Sedimentary geology of the Middle Carboniferous of the Donbas region (Dniepr-Donets basin, Ukraine)

Peer reviewedPublisher PD

Заява Спілки Археологів України щодо проекту Закону України “Про відродження унікального Символу православ’я — церкви Богородиці (Десятинної) в місті Києві” (№ 9196)

The Milankovitch theory of climate change is widely accepted, but the registration of the climate changes in the stratigraphic record and their use in building high-resolution astronomically tuned timescales has been disputed due to the complex and fragmentary nature of the stratigraphic record. However, results of time series analysis and consistency with independent magnetobiostratigraphic and/or radio-isotopic age models show that Milankovitch cycles are recorded not only in deep marine and lacustrine successions, but also in ice cores and speleothems, and in eolian and fluvial successions. Integrated stratigraphic studies further provide evidence for continuous sedimentation at Milankovitch time scales (10^4 years up to 10^6 years). This combined approach also shows that strict application of statistical confidence limits in spectral analysis to verify astronomical forcing in climate proxy records is not fully justified and may lead to false negatives. This is in contrast to recent claims that failure to apply strict statistical standards can lead to false positives in the search for periodic signals. Finally, and contrary to the argument that changes in insolation are too small to effect significant climate change, seasonal insolation variations resulting from orbital extremes can be significant (20% and more) and, as shown by climate modelling, generate large climate changes that can be expected to leave a marked imprint in the stratigraphic record. The tuning of long and continuous cyclic successions now underlies the standard geological time scale for much of the Cenozoic and also for extended intervals of the Mesozoic. Such successions have to be taken into account to fully comprehend the (cyclic) nature of the stratigraphic record

Supplemental Material: Long-eccentricity pacing of alluvial stratigraphic architecture in the Eocene Bighorn Basin, Wyoming, USA

Figure S1 (UAV-based photogrammetric model in the LIME software) and Figure S2 (comparison of channelized sandstone width and depth in Intervals II and III exhibiting varying dominance of river styles).  </p

Patterns and timing of loess-paleosol transitions in Eurasia: Constraints for paleoclimate studies

International audienceLoess-paleosol sequences are the most extensive terrestrial paleoclimate records in Europe and Asia documenting atmospheric circulation patterns, vegetation, and sedimentary dynamics in response to glacial-interglacial cyclicity. Between the two sides of the Eurasian continent, differences may exist in response and response times to glacial changes and finding these is essential to understand the climate systems of the northern hemisphere. Therefore, assessment of common patterns and regional differences in loess-paleosol sequences (LPS) is vital, but remains, however, uncertain. Another key to interpret these records is to constrain the mechanisms responsible for the formation and preservation of paleosols and loess layers in these paleoclimate archives. This study therefore compares LPS magnetic susceptibility records as proxies for paleosol formation intensity for selected sites from the central Chinese Loess Plateau and the Carpathian Basin in Europe over the last 440 kyr. Inconsistencies and crucial issues concerning the timing, correlation and paleoclimate potential of selected Eurasian LPS are outlined. Our comparison of Eurasian LPS shows generally similar patterns of paleosol formation, while highlighting several crucial differences. Especially for paleosols developed around 200 and 300 ka, the reported timing of soil formation differs by up to 30 ka. In addition, a drying and cooling trend over the last 300 ka has been documented in Europe, with no such evidence in the Asian records. The comparison shows that there is still uncertainty in defining the chronostratigraphic framework for these records on glacial-interglacial time scales in the order of 5-30 kyr for the last 440 ka. We argue that the baseline of the magnetic susceptibility proxy in loess from the Carpathian Basin is the most striking difference between European LPS and the Chinese Loess Plateau. In our opinion, many of the current timing/age differences may be overcome once a comparable stratigraphic interpretation is achieved

A late Eocene palynological record of climate change and Tibetan Plateau uplift (Xining Basin, China)

Climate models suggest that Asian paleoenvironments, monsoons and continental aridification were primarily governed by tectonic uplift and sea retreat since the Eocene with potential contribution of global climate changes: However, the cause and timing of these paleoenvironmental changes remain poorly constrained. The recently well-dated continental mudflat to ephemeral saline lake sedimentary succession, situated in the Xining Basin at the northeastern margin of the Tibetan Plateau (NW China), provides a unique opportunity to develop additional proxy successions in this area that are placed accurately in time. Here, a palynological record from this succession is reported. High abundances of desert and steppe-desert taxa such as Ephedripites and Nitrariadites/Nitraripollis are found, which can be differentiated by the presence of broad leaved deciduous forest taxa in the lower part of the section (particularly up to 36.4 Ma; magnetochron C16r), and a sudden increase of Pinaceae (Pinuspollenites, Piceaepollenites and Abiespollenites) which is dated at 36.1 Ma (C16n.2n). Coexistence Approach (CoA) indicates that from 39.9 to 36.4 Ma (C17n.1n) regional climate was warm and wet, while from 36.4 to 33.5 Ma (C16n.2n-C13r) climate tends to be cooler and drier. The data indicate that paleoenvironmental and palynological changes on the NE part of the Tibetan Plateau resulted from a combination of long-term tectonic uplift forcing and long- and short-term climate changes. The increase of taxa such as Piceaepollenites and Abiespollenites indicates not only a cooling and drying trend prior to the Eocene/Oligocene (E/O) boundary, but also the existence of high altitude mountain habitats in the periphery of the Xining Basin. The sudden Pinaceae event correlates closely in time with a marked aridification step as viewed from the lithology of the Xining Basin that was linked to the sea retreat out of the Tarim Basin. (C) 2012 Elsevier B.V. All rights reserved.Geography, PhysicalGeosciences, MultidisciplinaryPaleontologySCI(E)0ARTICLE16-3834

Identifying eolian dust in the geological record

Recognition of terrestrial dust in geological records is essential for reconstructing paleoenvironments and quantifying dust fluxes in the past. However, in contrast to eolian sands, silt-sized dust is difficult to recognize in pre-Quaternary records due to a lack of macroscopic features indicating eolian transport and mixing with alluvial sediments. Windblown dust deposits are commonly identified by comparing their sedimentological and petrological features with Quaternary examples of dust known as loess. Here, we review the characteristics of terrestrial dust deposits and conclude that most of these features are not exclusively windblown and may be formed by alluvial deposits as well. We therefore synthesize a set of criteria which enable a reliable identification and quantification of dust while acknowledging potential contributions of alluvial components. These methods include quartz-grain surface morphology analysis to distinguish eolian and alluvial transport modes, provenance studies to identify local and extrabasinal sources, grain-size-shape end-member modelling to quantify the various sedimentary contributions to the record, and a basin-scale stratigraphic approach to derive regional patterns and avoid interpretation of local phenomena. We reassess the Eocene to Pliocene records of the Chinese Loess Plateau and conclude that these strata represent both alluvial and eolian sediments deposited in extensive mudflat systems. Quaternary loess, by contrast, is almost exclusively composed of windblown dust. The early Pleistocene shift from mudflat to loess deposits is associated with a significant increase in accumulation rates, likely due to increased dust production upwind, overwhelming and blanketing the local mudflat systems in central China

Amplitude, frequency and drivers of Caspian Sea lake-level variations during the Early Pleistocene and their impact on a protected wave-dominated coastline

The Caspian Sea, the largest isolated lake in the world, witnessed drastic lake-level variations during the Quaternary. This restricted basin appears very sensitive to lake-level variations, due to important variations in regional evaporation, precipitation and runoff. The amplitude, frequency and drivers of these lake-level changes are still poorly documented and understood. Studying geological records of the Caspian Sea might be the key to better comprehend the complexity of these oscillations. The Hajigabul section documents sediment deposited on the northern margin of the Kura Basin, a former embayment of the Caspian Sea. The 2035 m thick, well-exposed section was previously dated by magneto-biostratigraphic techniques and provides an excellent record of Early Pleistocene environmental, lake-level and climate changes. Within this succession, the 1050 m thick Apsheronian regional stage, between ca 2·1 Ma and 0·85 Ma, represents a particular time interval with 20 regressive sequences documented by sedimentary and palaeontological changes. Sequences are regressing from offshore to coastal, lagoonal or terrestrial settings and are bounded by abrupt flooding events. Sediment reveals a low energy, wave-dominated, reflective beach system. Wave baselines delimiting each facies association appear to be located at shallower bathymetries compared to the open ocean. Water depth estimations of the wave baselines allow reconstruction of a lake-level curve, recording oscillations of ca 40 m amplitude. Cyclostratigraphic analyses display lake-level frequency close to 41 kyr, pointing to allogenic forcing, dominated by obliquity cycles and suggesting a direct or indirect link with high-latitude climates and environments. This study provides a detailed lake-level curve for the Early Pleistocene Caspian Sea and constitutes a first step towards a better comprehension of the magnitude, occurrence and forcing mechanisms of Caspian Sea lake-level changes. Facies models developed in this study regarding sedimentary architectures of palaeocoastlines affected by repeated lake-level fluctuations may form good analogues for other (semi-)isolated basins worldwide

Sedimentary architecture and depositional controls of a Pliocene river-dominated delta in the semi-isolated Dacian Basin, Black Sea

Sedimentological facies models for (semi-)isolated basins are less well developed than those for marine environments, but are critical for our understanding of both present-day and ancient deltaic sediment records in restricted depositional environments. This study considers an 835 m thick sedimentary succession of mid-Pliocene age, which accumulated in the Dacian Basin, a former embayment of the Black Sea. Detailed sedimentological and palaeontological analyses reveal a regression from distal prodelta deposits with brackish water faunas to delta-top deposits with freshwater faunas. Sediments contain frequent hyperpycnal plumes and an enrichment in terrestrial organic material, ichnofossils and in situ brackish and freshwater faunas. Deltaic progradation created thin, sharply-based sand bodies formed by multiple terminal distributary channels, covering a wide depositional area. The system experienced frequent delta-lobe switching, resulting in numerous thin parasequences. Parasequences are overlain by erosive reddish oxidized sand beds, enriched in broken, abraded brackish and freshwater shells. These beds were formed after sediment starvation, on top of abandoned delta lobes during each flooding event. A robust magnetostratigraphic time frame allowed for comparison between the observed sedimentary cyclicity and the amplitude and frequency of astronomical forcing cycles. Our results indicate that parasequence frequencies are significantly higher than the number of time equivalent astronomical cycles. This suggests that delta-lobe switching was due to autogenic processes. We consider the observed facies architecture typical for a delta prograding on a low-gradient slope into a shallow, brackish, protected, semi-isolated basin. Furthermore, in the absence of significant wave and tidal influence, sediment progradation in such a protected depositional setting shaped a delta, strongly river-dominated

Identifying eolian dust in the geological record

Recognition of terrestrial dust in geological records is essential for reconstructing paleoenvironments and quantifying dust fluxes in the past. However, in contrast to eolian sands, silt-sized dust is difficult to recognize in pre-Quaternary records due to a lack of macroscopic features indicating eolian transport and mixing with alluvial sediments. Windblown dust deposits are commonly identified by comparing their sedimentological and petrological features with Quaternary examples of dust known as loess. Here, we review the characteristics of terrestrial dust deposits and conclude that most of these features are not exclusively windblown and may be formed by alluvial deposits as well. We therefore synthesize a set of criteria which enable a reliable identification and quantification of dust while acknowledging potential contributions of alluvial components. These methods include quartz-grain surface morphology analysis to distinguish eolian and alluvial transport modes, provenance studies to identify local and extrabasinal sources, grain-size-shape end-member modelling to quantify the various sedimentary contributions to the record, and a basin-scale stratigraphic approach to derive regional patterns and avoid interpretation of local phenomena. We reassess the Eocene to Pliocene records of the Chinese Loess Plateau and conclude that these strata represent both alluvial and eolian sediments deposited in extensive mudflat systems. Quaternary loess, by contrast, is almost exclusively composed of windblown dust. The early Pleistocene shift from mudflat to loess deposits is associated with a significant increase in accumulation rates, likely due to increased dust production upwind, overwhelming and blanketing the local mudflat systems in central China