Terminal Mesozoic Events

As a result of plate tectonics and continental drift during the last few million years of the Cretaceous period, mountains were forced upwards (the Laramide revolution), volcanoes erupted throughout the world (the Deccan plateau of India was formed by volcanic action), plateaux replaced floodplains and swamps, and the seas retreated. Many authorities believe that a period of global cooling occurred then, as a result of the reduction of solar heat by sulphurous gases; others that, due to increased carbon dioxide and methane in the atmosphere creating a 'greenhouse' effect, the temperature of the world increased.

During the Jurassic period, climatic conditions favoured a bloom of calcareous algae in the ocean. This must have depleted the atmosphere of carbon dioxide (CO2) and the seas of calcium and bicarbonate ions. In deeper water, a zone of calcium carbonate (CaCO3) under saturation probably developed. If this moved to the surface during the Late Cretaceous (Hallam and Wignall 1997), it would have destroyed the phytoplankton, triggered an ecological collapse, which is recorded as a gap of at least a million years in calcareous sedimentation, and engendered a consequent increase in atmospheric CO2 resulting in global warming. Shortage ofbasic food materials might then have been responsible for increased competition and reduction in the numbers of molluscs, fishes and other marine animals upon which top predators, the mosasaurs and plesiosaurs, depended. Consequently, these too would have become extinct. The decline of the ammonites, inoceramid and rudist molluscs had, however, begun at least 6 my before the K-T boundary. In contrast, plant communities did not experience massive exterminations on land, although they did suffer some disturbances (as pointed out by Spicer and Parrish 1990 and by many other authors). The earlier extinction of the pterosaurs has already been discussed (Sect. 6.7.1).

Oxygen isotope studies of plankton and benthic Foraminifera suggest long-term cooling of the seas during the final Maastrichtian stage of the Late Cretaceous period. At the same time, there is evidence for K-T boundary warming in some parts of the world, just before the cooler period that may have followed. If temperatures did increase, a decrease in the solubility of oxygen might have reduced the amount of this gas in the oceans. There are deposits of black shale, which is caused by anoxic conditions, at the K-T boundary in several regions of the world (Hallam and Wignall 1997).

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