Reproductive behaviour of benthic foraminifera: A key to palaeoclimate

Abstract

Many benthic foraminiferal species exhibit dimorphism associated with reproduction. The two resultant forms are known as microspheric and megalospheric forms. Culture studies, though limited in number, show that ratios of these forms are affected by extremes of temperature. This property can be utilized to study palaeoclimates. To test this hypothesis variation in dimorphic ratios (microspheric/megalospheric) of Bolivina skagerrakensis at different levels in a core spanning 1000 years B.P. from the Skagerrak, E. North Sea was compared with a few other independent scales. It is concluded that higher ratios indicate a warmer climate. However, distinction between dimorphic forms is not always possible in many species. In such cases, mean proloculus size would be useful. Higher ratios of dimorphic forms mean more number of microspheric forms and thus lower mean proloculus size. Therefore mean proloculus size can also be used for palaeo-climatic studies. This concept is again tested by studying variation in mean proloculus size in Cavarotalia annectens in a number of modern surface sediment samples from the inner shelf of the west coast of India. It is observed that mean proloculus size is inversely proportional to temperature and salinity. In view of the above, it is proposed that the reproductive behaviour of benthic foraminifera can be utilized as a palaeoclimatic index