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, the positive spike at the boundary was attenuated by diagenesis and represents a minimum value. This shift is primarily attributed to abrupt and severe cooling, possibly accompanied by increased salinities of the surface mixed layer. A reversal in the δ¹⁸O signal from −1.2‰ to −4.6 ‰ at 1 mm above the boundary is interpreted to be indicative of marked warming and decreased salinities. The echinoids and benthonic foraminifera exhibit a modest shift in δ¹⁸O, suggesting much less pronounced temperature and salinity changes of the bottom water.

Independent geological evidence indicates that the terminal Cretaceous temperature decline was coeval with widespread and intense volcanism, which peaked at the close of the Mesozoic Era. It is proposed that volatile emissions from massive volcanic eruptions led to acid rain, which depressed the surface-water pH, temporarily prohibiting calcite nucleation of the surface-dwelling warm-water plankton. Superimposed upon severe and rapid climatic changes, decreased alkalinity caused the extinction of calcareous phyto- and zooplankton. The extinction appears to have extended over several hundred thousand years.