The Eocene-Oligocene climate transition in the Central Paratethys

The Eocene-Oligocene climate transition in the Central Paratethys

Authors:

Ozsvárt et al

Abstract:

We studied two boreholes (Cserépváralja-1 and Kiscell-1) with continuous sedimentary records across the Eocene-Oligocene climate transition from the Central Paratethyan area. Assemblages of benthic foraminifera display a shift in dominance by epifaunal taxa in the late Eocene to shallow and deep infaunal taxa in the early Oligocene. Using the benthic foraminiferal oxygen index (BFOI), a decreasing trend of bottom-water oxygen levels is established across the Eocene-Oligocene transition (EOT), leading to the development of dysoxic conditions later in the early Oligocene.

Trends in δ18O and δ13C values measured on tests of selected benthic and planktic foraminifera roughly parallel those of the global record of stepped EOT δ18O increase and deviate only later in the early Oligocene, related to the isolation of the Paratethys. The overall similarity of the isotope curves and the presence of a planktic-benthic ecological offset suggest that the original isotope trends are preserved, despite the systematically more negative δ18O values. Of different scenarios, a quasi-uniform diagenetic overprint by fluids with low δ18O values, during burial or uplift, appears best supported. We conclude that the globally established isotopic expression of Antarctic ice sheet growth across the EOT may be recognizable in the Paratethys. Deviations from the global trends after the EOT were caused by regional paleoceanographic changes induced by the progressing Alpine orogeny and sea-level change, which led to a restricted connection with the open ocean, freshwater influx from increased precipitation, and gradual development of bottom-water oxygen depletion.

Connectivity of the Portions of th Paratethys During the Miocene Neogene

Miocene connectivity between the Central and Eastern Paratethys: Constraints from the western Dacian Basin

Authors:

ter Borgh et al

Abstract:

The Dacian Basin formed an important link between the central and eastern parts of the Paratethys, a chain of late Tertiary inland seas and lakes. This study presents constraints on Miocene sea and lake level fluctuations in the Dacian Basin and on the connectivity between it and other Paratethys basins, based on the interpretation of seismic lines, a micropalaeontological study and lithofacies analysis of a large number of outcrops. It is shown that relative sea level fluctuations in the western part of the Dacian Basin during the Middle Miocene were primarily driven by tectonic activity in the nearby Carpathian Mountains. From the Maeotian (Late Miocene) onwards, however, tectonic activity was minor and relative sea level fluctuations were primarily driven by changes in basin connectivity and climate. The connection between the Central and Eastern Paratethys was broken at the end of the Middle Miocene, leading to the development of an endemic fauna in the former, but new data presented here suggest that isolation was not sustained completely as Central Paratethys species appeared in the Dacian Basin during the Maeotian (Late Miocene). Besides the isolation two falls in water level occurred in the basin during the latest Miocene: Of these, the intra-Pontian sea-level drop is the best known. We show, however, that this drop was preceded by a larger sea or lake-level drop in the late Sarmatian/Maeotian. This latter event may have affected much larger parts of the Paratethys, and we recommend more study of the bordering basins. The hypothesis that the connection between the Dacian and Central Paratethys basins was located in the region where the Danube River presently crosses the Carpathians was tested, but no supporting evidence was found.

The Consequences of the Tethys Marine Regression From Tarim, China During the Eocene Paleogene

Timing, cause and impact of the late Eocene stepwise sea retreat from the Tarim Basin (west China)

Authors:

Bosboom et al

Abstract:

A vast shallow epicontinental sea extended across Eurasia and was well-connected to the Western Tethys before it retreated westward and became isolated as the Paratethys Sea. However, the palaeogeography and the timing of this westward retreat are too poorly constrained to determine potential wider environmental impacts, let alone understanding underlying mechanisms of the retreat such as global eustasy and tectonism associated with the Indo-Asia collision. Here, an improved chronostratigraphic and palaeogeographic framework is provided for the onset of the proto-Paratethys Sea retreat at its easternmost extent in the Tarim Basin in western China is provided. Five different third-order sea-level cycles can be recognised from the Cretaceous–Palaeogene sedimentary record in the Tarim Basin, of which the last two stepped successively westwards as the sea retreated after the maximum third incursion. New biostratigraphic data from the fourth and fifth incursions at the westernmost margin of the Tarim Basin are compared to our recent integrated bio-magneto-stratigraphic results on the fourth incursion near the palaeodepocentre in the south-western part of the basin. While the fourth incursion extended throughout the basin and retreated at ~ 41 Ma (base C18r), the last and fifth incursion is restricted to the westernmost margin and its marine deposits are assigned a latest Bartonian–early Priabonian age from ~ 38.0 to ~ 36.7 Ma (near top C17n.2n to base C16n.2n). Similar to the fourth, the fossil assemblages of the fifth incursion are indicative of shallow marine, near-shore conditions and their widespread distribution across Eurasia suggests that the marine connection to the Western Tethys was maintained. The lack of diachronicity of the fourth incursion between the studied sections across the southwest Tarim Basin suggests that the sea entered and withdrew relatively rapidly, as can be expected in the case of eustatic control on a shallow epicontinental basin. However, the westward palaeogeographic step between the fourth and fifth incursions separated by several millions of years rather suggests the combined long-term effect of tectonism, possibly associated with early uplift of the Pamir-Kunlun Shan thrust belt. The fourth and fifth regressions are time-equivalent with significant aridification steps recorded in the Asian interior, thus supporting climate modelling results showing that the stepwise sea retreat from Central Asia amplified the aridification of the Asian interior.