Morphometric response of late Aptian planktonic foraminiferal communities to environmental changes: A case study of Paraticinella rohri at Poggio le Guaine (central Italy)

The Aptian-Albian boundary interval represents a pivotal time of pronounced climate/ocean changes associated with Oceanic Anoxic Event 1b (OAE 1b) that seriously influenced marine pelagic ecosystems. The sedimentary successions record the first and most significant species turnover in the evolutionary history of planktonic foraminifera, consisting of the selective extinction/disappearance of large Aptian species and resulting in inter-specific size reduction (the post-extinction dwarfing as in the Lilliput effect) in the earliest Albian assemblages. In this study we investigate how the planktonic foraminifera respond to crucial environmental stress and if any intra-specific decrease in test size predate the extinction (pre-extinction dwarfing). Paraticinella rohri was chosen as the most representative of the Aptian planktonic foraminiferal taxa and its test-size evolution was compared with the high-resolution record of redox and productivity geochemical proxies through the uppermost Aptian - lowermost Albian pelagic succession of Poggio le Guaine (central Italy). Our geochemical data document long-term fluctuations characteristic of a system with transitions among different states (oligotrophic-mesotrophic-eutrophic and oxic-dysoxic-anoxic). The strong correlation of the large test-size changes of Pa. rohri during its total range with higher productivity (increased Ba-excess concentrations) suggests that the trophic regime was the key factor controlling robust abundance and size variability in this taxon. The progressive increase in surface water fertility during the late Aptian offered optimum ecological conditions for planktonic foraminifera, that increased in abundance, size, and specific diversity. After the transient anoxic 113/Jacob sub-event, smaller to dwarfed specimens of Pa. rohri and depauperate planktonic foraminifera assemblages faced high selective pressures due to more oligotrophic conditions. Transition to a more eutrophic ocean in the latest Aptian irreversibly eroded their resilience, and Aptian dwarfed k-strategist taxa became extinct. The planktonic foraminifera turnover has been therefore the result of an irreversible and drastic regime shift toward a more eutrophic ecosystem that promoted the thriving of small-sized opportunistic/disaster planktonic foraminiferal species and the siliceous radiolaria.