Calcareous nannofossils and Mesozoic oceanic anoxic events

Abstract

Twenty-five years ago, Mesozoic oceanic anoxic events (OAEs) were documented and formalised as intervals of widespread to global deposition of organic matter. The Toarcian, Early Aptian (OAE1a) and latest Cenomanian (OAE2) OAEs are truly global in nature, commonly carbonate-poor, and typically represented by organic carbon-rich black shales. In some areas, these OAEs are also characterised by abundant radiolarian-sands and silts. They are associated with negative and positive excursions in the 87Sr/ 86Sr record, in addition to large global carbon-isotope anomalies in carbonate and/or organic matter, caused by a major perturbation of the global carbon budget. Increased rates of volcanism during the formation of the Ontong Java (and Manihiki) and Caribbean Plates, and the Karoo-Ferrar Traps, are believed to have caused the geological responses associated with OAE1a, OAE2, and the Toarcian OAE, respectively. Excess volcanogenic CO 2 in the atmosphere most probably turned the climate into a greenhouse mode, accelerating continental weathering and increasing nutrient content in oceanic surface-waters via river run-off. Higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes. Because Mesozoic OAEs are often represented by carbonate-poor sediments, quantitative studies of calcareous nannofossils have been applied to explore (a) the causes and effects of igneous/tectonic events and climate changes, relative to nannofloral increases and crises, as well as (b) dissolution events, and (c) diagenetic modifications. Characterization of calcareous nannofloras in OAE intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO 2 in the oceans. Types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events. In selected sections, it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pCO 2 by drastically reducing calcification. As in the modern oceans, during OAEs the increase of nutrients and atmospheric CO 2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep-photic zone nannoconids and schizosphaerellids. Similarly to the ‘nannoconid crisis’ preceding deposition of the Early Aptian OAE1a black shales, a ‘schizosphaerellid crisis’ is detected prior to the Toarcian OAE. Both OAEs are further characterised by a rapid nannofloral speciation, beginning approximately 1.5 myr before the OAE, but without extinctions. Global changes during the latest Cenomanian OAE 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions. This event, in fact, was a time of extinctions followed by originations within calcareous nannofossils. Precise timing of the events before, during and after OAE1a, OAE2 and the Toarcian OAE indicates that they were intervals of enhanced oceanic productivity and that anoxia/dysoxia post-dated biotic changes.