No abstract

Abundant and well‐preserved marine macrofossils on Seymour Island, Antarctica, provide a valuable resource to establish paleoenvironmental conditions at high southern latitudes during the warm Paleogene. Stable isotope, minor element, and 87Sr/86Sr compositions have been measured for the aragonite bivalve Cucullaea from the Eocene La Meseta Formation. The 87Sr/86Sr stratigraphy suggests an early to late Eocene age for the La Meseta Formation. Sea surface temperature estimates based on δ18O values range from 14.5° to 15.5°C in the early and middle Eocene to 10.5°C in the late Eocene. High‐resolution sampling along growth structures provides estimates of seasonality in temperature, which decrease considerably up section. To address the relative contribution of temperature and ice volume or salinity to the δ18O record, Sr/Ca was measured and evaluated as a potential independent paleothermometer for Cucullaea.

The discovery of a fish bone layer immediately overlying the K-T iridium anomaly on Seymour Island, Antarctic Peninsula, which may represent the first documented mass kill associated with the impact event, together with new faunal data across the boundary has provided new insight into events at the end of the Cretaceous. The utilization of a geographical approach and a new graphical representation of range data has revealed that events at the end of the Cretaceous were not instantaneous, but occurred over a finite period of time. Although the fish bone layer may contain victims of the impact event, the absence of ammonites in either the iridiumbearing layer or the overlying fish layer suggests that the extinction event at the end of the Cretaceous was the culmination of several processes beginning in the late Campanian. The impact was the proverbial “straw that broke the camel's back,” leading to the extinction of many others forms of life that might have survived the period of global biotic stress during the waning stages of the Mesozoic if there had not been an impact. The absence of mass extinction following catastrophic geologic events in a biotic robust world, such as the Middle Ordovician Millbrig-Big Bentonite volcanic event suggests that the biosphere is remarkably resilient to major geologic catastrophes with mass extinction events occurring only when there is a conjunction of geologic events none of which might be capable of producing a global mass extinction by itself.

Seymour Island, Antarctica (64°17'S), offers the first opportunity to examine the crisis at the end of the Cretaceous from the high southern latitudes. The K/T boundary sequence on Seymour Island consists of a nearly continuous sequence of siliciclastic sediments deposited in a mid-shelf environment. The faunal changes across the boundary occur through a 30-m interval with no single extinction horizon, in contrast to other well-studied K/T boundary sections. The “expanded” nature of the Seymour Island section makes placement of the K/T boundary difficult because boundary indicators such as planktic foraminifera, ammonites, and dinocysts disappear at different levels within the section.

Discovery of 11 genera, in five classes within the Mollusca, Echinodermata, and Arthropoda in upper Eocene rocks on Seymour Island, Antarctica, previously known only from Late Cenozoic in mid-latitudes, suggests that the high latitude region of the Southern Hemisphere acted as a center of origin and dispersal for a broad spectrum of taxa. Precursors to modern deep- and shallow-water mid-latitude forms evolved and flourished in the high latitudes until conditions in lower latitudes favored their dispersal. These observations of Antarctic marine invertebrates corroborate those recently made about terrestrial mammals and plants in the Arctic.