New age constraints support a K/Pg boundary interval on Vega Island, Antarctica: implications for latest Cretaceous vertebrates and paleoenvironments

A second K/Pg boundary interval in the northern sector of the Antarctic Peninsula on Vega Island has been proposed, yet current temporal resolution for these strata prohibits direct testing of this hypothesis. In order to not only test for the existence of a K/Pg boundary on Vega Island, but to provide increased age resolution for the associated vertebrate fauna (e.g., marine reptiles, non-avian and avian dinosaurs), the Vega Island succession was intensively re -sampled. Stratigraphic investigation of the Cape Lamb Member of the Snow Hill Island Formation, and in particular, the overlying Sandwich Bluff Member of the López de Bertodano Formation, was conducted using biostratigraphy, strontium isotope stratigraphy, magnetostratigraphy and detrital zircon geochronology. These data indicate a late Campanian-early Maastrichtian age for the Cape Lamb Member and present three possible correlations to the global polarity time scale (GPTS) for the overlying Sandwich Bluff Member. The most plausible correlation, which is consistent with biostratigraphy, detrital zircon geochronology, sequence stratigraphy, and all but one of the Sr-isotope ages, correlates the base of the section to C31N and the top of the section with C29N, indicating that the K/Pg boundary passes through the top of the unit. A second, less plausible option conflicts with the biostratigraphy and depends on a series of poorly-defined magnetic reversals in the upper part of the stratigraphy that also correlates the section between C31N and C29R, again indicating an inclusive K/Pg boundary interval. The least likely correlation, one dependent on favoring only a single Sr-isotope age at the top of the section over biostratigraphy, correlates the section between C31N and C30N and is inconsistent with an included K/Pg boundary interval. Although our preferred correlation is well supported, we failed to identify an Ir-anomaly, spherules/impact ejecta, or other direct evidence typically used to define the precise position of a K/Pg boundary on Vega Island. This study does however confirm that Vegavis, from the base of the Sandwich Bluff Member, is the oldest (69.2-68.4 Ma) phylogenetically-placed representative of the avian crown clade, and that marine vertebrates and non-avian dinosaurs persisted in Antarctica up to the terminal Cretaceous.