Morphological changes of calcareous nannofossils during Oceanic Anoxic Event 2

The Cenomanian - Turonian Oceanic Anoxic Event 2 (OAE2) is one of the most pronounced perturbations of the Cretaceous that induced major changes in the marine environment and severe disturbances of the biosphere. It is thought to be linked to extensive volcanism and particularly to the emplacement of a Large Igneous Province (LIP) that acted as a natural source of excess CO2. Particularly, OAE2 seems to be strictly connected with the formation of the Caribbean Plateau, which triggered a chain of events leading to enhance ocean fertility, accelerated organic carbon burial, widespread ocean anoxia, and acidification. Calcareous nannoplankton is sensitive to chemical - physical - trophic changes and must have reacted to the OAE2 extreme environmental conditions. Indeed, nannofossil data show that they experienced a turnover, a general decrease in species richness and locally a decline in abundance of the fertility indicator species. In this study we present new results of a detailed morphometric analysis of 4 nannofossil species during OAE2 from two different areas: the Mediterranean area (Novara di Sicilia section, North-eastern Sicily) and the Sussex area (Eastbourne section, United Kingdom). These sections have been chosen based on integrated high-resolution stratigraphy: the C isotopic anomaly is taken as primary tool of dating and correlation of OAE2 events. Furthermore the Novara di Sicilia section represents only the lower portion of OAE2, whereas the Eatsbourne section, covering the entire OAE2 interval, provides the opportunity to characterize changes before, during and after the paleoenvironmental perturbation. In analogy to the work of (ERBA et alii, 2010) on the Early Aptian OAE1a, biometric analyses were performed at high resolution on B. constans, D. rotatorius, Z. erectus and W. barnesiae in order to evaluate the influence of paleoenvironment on coccolith size and morphology. Size measurements of calcareous nannofossils were carried out on smear slides with a light microscope: in each sample 30 specimens of B. constans, D. rotatorius, Z. erectus and 50 specimens of W. barnesiae have been measured by using the Qcapture Pro suite. Morphometric analyses highlight that: 1. In both sections dwarf coccoliths are present even before the OAE2 onset. 2. There is a species - specific response in both section: B. constans displays the most pronounced reduction in size, D. rotatorius records a well express reduction in size too as opposed to Z. erectus which diminishes in size to a lesser extend. 3. In both sections, the mean size of W. barnesiae is within the holotype and normal range size 4. In the Novara di Sicilia section dwarf coccoliths record the strongest reduction in size relative to Eastbourne samples. 5. Pronounced dwarfism is observed in discrete intervals before and within the carbon isotope excursion. 6. During OAE2 coccoliths return to normal sizes around the carbon isotopic peak “A”. Comparison of morphometric analyses through OAE1a (ERBA et alii, 2010) and OAE2 shows analogies and differences. During OAE1a dwarfism and malformation are restricted to the onset of the carbon isotopic anomaly, in the core of the negative shift coeval with most profound paleoenvironmental perturbations. In the OAE2 interval dwarfism is most pronounced in the last part of the C isotopic anomaly, and coccolith deformation is negligible. During the two major Cretaceous OAEs, the ocean experienced extreme CO2 concentrations, acidification and an increase in fertility. Coccolith dwarfism suggests a link between nannoplankton calcification and high pCO2, although enhanced fertility and/or presence of toxic metals in seawater might have been instrumental as well. We speculate that during OAE1a and OAE2 excess CO2 played a fundamental role in ruling nannoplankton calcification efficiency, and that coccolith dwarfism might be a proxy of ocean acidification. Different patterns and degree of dwarfism and malformation during OAE1a and OAE2 suggest unequal volcanic CO2 emissions (rates, pulses, amount). We conclude that analogous causes (LIPs emplacement) have induced partially similar response at different times. REFERENCE ERBA E., BOTTINI C., WEISSERT H.J. & KELLER C.E., L., (2010) – Calcareous nannoplankton response to surface- water acidification around Oceanic Anoxic Evenet 1. Science, 329, 428-432.