Ocean circulation and calcareous nannoplankton evolution during the jurassic and cretaceous

Abstract

Spatial and temporal distribution of calcareous nannoplankton aid in the reconstruction of ocean circulation during the Cretaceous. Biogeographic patterns of calcareous nannoplankton during the late Cretaceous are mostly zonal, reflecting mostly latitudinal gradients of surface water temperature. During the middle Cretaceous calcareous nannofossil assemblages are more strongly affected by surface water fertility (concentration of limiting nutrients) especially in a wide tropical belt between 50° and 45°N latitudes. Major upwelling regions off northwest Africa, in the south Atlantic, and the equatorial Pacific divergence are apparent. Temporal changes in the composition of middle Cretaceous nannofloras are indicative of changes in surface water fertility and vertical mixing at frequencies of tens to hundreds of kiloyears (ka). Changes in the intensity of atmospheric circulation with frequencies close to the Milankovitch cycles appear to be the ultimate cause of these minor changes in the circulation of the upper ocean. Calcareous nannofossil preservation patterns show that during the Middle Cretaceous deep waters were quite homogeneous with respect to carbonate saturation in the Atlantic Ocean but not in the Indo-Pacific Ocean where a more distinctive calcite compensation depth (CCD) is apparent. Water-mass structure of the major oceans during the Mesozoic was thus quite different. Taxonomic evolution of calcareous nannofossils was also strongly affected by ocean circulation and fertility. Major peaks in the rates of diversification coincide with anoxic events, that is periods of increased preservation of organic carbon in the oceans which coincide with high sea level stands. During anoxic events ocean circulation was sluggish, vertical mixing was slow, deep ocean ventilation poor, and the concentration of limiting nutrients in surface waters was low. Increased competition for limiting nutrients and possibly preferential cropping of the dominant forms by zooplankton resulted in increased rates oof speciation during anoxic events. Periods of major production of nannofossil carbonate coincided with low sea-level stands when oceans were more vigorously stirred and surface waters were more fertile. The first such period was the latest Jurassic when calcareous nannoplankton invaded the open oceans and became the major carbonate producers in the pelagic realm during the Mesozoic. The frequency of peaks and troughs in evolutionary rates is on the order of 10–30 Ma and appears to be irregular. The coincidence of periods of anoxia with high sea-level stands, which in turn appear to be caused by more rapid sea-floor spreading indicates that the ultimate diving forces of nannoplankton evolution are processes in the earth's mantle. These major tectonically driven fluctuations in ocean-atmosphere circulation with frequencies of a few million to a few tens of millions of years are hereby named Fischer cycles. Superimposed astronomically driven Milankovitch cycles affected relative abundance patterns but not taxonomic evolution of calcareous nannoplankton. Rates of nannoplankton extinction are about five times higher than background rates at the end of the Jurassic and orders of magnitude higher in the latest Cretaceous possibly indicating a terrestrial or extraterrestrial catastrophe.