Floral, faunal, and stable isotope evidence in a continuous sequence of latest Cretaceous and earliest Tertiary shallow-water marine carbonates in the Mangyshlak Peninsula, northeast of the Caspian Sea, USSR, suggest severe environmental changes at the Cretaceous/Tertiary (K/T) boundary. Time frame is provided by nanno-, micro-, and macrofossils as well as by magnetic stratigraphy and an iridium spike.

Oxygen-isotopic analyses of the bulk sediments, composed of nanno- and micro-plankton skeletal remains, show a sharp positive spike from −4.2 ‰ to −1.2 ‰ at the K/T boundary. Since the sediments have undergone diagenesis, a process that results in depletion of oxygen 18,...