Biometry of Cenomanian-Turonian placoliths: a proxy for changes of fertility and surface-water temperature?

Most publications discussing CenomanianTuronian calcareous nannofossils focus on abundance fluctuations across the boundary interval. So far, there have been no studies that deal with the influence of palaeoenvironmental changes on the size of common CenomanianTuronian nannofossil taxa. The genera Biscutum, Broinsonia, Prediscosphaera, Retecapsa and Watznaueria have therefore been analysed from 19 samples of CenomanianTuronian age from the Goban Spur, northeast Atlantic. The genus Biscutum shows a slight decrease of mean length from 4.14 mu m in the Cenomanian to 3.94 mu m in the Turonian. Broinsonia is marked by a decrease from 6.07 mu m in the Cenomanian to 5.64 mu m in the Turonian. On the other hand, Prediscospheara increases in size from 4.98 mu m in the Cenomanian to 5.61 mu m in the Turonian. Two genera (Retecapsa, Watznaueria) show no significant changes in their mean length. The mean size of Biscutum is perhaps controlled by nutrients, where larger specimens may have preferred the more fertile palaeoenvironment of the Late Cenomanian. The size decrease of Biscutum in the Turonian is probably related to reduced nutrient availability. The genus Prediscosphaera spp., may have favoured low-fertility conditions, as its mean size increases in the Turonian. A worldwide decline of the frequency of Broinsonia spp. during the CenomanianTuronian transition implies that this genus is not solely controlled by the nutrient content. The size of Broinsonia spp. may have been therefore influenced by the latest Cenomanian warming event. The increase in sea-surface temperature may have been unfavourable for Broinsonia spp. as reflected by decreasing mean size and frequency.