Short and long-term size changes of calcareous nannofossil Watznaueria barnesiae across the latest Barremian- late Cenomanian interval (Cretaceous) in the western Tethys.

Calcareous nannofossil Watznaueria barnesiae is a resistant and tolerant species and previous morphometric studies have evidenced minor (Erba et al., 2010; Lubke et al.2016) or no size changes during Oceanic Anoxic Events (OAEs) (Bornemann et al. 2006; and Faucher et al. 2017b) as well as during the Cenomanian to Maastrichtian (Linnert et al. 2014). Here, we present the morphometric record from the western Tethys (Cismon core from the Belluno Basin; Piobbico core and the Monte Petrano section from Umbria-Marche Basin, Italy) across the latest Barremian- late Cenomanian which represents a relatively long-time interval (ca. 26 Myrs) marked by interludes of profound changes in the ocean and climatic conditions named (OAE 1a, 1b and 1d). Our study shows that indeed W. barnesiae experienced coccolith and central unit variations during the studied interval. Specifically, smaller coccoliths with also smaller central unit marked the early Aptian to early Albian. The middle Albian to the early Cenomanian was characterized by larger and rounded coccoliths without significant changes in the central unit dimension. Short-term size changes are detected in correspondence of OAE 1a and OAE 1b marked by W. barnesiae size decrease. Particularly, OAE 1a was characterized by the smallest and most elliptical specimens of the studied interval. During the OAE 1b W. barnesiae coccoliths were characterized by a moderate decrease in the mean size, because specimens were already smaller before the event. The morphometric data were correlated with calcareous nannofossil temperature and nutrient indices calculated on the same samples investigated for morphometry. Long-term size changes do not show a clear correspondence with temperature and nutrient variations. W. barnesiae size changes across the early Aptian to early Albian may be related to large igneous province activity. Short-term size variations during OAE 1a and OAE 1b were influenced by the combination of large CO₂ emissions, temperature and fertility increase. In conclusion, although W. barnesiae is a resistant and tolerant species it shows morphometric changes both in short- and long-term, probably in response to partially different causes.