Shell morphology of cretaceous planktonic foraminifers as a means for paleoenvironment reconstructions

Abstract

During the Cretaceous, there were several relatively short intervals with a relatively low oxygen content in the water column. They were named “Oceanic anoxic events.” These events changed essentially the characteristics of the water masses, which had a significant effect on the morphology of planktonic foraminifers. The proportion of taxa with elongated chambers of the last whorl periodically increased. This structural feature probably provided shells with certain advantage under nonstandard oceanographic conditions. The elongated chambers were probably advantageous for dwelling in conditions of a low oxygen content in the water column. The small size, extended last whorl chambers, and thin and light wall facilitated floating in the upper water layers, which are particularly rich in oxygen. This hypothesis is corroborated by widespread periodic lateral distribution of the morphotypes with extended chambers of the last whorl during rather short intervals with the low oxygen content in sea water, making them an important marker of paleoenvironments. Returning to the normal environmental parameters promoted an increase in the taxonomic diversity of planktonic foraminifers and appearance of new morphological features. At the same time, each of these events had certain distinctive features.