ASSESSING THE IMPORTANCE OF SAMPLING DEPTH ON δ13CORG VALUES IN CONTINENTAL DEPOSITS ACROSS THE K-PG BOUNDARY

A disturbance in the carbon cycle at the K-Pg boundary is inferred from an excursion in the δ¹³C record of both inorganic and organic carbon. In locations without evidence for impact debris or similar markers, the δ¹³C excursion has been successfully used to identify the location of the K-Pg boundary. In the Hell Creek region of Montana (USA), previous studies have successfully used δ¹³C_org curves that record an excursion associated with the K-Pg boundary for correlation within this region (Arens et al., 2014; Arens and Jahren, 2000).

To ensure the accuracy of these curves, care is taken to avoid contamination from modern sources of organic carbon, typically by digging trenches to obtain samples from below the modern weathering profile. We have noticed that despite hand trenching in this region, modern roots can still be found at depths greater than 0.75 m in apparently undisturbed material. To test the importance of sampling depth on measured δ¹³C_org values, we collected samples from three sections spanning the K-Pg boundary. One was obtained from a traditional hand trench, another from a deep (>1m) trench excavated using a backhoe, and a third from a sediment core. We presume that the sediment core should exhibit the least susceptibility to modern alteration, while the hand trench should be most susceptible. In addition to stratigraphic overlap, all sections were collected within 25 meters of each spatially, so variability between them should largely be explained by sampling technique.

In addition to modern contamination, the lithology of a sample may exert some control on δ¹³C_org values. This presents a particular challenge in the Hell Creek region because the K-Pg boundary is associated with lithological change between Cretaceous Hell Creek Formation and the overlying Paleogene Tullock Member of the Fort Union Formation. To ensure that any excursions we identify at the K-Pg boundary are not simply the result of this change, we have sampled an additional section through a longer Paleogene interval to examine the relationship between lithology and δ¹³C_org.

To date we have measured δ¹³C_org in 119 samples, from three sections and found a negative excursion of roughly 2‰ just above the K-Pg boundary, similar to previous studies. Results from an additional 170 samples will be presented at the conference.