Open Access

Alexandra Hellwig

• Enhanced aridification of Central Asia driven by the combined effects of orogenic surface uplift, Paratethys retreat, changes in atmospheric moisture transport and global cooling is one of the most prominent Cenozoic climate change events of the Northern Hemisphere. Deciphering regional long-term patterns of Central Asian hydrology is, therefore, a key element in understanding the role of Northern Hemisphere mid-latitude drying in the global hydrological system. Pedogenic carbonates record information of the paleoecosystem, the paleogeography, hydrology, tectonic and climatic conditions as well as the sedimentary regime during the time in which they formed. The calcrete-bearing paleosols in continental sedimentary basins yield the possibility for understanding these processes, mechanisms and controlling factors. This study characterizes long-term paleoenvironmental conditions between the late Oligocene and early Miocene in SE Kazakhstan based on stable isotopes, elemental geochemistry and laser ablation U-Pb geochronology from alluvial, fluvial and pedogenic deposits. The main topics addressed in this thesis are paleosol and calcrete formation processes in the light of geochemistry, tectonic and climatic influences on both, as well as the paleoenvironmental and hydrological conditions which led to these processes. The sedimentological succession of the 14 km-long Kendyrlisai Valley in the Ili Basin, SE Kazakhstan in Central Asia, yields exceptional well-developed paleosols which provide an insight into the relationship between tectonics and sedimentation as well as soil formation processes. The valley accommodates more than 600 m of siliciclastic sediments deposited in a fluvial to alluvial environment in the lower part which grade into lacustrine to fluvial deposition with minor gypsum accumulation in the upper part. The yet undated sediments of Kendyrlisai Valley are compared with a biostratigraphically dated and well described succession in an adjacent location. The correlation of the two successions revealed a proximal-distal relation with the Kendyrlisai Valley succession represents the basin margin position with coarse alluvial-fluvial sediments. The calcrete nodules in Kendyrlisai Valley paleosols give the opportunity of U-Pb dating with LA-ICP-MS which uses the variation in both U and Pb to directly date the calcite. The U-Pb dating revealed a late Oligocene-early Miocene age for the investigated section. The analysis of facies and geochemistry of the paleosol profiles in Kendyrlisai Valley gives insight into calcretization stages, pedogenic processes and landscape stability. Kendyrlisai Valley paleosols show a low to moderate weathering intensity with the highest weathering intensity between 0.5 and 0.8 m depth in the paleosol profile. The comparison between acid leachable and non-acid leachable fraction indicate an incorporation of Ca and Ba in incipient calcrete calcite, whereas clay minerals adsorb Mg and Sr. During the evolution from early to more mature calcrete stages, i.e. calcrete nodules, Ba is lost by dissolution and subsequently adsorbed on clay minerals. The nodules consist almost exclusively of pure calcium carbonate with rare substitution of Ca by Mg indicating a constant supply of Ca by weathering and solution during calcrete formation. The occurrence of Mg-bearing clay minerals indicates high evaporative conditions with alkaline waters (pH >9). Sampled calcrete nodule cross sections reveal more than one weathering and calcrete formation phase with a higher supply of Ca, Mg and Fe during early phases. The organic and inorganic carbon isotope composition of the calcrete nodules reflects C3 metabolism under occasionally moisture stress, resulting in higher δ13C values and lower respiration rates. This study also presents calculated atmospheric pCO2 values from the calcrete nodules with the equation from Cerling et al. (1999). The average calculated CO2 concentration for Kendyrlisai Valley paleosols is 313 ± 110 ppmv. The comparison with data from other studies during the late Oligocene–early Miocene reveal lower values for Kendyrlisai Valley paleosols, which may stem from an underestimation of the plant-respired CO2. The knowledge of the variability within the paleosols and calcrete nodules enables a reliable paleoclimatic interpretation for the Kendyrlisai Valley and beyond the Ili Basin. Sedimentary facies and geochemical weathering indices suggest an increased surface and groundwater discharge fed by orographically-enhanced precipitation in the Tien Shan hinterland. In contrast, pedogenic stable isotope data and elevated rates of Mg fixation in clay minerals mirror enhanced rates of evaporation in the vadose zone due to protracted aridification. This study posits that pronounced surface uplift of the Tien Shan Mountains during the Oligocene–Miocene transition promoted regionally increased orographic precipitation and the development of fluvial discharge systems. The comparison with other studies in adjacent basins creates a precipitation pattern for Central Asia during the late Oligocene to early Miocene. The westerlies supplied Central Asia with atmospheric moisture from the West, possibly from the Eastern Paratethys. The uplifting Tien Shan Mountain ranges captured this moisture on their luv-side, resulting in a pronounced orographic rainfall in the adjacent Ili and Issyk Kul Basins. The Tarim Basin and the Valley of Lakes experienced drier conditions due to the rain shadow effect on the lee-side of the Tien Shan and Altai Mountains. In course of this findings, the thesis highlights the crucial role the Tien Shan Mountains play and had been playing in former times as an orographic barrier for the distribution of atmospheric moisture across Central Asia.
• Pedogene Karbonate zeichnen Informationen über das Paläoökosystem, die Paläogeographie, Hydrologie, tektonische und klimatische Bedingungen sowie das sedimentäre System während ihrer Bildungszeit auf (Alonso-Zarza 2003). Obwohl das wissenschaftliche Interesse an pedogenen Karbonaten in den letzten Jahren gestiegen ist, gibt es noch immer zahlreiche offene Fragen und die Thematik ist weit davon entfernt, verstanden zu sein (Zamanian et al. 2016). Es fehlt an Arbeiten zu Konzepten und Modellen von Bildungsmechanismen pedogener Karbonate sowie deren Beziehung zu Umweltfaktoren. Das Wissen um die abhängigen Einflussfaktoren würde das Verständnis der Bildungsphasen und -bedingungen verbessern. Die Bodenkarbonat führenden Paläoböden in kontinentalen Sedimentbecken ermöglichen es, diese Prozesse, Mechanismen und Kontrollfaktoren zu verstehen. Ein Sedimentbecken weit weg von jeglichen marinen Einflüssen bietet eine wertvolle Chance, das Zusammenspiel von Sedimentologie, Klima und Tektonik zu untersuchen. Diese Arbeit stellt die sedimentologische Abfolge des 14 km-langen Kendyrlisai Tals im Ili Becken, im Südosten von Kasachstan in Zentralasien, vor. Die Lokation im Vorland der Tien Shan Berge und dessen andauernde tektonische Aktivität infolge der Indien-Asien-Kollision vor 55 Millionen Jahren birgt nicht nur das Potenzial, die Beziehung von Tektonik und Sedimentation zu untersuchen. Die außergewöhnlich gut ausgebildeten Paläoböden im Tal bieten einen Einblick in die Prozesse während deren Bildung. Die Hauptthemen, die in dieser Arbeit angesprochen werden, sind die Vorgänge der Paläoboden- und Bodenkarbonatbildung im Licht von Geochemie, tektonischen und klimatischen Einflüssen auf beide sowie die Paläoumwelt- und Hydrologiebedingungen, die zu diesen Prozessen führten. Das tiefere Wissen der einzigartigen Bodenkarbonatbildung im Kendyrlisai Tal könnte die paläoklimatische Geschichte des Beckens und darüber hinaus aufklären...