The effects of paleoenvironmental changes on nannoplankton biocalcification: mid-Cretaceous size variations of Biscutum constans

The investigation of the geological record allows to decipher the response of organisms to paleoenvironmental changes occurring at a time scale larger than human observation. One open issue regards the effects of stressing factors on coccolithophore algae calcification process. These planktonic organisms are important primary producers and largely contribute to the organic and inorganic carbon cycle. In the last decades, evidence for a direct response of coccolithophores to stressing factors were provided by experiments on living forms and by studies of extreme Cretaceous events, such as the early Aptian Oceanic Anoxic Event (OAE) 1a and latest Cenomanian OAE 2. These OAEs were marked by altered carbon cycle, trace elements anomalies, rapid and intense warming and peaks in surface water fertility. The studies evidenced fluctuations in the mean size of some selected nannofossil species, among which, Biscutum constans resulted to be the most sensitive. B. constans reached the smallest mean size under the climax of the OAEs coinciding with super greenhouse climate, excess CO2, accelerated nutrient recycling and trace metals peaks. In order to better understand the role of these stressing factor/s on B. constans calcification, we decided to investigate B. constans size variations through a longer time interval (ca. 27 Myrs) spanning the Aptian to the Cenomanian. This time interval includes either periods of stability and episodes of global environmental perturbations such as OAE 1a, OAE 1b, OAE 1d, the Mid-Cenomanian Event and OAE 2. Prior to this study, it was unknown if B. constans was affected by size changes during interludes of “stable” paleoenvironmental conditions. We focused on the Umbria-Marche Basin (central Italy) by investigating the Piobbico core and the Monte Petrano section which are stratigraphically well constrained and a complete characterization of paleotemperature and paleofertility is available. By performing morphometrics analyses we intended to better understand which factor (or combination of factors) was directly altering the biocalcification process in this species. The results revealed indeed changes in the mean size of B. constans. A relatively prolonged interval of smaller specimens was detected after OAE 1a. A recovery in B. constans average size was identified in the Albian although the largest specimens were found in the middle Albian. A relative decrease in size is detected just prior to OAE 1d. The subsequent main shift coincides with OAE 2 marked by dwarf specimens. Statistical analyses were performed to detect any possible dependence from temperature or nutrient variations suggesting no direct connection between these parameters and size, whilst trace elements peaks and phases of most intense volcanism resulted to correlate with minimum B. constans size.