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

International audienceA 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

Paratethys pacing of the Messinian Salinity Crisis:Low salinity waters contributing to gypsum precipitation?

During the so-called Messinian Salinity Crisis (MSC: 5.97-5.33 Myr ago), reduced exchange with the Atlantic Ocean caused the Mediterranean to develop into a “saline giant” wherein ∼1 million km3 of evaporites (gypsum and halite) were deposited. Despite decades of research it is still poorly understood exactly how and where in the water column these evaporites formed. Gypsum formation commonly requires enhanced dry conditions (evaporation exceeding precipitation), but recent studies also suggested major freshwater inputs into the Mediterranean during MSC-gypsum formation. Here we use strontium isotope ratios of ostracods to show that low-saline water from the Paratethys Seas actually contributed to the precipitation of Mediterranean evaporites. This apparent paradox urges for an alternative mechanism underlying gypsum precipitation. We propose that Paratethys inflow would enhance stratification in the Mediterranean and result in a low-salinity surface-water layer with high Ca/Cl and SO4/Cl ratios. We show that evaporation of this surface water can become saturated in gypsum at a salinity of ∼40, in line with salinities reported from fluid inclusions in MSC evaporites

The dinoflagellate cyst genera Achomosphaera Evitt 1963 and Spiniferites Mantell 1850 in Pliocene to modern sediments: a summary of round table discussions

Source at https://doi.org/10.1080/01916122.2018.1465739. We present a summary of two round-table discussions held during two subsequent workshops in Montreal (Canada) on 16 April 2014 and Ostend (Belgium) on 8 July 2015. Five species of the genus Achomosphaera Evitt 1963 and 33 of the genus Spiniferites Mantell 1850 emend. Sarjeant 1970 occuring in Pliocene to modern sediments are listed and briefly described along with remarks made by workshop participants. In addition, several holotypes and topotypes are reillustrated. Three species previously assigned to Spiniferites are here considered/accepted as belonging to other genera: Impagidinium inaequalis (Wall and Dale in Wall et al.1973) Londeix et al. 2009, Spiniferites? rubinus (Rossignol 1962 ex Rossignol 1964) Sarjeant 1970, and Thalassiphora balcanica Baltes ̧ 1971. This summary forms the basis for a set of papers that follows, where points raised during the workshops are explored in greater detail

Strontium isotopes reveal large connectivity changes during the late Miocene-Pliocene in restricted Dacian Basin (Romania) of the Eastern Paratethys

During the late Miocene-Pliocene the Dacian Basin was the western appendix of the Black Sea. The restricted paleogeographical location of the Dacian Basin within the larger Paratethys-Mediterranean system made possible unique recordings of hydrological changes governed by short lived connections to the open marine realm. The data set presented here is based on strontium isotopes (87Sr/86Sr) measured on ostracod shells collected from a well dated Miocene to Pliocene succession exposed along Slanicul de Buzau Valley, located in the Carpathian Foredeep of Romania (part of the Dacian Basin). We measured the 87Sr/86Sr only on ostracods. The choice for ostracods is twofold: 1) they occur abundantly in the fresh to brackish deposits of the Paratethys, while these environments are generally not suitable for the occurrence of foraminifera or other typical marine organisms; 2) ostracods bear in general low ornamentation and they molt their entire shell in one event. We used ostracods from the Slanicul de Buzau section and added 39 new 87Sr/86Sr data points to the earlier acquired results from Rîmnicu Sărat, extending and refining in this way the Dacian Basin 87Sr/86Sr record from the late Miocene (Khersonian) to the Pleistocene (Romanian Stage). Age constraints for Slanicul de Buzau section are provided by biostratigraphy coupled to magnetostratigraphy. Additionally, the section is correlated by magneto- biostratigraphy to the previously investigated Rîmnicu Sărat section, 15 km to the north. The detailed 87Sr/86Sr record for the Dacian Basin provides an exceptional record of basin connectivity from the latest Tortonian (ca. 7.7 Ma) until the early Pleistocene (ca. 1.8 Ma). Relatively high 87Sr/86Sr originated either from marine incursions from water supplied by the Balta delta in the East Carpathian Foreland, or from outflow waters of Lake Pannon and local rivers, while low 87Sr/86Sr likely originated from connectivity with the Black Sea region. Data 87Sr/86Sr record indicates that a late Tortonian transgression (7.6‒7.4 Ma) started with an incursion of Eastern Paratethys waters into the Dacian Basin. Afterwards, local rivers became the dominant source for the mostly freshwater environments of the early Messinian. The regional Maeotian-Pontian transitional interval, between 6.3‒5.9 Ma, is marked by a second incursion of Eastern Paratethys waters, but during this event, an additional marine (Mediterranean) influx coincides with a short-lived salinity incursion. During the Messinian Salinity Crisis of the Mediterranean, the Dacian Basin connected progressively to the Eastern Paratethys (5.9‒5.5 Ma). The Dacian Basin became restricted during the peak Mediterranean lowstand at ~5.5 Ma and filled with Lake Pannon and local river water between 5.5‒5.3 Ma. During the Plio-Pleistocene, the Dacian Basin reconnected with the Black Sea (at that time isolated from the ocean ocean), which shows similar 87Sr/86Sr as in the Last Glacial Maximum. When evaluating the application of 87Sr/86Sr in assessing the connectivity (i.e. to open ocean) we conclude that the Dacian Basin (between 7.6 and 1.7 Ma) is an opposite counterpart to the Mediterranean during the Messinian Salinity Crisis. While the latter was a marine basin affected by relatively short term disconnection form the ocean, the former (the Dacian Basin) was a part of a large brackish-lacustrine isolated domain affected by a very short connection to the marine realm. Samples were collected from Slanic de Buzau (Romania) deposits exposed on the left side of the Slănicul river, along the Mânzăleşti (GPS 45°29'18.74 N; 26°39'10.30E). The dataset include all 87Sr/86Sr measured on ostracods and one foraminifera level

Linking Tarim Basin sea retreat (west China) and Asian aridification in the late Eocene

International audienceThe Tarim Basin in western China formed the easternmost margin of a shallow epicontinental seathat extended across Eurasia and was well connected to the western Tethys during the Paleogene.Climate modelling studies suggest that the westward retreat of this sea from Central Asia may havebeen as important as the Tibetan Plateau uplift in forcing aridification and monsoon intensificationin the Asian continental interior due to the redistribution of the land-sea thermal contrast. However,testing of this hypothesis is hindered by poor constraints on the timing and precise palaeogeographicdynamics of the retreat. Here, we present an improved integrated bio- and magnetostratigraphicchronological framework of the previously studied marine to continental transition in the southwestTarim Basin along the Pamir and West Kunlun Shan, allowing us to better constrain its timing,cause and palaeoenvironmental impact. The sea retreat is assigned a latest Lutetian–earliest Bartonianage (ca. 41 Ma; correlation of the last marine sediments to calcareous nannofossil Zone CP14and correlation of the first continental red beds to the base of magnetochron C18r). Higher up in thecontinental deposits, a major hiatus includes the Eocene–Oligocene transition (ca. 34 Ma). This suggeststhe Tarim Basin was hydrologically connected to the Tethyan marine Realm until at least theearliest Oligocene and had not yet been closed by uplift of the Pamir–Kunlun orogenic system. Thewestward sea retreat at ca. 41 Ma and the disconformity at the Eocene–Oligocene transition are bothtime-equivalent with reported Asian aridification steps, suggesting that, consistent with climatemodelling results, the sea acted as an important moisture source for the Asian continental interior

Mediterranean-Paratethys connectivity during the Messinian salinity crisis: The Pontian of Azerbaijan

Prior to the onset of the Messinian Salinity Crisis, a connection was established between the Mediterranean Sea and the Paratethys region to the north. Rivers currently draining into the Caspian Sea thereby became important for the Mediterranean hydrological budget. The role of this connection and the influence of the Paratethys on the hydrological budget of the Mediterranean Sea during the Messinian Salinity Crisis is however poorly understood because of a lack of records in the Paratethys with a high-resolution (cyclostratigraphic) age model. Here, we present a high-resolution integrated stratigraphic study of a key section in the Caspian Sea region (Azerbaijan), to assess the connectivity of the Caspian Sea during the salinity crisis. The studied section spans the time interval between ~ 6.16 Ma an

Quantitative analysis of Paratethys sea level change during the Messinian Salinity Crisis

At the time of the Messinian Salinity Crisis in the Mediterranean Sea (i.e., the Pontian stage of the Paratethys), the Paratethys sea level dropped also. Evidence found in the sedimentary record of the Black Sea and the Caspian Sea has been interpreted to indicate that a sea level fall occurred between 5.6 and 5.5 Ma. Estimates for the magnitude of this fall range between tens of meters to more than 1500 m. The purpose of this study is to provide quantitative insight into the sensitivity of the water level of the Black Sea and the Caspian Sea to the hydrologic budget, for a scenario in which the Paratethys is disconnected from the Mediterranean. Using a Late Miocene bathymetry based on a palaeographic map we quantify the fall in sea level, the mean salinity, and the time to reach equilibrium for a wide range of negative hydrologic budgets. By combining our results with (i) estimates calculated from a set of recent global Late Miocene climate simulations and (ii) reconstructed basin salinities, we are able to rule out a drop in sea level of the order of 1000 m in the Caspian Sea during this time period. In the Black Sea, however, such a large sea level fall cannot be fully discarded

Flooding of the Caspian Sea at the intensification of Northern Hemisphere Glaciations

The semi-isolated epicontinental Paratethys Sea in the Eurasian continental interior was highly sensitive to changes in basin connectivity and hydrological budget. The Caspian Sea, the easternmost basin experienced a five-fold increase in surface area during the Plio-Pleistocene climate transition, but a basic process-based understanding is severely hampered by a lack of high-resolution age constraints. Here, we present a magnetostratigraphic age model supported by 40 Ar/ 39 Ar dating of volcanic ash layers for the 1600 m thick Jeirankechmez section in Azerbaijan that comprises a sedimentary rock succession covering this so-called Akchagylian flooding. We establish the age of this major change in Caspian paleohydrology at around 2.7 Ma. The presence of cold water foraminifera, rising strontium isotope ratios and the possible arrival of the enigmatic Caspian seal in the basin hints at an Arctic marine source for the Akchagylian waters. The new age model indicates a direct link to the intensification of northern hemisphere glaciations at the end of the Pliocene and to concurrent hydrological shifts across Eurasia, such as the onset of cyclic Chinese Loess deposits. The transformation of the Paratethys region around 2.7 Ma from a series of small Pliocene endorheic lake basins to a large Early Pleistocene epicontinental water mass coincides with a more positive hydrological budget for the Eurasian continental interior. The drainage of additional high latitude, low salinity water to the Mediterranean, may have contributed towards variability in global paleoceanography, and could potentially provide a positive feedback towards Pleistocene climate cooling