engleski

The multidisciplinary approach of studying the Middle Eocene warming episodes in Dinaric foreland basin: new data or unfulfilled promises?

In the central Neo-Tethys (Dinaric foreland basin, Croatia) the middle Eocene sediments are arranged in a NW-SE oriented facies belt that differs in thickness, age attribution and facies. The carbonate ramp facies passed into Transitional beds (including “Globigerina marls”) and basin sediments (flysch). Tectonic influences associated with the ongoing orogen, sea-level changes, and warming events (Early Eocene Climate Optimum, EECO, and Middle Eocene Climate Optimum, MECO) had a major control on sedimentation. Thus, these sedimentary sequences are a reliable tool for studying biotic response to climate change for platform to basin assemblages. The multidisciplinary study includes the taxonomic classification of different fossil groups, microfacies and petrologic interpretation of the rocks and interpretation of the biodiversity of the foraminiferal assemblages, geochemical composition of the sediments, and isotopic analysis of different elements of the skeletons. The limestones, rich in larger benthic foraminifera (LBF), were deposited on the transient carbonate ramps. The successions are rarely complete in terms of depositional settings (missing outer ramp, Španiček et al., 2017 ; inner ramp, Ćosović et al., 2022) and very often stratigraphically incomplete (Ćosović et al., 2018), whereas the sediments are diagenetically altered. The LBF tests that contribute most to sediment production are recrystallized and cemented and therefore unsuitable for isotopic analysis. Great proportion of tests of the Bartonian (nummulitids of genus Nummulites and orthophragminids) LBF show traces of bioerosion, and in some cases, it is unclear when bioerosion occurred (burrowing traces in large orthophragminids follow the growth pattern). The morphology of the basin(s) was also strongly tectonic. The depth of the basin varies from NW (Lutetian) to SE (Bartonian), with estimates ranging from 900 - 1200 m (Živković & Babić, 2003) down to 1800 m. The proportion of smaller benthic foraminifera follows this trend, as they are more abundant in sediments from the NW part and almost absent in sediments in the SE. This could be an indication of lower oxygen levels at the bottom of the basin due to increased stratification from warming. Unfortunately, planktonic tests, whether or not they occur in organic-rich marls, have always been recrystallized. Tests deposited near the EECO were also highly dissolved. Could this be the results of supra-lysoclinal dissolution or due to stratification? In shallower basins significant shoaling of the lysocline and thermocline follows local circulation patterns and local tectonics (Cramwinckel et al., 2020). The shallowing of the thermocline considered as a global event due to warming, resulted in the migration of certain species from their habitats, and the changes in their size (gigantism or dwarfism). How then might we interpret the conditions? In addition to interpreting the microfacies interpretation using the standard criteria for carbonate environments, bulk-sediment geochemical analysis proved to be the key to interpreting conditions on the bottom. The ratio of certain elements indicates the intensity of weathering and terrestrial influx in carbonate environments.

Foraminifera, Weathering indices, Eocene, Dinaric foreland basin

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