A biostratigraphically complete but intensely bioturbated Cretaceous/Tertiary (K/T) boundary section was taken during drilling at Ocean Drilling Program Leg 113 Site 690 on the Maud Rise (65°S) in the Weddell Sea off East Antarctica. The boundary, which is contained in a relatively undisturbed core, has been delineated by lithostratigraphic, paleontological, and geochemical methods. The first occurrence of the calcareous nannofossil Biantholithus sparsus is used to biostratigraphically estimate the boundary horizon, and a distinct color change between dark brown, clay-rich Tertiary sediments and light-colored Cretaceous chalks is used to more precisely delimit the boundary between 41.5 and 41.8 cm in section 4 of core 15. An iridium peak of 1,566 ± 222 parts per trillion (ppt) was detected in the same section at 39 to 40 cm. Multiple but less intense Ir peaks were also detected below the boundary.

Dark-colored burrows sampled below the boundary contain up to 17 percent Tertiary nannofossils estimated to have been displaced at least 1.3 m by large-bodied bioturbators. In addition, the multiple peaks in Ir abundance below the boundary are attributed to the redeposition of Ir by this intense bioturbation. Such processes may account for some multiple Ir peaks reported at other K/T boundary sections.

We suggest that serious consideration should be given to the problems of bioturbation when attempting to biostratigraphically determine any marine boundary horizon and, in reference to the K/T boundary, when interpreting multiple Ir peaks as being the result of multiple extraterrestrial impacts. Similar caution should be exercised during micropaleontological studies to determine the succession of biological extinctions and evolutionary first appearances across any boundary interval.