FLORAL TURNOVER AT THE K/T BOUNDARY: AN ISOTOPIC AND MOLECULAR STUDY FROM THE WESTERN INTERIOR OF NORTH AMERICA

Stable isotope and organic geochemical analyses have been carried out on two terrestrial K/T boundary sections from the Western Interior of the USA. The study is part of a project aiming to better understand the nature of environmental changes resulting from events at the boundary, including the formation of the Chicxulub impact structure off the Yucatan Peninsula. The Raton Basin on the New Mexico-Colorado border and Brownie Butte in Montana are located ~2000 km and ~3000 km from the crater, respectively. An ejecta layer is found followed by an Ir-rich fireball layer in each section, stratigraphy that is common to sites proximal to Chicxulub.

Carbon isotopic data, expressed as δ¹³C values in ‰, of bulk organic matter and of specific molecules are presented, which reflect the state of the carbon cycle at the time of organic matter deposition. Study of the molecular fossils contained within this matter allows constraint to be placed on the causes of any isotopic disturbances, thus pyrolysis and GC-MS techniques are employed.

The asynchronous recoveries of δ¹³C values and terrestrial ecosystems indicate that the carbon isotopes reflect changes to the carbon cycle independent of terrestrial plant succession. The shape of the excursion provides information on the source of the isotopically light carbon in the atmosphere coincident with the KTB.

Further samples from Eurasia and New Zealand are being studied in order to assess the wider environmental effects of events at the K/T boundary.