_org across the K-Pg boundary in the area (Arens et al., 2014; Arens and Jahren, 2000). However, modern organic contamination, lithological control, and local spatial variability may affect δ¹³C_org values.

To test the effects of modern contamination and lithological control, we collected samples from a traditional hand trench, a coring device, and a deep (>1m) trench dug using a backhoe. Samples obtained using each method are collected across the same stratigraphic interval within 20 m (geographically) of each other, so their relative reliability can be assessed. Results show that core samples values are more negative, but the general trends, including the negative K-Pg excursion, are preserved in all sampling methods. There does not appear to be a significant lithological bias to δ¹³C_org values. Additionally, samples were also collected from hand trenches over a greater, but still fairly small (<10 km²) area to compare the effects of local spatial variability in δ¹³C_org signals. Results of these samples will be presented at the conference.