engleski

Mid-Eocene thermals record in the Istrian Paleogene Basin (Outer Dinarides, Croatia), Neotethys

Several short sections from Istria, northern Adriatic (Racani––Rac, <5 m ; Jakomići––J, ~6 m and Jak, ~2 m), were studied micropalaeontologically and geochemically. The top Nannotetrina spp. (N. cristata) (42.97 Ma) and base common Reticulofenestra umbilicus (42.3 Ma) point to Zone NP16 (Martini, 1971) and the Lutetian MNP16A (Fornaciari et al., 2010 ; Agnini et al., 2014), which agree with the foraminiferal E10/E11 zonal boundary in the Racani section (Berggren & Pearson, 2005). This interval is known as the Late Lutetian Thermal Maximum (Westerhold et al., 2020). The interpreted peak climatic condition is reflected in the relatively high species richness (73) and the rapid deposition of two organic rich intervals (7.25% TOT/C), which may be associated with the increase in the concentration of sulphur (0.29% TOT/S) and low concentration of Mn (0.09%), implying low-oxygen, possibly dysoxic, bottom- water conditions (Spofforth et al., 2010 and references therein). The warm-water species Cyclicargolithus floridanus dominated (up to 52%) the assemblages, indicating meso-oligotrophic conditions (Toffanin et al., 2011). In the Mediterranean, the base Furcatolithus obtusus (39.63 Ma) coincides with the top Sphenolithus spiniger (39.63 Ma), which was recorded at the top of Section J, indicating Bartonian Subzone MNP16Bc (= NP17), which indicates Chron C18n2n (Fornaciari et al., 2010). The domination of the warm-water Reticulofenestra bisecta (21%) points to the enhanced eutrophic conditions that characterised the carbon isotope excursion phase of the Middle Eocene Climatic Optimum (MECO) (D’Onofrio et al., 2021 and reference therein), with continued high (71) species richness. The base Furcatolithus obtusus at the base (in the first metre) of the Jak section defines the base of Subzone MNP17A, which agrees with the top S. spiniger (which occurred in the second metre of the section). The warm-water Reticulofenestra producta increased (by up to 25%), suggesting more shallow and mesotrophic conditions (Wade & Bown, 2006), with the decreased species richness (65) characteristic of the post- MECO phase (D’Onofrio et al., 2021). The composition of the planktonic foraminiferal assemblages (predominantly thermocline-dwelling Subbotina and common subsurface-dwelling Turborotalia) and the low proportion of smaller benthic foraminifera (15% in J, 3.8–5.4% in Jak and 20% in Rac) indicate eutrophic to mesotrophic conditions in the lower bathyal (Murray, 1991), stratified water column (Pearson et al., 2001). This study was undertaken within the framework of the Croatian Science Foundation Project IP-2019- 04-5775 BREEMECO.

Late Lutetian Thermal Maximum ; MECO ; microfossils

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