Response of larger benthic foraminifera to Eocene warming episodes in the Dinaric foreland basin

The study of Eocene deposits from the Dinaric foreland basin (Outer Dinarides, eastern Adriatic coast) documents a 300 m thick sequence of shallow-water carbonates originated on ramps that formed intermittently during the tectonic evolution of the basin. During the intense tectonic activity in this area, global sea-level changes, and warming events occurred (Early Eocene Climatic Optimum, EECO, Late Lutetian Thermal Maximum, LLTM and Middle Eocene Climatic Optimum, MECO). Up to 60% of all skeletal debris in these carbonates are larger benthic foraminifera (LBF). Ramp successions are rarely complete in terms of inner-, mid- and outer-ramp environments (availability of suitable benthic habitats for different LBF group) and very often stratigraphically incomplete. The LBF rich deposits document three distinctive groupings: i) the post - PETM phase (SBZ4 - SBZ10); ii) the EECO phase (from pre - to post - EECO phase, SBZ 10/11 - SBZ12/13); and iii) the late post - EECO phase including short-lived LLTM (SBZ13 - SBZ17). The studies of the dominant LBF (Drobne & Ćosović, 2009, 2010; Drobne et al., 2011; Pavlovec, 2012), showed no significant generic and species diversity across the foreland basin, from SBZ4 to SBZ17, but differences in species richness were found. The LBF assemblages included representatives of alveolinids, nummulitids, complex miliolids, conical agglutinated foraminifera, orthophragiminids, encrusting foraminifera and larger rotaliids. For this study, the diversity of the genera Alveolina, Assilina, Nummulites and conical foraminifera and complex miliolids was compared. The post-PETM stage showed similar species diversity of Alveolina, Nummulites, and Assilina (14-18-16) in contrast to the presence of three species of complex miliolids. In the EECO stage, representatives of Nummulites (32 species) were preferred over eighteen species of Alveolina, while the low species diversity showed conical foraminifera (8 species), Assilina (5 species) and complex miliolids (4 species). In the post - EECO and LLTM stages, Nummulites species were most abundant (39 species), Assilina less abundant (16 species), while the diversity of other groups decreased sharply (3 species of Alveolina and six species of complex miliolids). From the Ypresian to the Bartonian, LBF maintained high species diversity, with differences in species richness most likely due to competition (e.g., alveolinids vs. complex miliolids), local shifts in the paleoenvironment and long-term evolution, rather than global atmospheric or oceanic conditions.

The study was funded by the Croatian Science Foundation (HRZZ) through Project IP-2019-04-5775 (BREEMECO).

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