Connecting morphological changes in planktonic foraminifera to late Eocene cooling: a case study on Pseudohastigerina micra (Cole 1927) from the Dinaric foreland basin, Adriatic Sea

We present here the analysis of planktonic foraminiferal assemblages from two stratigraphic sections cropping out at the Podstine and at Zaraće coves, at Hvar Island (Croatia), recording the flysch sediments in the Dinaric foreland basin during the late Eocene. The planktonic foraminiferal biostratigraphy reveals a Priabonian age of the sediments analyzed (Biozone E15 in Podstine cove section and Biozones E15 and E16 in Zaraće cove section). We focus on the small-sized, planispirally–coiled Pseudohastigerina micra (Cole 1927) providing several morphometric measurements on specimens. The test diameter of P. micra tests reveals a decrease during the Biozone E15 as observed at the Podstine cove section, with stabilization at such smaller test sizes observed in Biozone E16 at the Zaraće cove section. We attribute this size decrease as mainly due to the global sea surface cooling occurring during the late Eocene and possibly to enhanced surface water eutrophication. The cooling trend is supported by the increase in relative abundances of planktonic foraminifera cold-water taxa recorded in the assemblages whereas increased eutrophication appears largely controlled by nutrient input due to enhanced weathering, as suggested by lithological changes. Superimposed to the test-size decrease, the degree of test compression (ratio of test diameter and width) in P. micra proves to change irregularly, indicating intervals of ecological instability. The variations in abundance of P. micra, in phase with the genus Globoturborotalita, known to inhabit the surface water layers, suggest a similar habitat for P. micra for which the late Eocene paleoecology was uncertain.