Sedimentological and morphological analysis of highstand sediments from lake Heihai (China) and their chronostratigraphical interpretation

Die vorliegende Dissertation untersucht die Seespiegelschwankungen des Heihai. Dafür wurden morphologische und sedimentologische Arbeiten an den Hochstandssedimenten des Sees durchgeführt und deren Ergebnisse, mit Hilfe eines Bohrkern aus der Mitte des Sees, in einen chronostratigraphischen Kontext gestellt. Der tiefste Seespiegel konnte zwischen -17 bis -18 m verortet werden und geht vermutlich auf das Spätglazial/Frühholozän zurück. Das höchste Niveau erreichte der See in der ersten Hälfte des Holozäns mit einer Höhe von +5 bis +15 m oberhalb des heutigen Stands. Die heute teils periglazial überprägte Hochstandsfläche konnte mit Hilfe einer Hot-Spot- Analyse in Hebungs- und Absenkungsbereiche unterteilt werden. In den weichen Hochstandsablagerungen hinterließ der See morphologische Spuren, die zu vier weiteren Seespiegelständen zusammengesetzt wurden (S1/T0 = +0,3 m, T1 = +1,1 m, T2 = +2,2 m, T3 = +3,2 m). Ein Vergleich von Fernerkundungsdaten der letzten 40 Jahre zeigte, dass der exorheische Heihai nach einem Starkregenereignis (2010), maximal das Level der T1 erreichen konnte, ansonsten jedoch ein stabiles Gleichgewichtsniveau einnahm. Um die Prozessdynamik, die während und im Anschluss an den Hochstand herrschte, abbilden zu können, wurde die geochemische Zusammensetzung der Hochstandssedimente mittels einer Faktorenanalyse analysiert. Da sich in den Profilen jedoch unterschiedliche Ablagerungsmilieus mischten, wurden die Sedimente zuerst mit einer Clusteranalyse in dominante Korngrößenklassen aus lakustrinen Schluffe und terrestrischen Sanden getrennt. Mineralbestimmungen mittels XRD-Analyse deckten singuläre Mineralbildungen auf, die die Elementkorrelation beeinflussten. Nach Entfernung der halithaltigen Proben aus dem Datensatz ergaben sich bei der Faktorenanalyse der Elementdaten aus den lakustrinen Ablagerungen Faktoren, die stellvertretend für die Ab- und Umlagerungsprozesse im See stehen. Der Heihai weist einen der höchsten bekannten modernen Reservoireffekte auf dem Qinghai-Tibet-Plateau auf (6.465 ± 75 14C-Jahre). Dieser war im Laufe der Ablagerungsgeschichte jedoch starken Schwankungen unterworfen, welche größtenteils an den Eintrag exogener Karbonate gebunden waren. Während in der ersten Phase der Seeentwicklung alter Kohlenstoff durch den Zufluss aus einem Kalksteineinzugsgebiet in den See gelangte, war es in der Folge die Remobilisierung der kalzithaltigen Hochstandssedimente, die der Reservoireffekt dominierten. Die terrestrischen Sandablagerungen, die in den Hochstandssedimenten gefunden wurden, konnten als Markerhorizont verwendet werden und weisen auf eine Trockenperiode im Einzugsgebiet des Sees hin. Diese Trockenphase kann um 3,5 ka BP angenommen werden und stellt die zeitliche Mindestgrenze für die Hauptphase der Seeentwicklung dar. Diese fiel vermutlich mit der Ausbildung von Monohydrokalzit im Kern zusammen, was den Höchststand stratigraphisch auf ein Alter zwischen 4875 ± 1650 und 8.360 ± 1.650 cal a BP begrenzt.This thesis examines lake level variations in the Heihai lake system (Northern Tibetan Plateau). These changes were derived from landforms and sediments along the littoral zone of the lake. In addition an age-depth-model from a piston long core from the deepest part of the lake was established to put the results in a chronostratigrapical context. The deepest terrace generation dates from the Late Glacial to Early Holocene and was identified between -17 to -18 m below the modern lake level. The highest reconstructable lake stand was established in the first part of the Holocene at a height +5 to +15 m above the modern level. The high uncertainty in the estimation was caused by periglacial uplift that overprinted the former depositional height. These uplifted areas were revealed by hot spot analysis in combination with a detailed littoral mapping approach. Inside the soft sediments of the highstand, different terrace levels could be identified, indicating four lake stages at +0,3 m (S1/T0), +1,1 m (T1), +2,2 m (T2) and +3,2 m (T3) above the modern lake level. Thereby the level T1 was associated with an extreme rainfall event in 2010. Apart from that, the exogenous Heihai had remained constant for at least the last 40 years. The sediments around the lake were sampled in eight profiles and can be divided into two dominant grain size classes (lacustrine silt and terrestrial sand). The mineral content was analyzed on the basis of XRD measurements, revealing isolated minerals which influenced the elemental correlation within the deposits. After removal of the halite-bearing samples, a factor analysis was performed, indicating processes of internal carbonate production, sediment mixing and periglacial weathering activity. Lake Heihai shows one of the highest known modern reservoir effects on the Tibetan Plateau (6.465 ± 75 14C years). However this effect was not constant over time and fluctuated depending on the input of exogenic carbonate from the catchment. During the first period of lake development, dead carbon from a carbonate-bearing catchment had a massive impact on the reservoir effect. Thereafter the remobilisation of calcite leached out from the former highstand sediments led to a homogenization of 14C-ages. The terrestrial sand layers from the highstand sediments were linked to a period of aridity in the catchment at about 3,5 ka BP. On the basis of a process and provenance genetic age-depth-model, the calibrated age of the highest lake level at Lake Heihai was limited to between 4875 ± 1650 and 8.360 ± 1.650 cal a BP

A process- and provenance-based attempt to unravel inconsistent radiocarbon chronologies in Lake Sediments: An example from Lake Heihai, north Tibetan Plateau (China)

Aquatic macrophytes from a lacustrine environment are highly prone to a reservoir effect, resulting in an overestimation of age. This is often caused by the incorporation of dissolved carbon (CO2 and HCO3–) through photosynthesis from lake waters that have a different 14C activity than the atmosphere. The atmosphere-water disparity is often produced by a mixing of carbon between the water body and its terrestrial surroundings, a process highly prone to temporal variations. Thus, only a comprehensive understanding of the 14C budget over time enables a reliable chronology of lacustrine records. We studied lacustrine sediments from Lake Heihai on the northern Tibetan Plateau with a recent reservoir effect of 6465 ± 75 14C yr as estimated from accelerator mass spectrometry (AMS) dating of three living aquatic plants. Age inversions in a well-laminated composite core from the lake suggest that the reservoir effect markedly changed over the depositional period. In the lower part of the core, an excellent correlation was observed between the allochthonous input of dolomite and the inverse 14C ages, indicating the incorporation of dissolved 14C-dead carbon from a limestone catchment in the plant material. For the upper part of the core, sediment recycling of Holocene high-stand deposits may have further contributed to the reservoir effect. These findings give rise to a reliable process- and provenance-based chronology within a confidence interval supported by 137Cs measurements and magnetostratigraphic investigations. Our results highlight the need to identify the interactions of lakes with their surroundings to estimate reservoir-corrected ages in lacustrine settings

Influence of HCl pretreatment and organo-mineral complexes on laser diffraction measurement of loess–paleosol-sequences

The influence of different sample pretreatment methods on grain size distributions in particle size analysis has been subject to controversial discussions. Standard sample preparation typically comprises the disaggregation of aggregated and agglomerated particles into single primary particles, i.e., the organic binding material is oxidized by hydrogen peroxide (H2O2) and the contained carbonates are dissolved by hydrochloric acid (HCl). The aim of this study is to evaluate the effects of HCl treatment on grain size analyses of Late Pleistocene and Holocene loess-paleosol-sequences investigated by a Beckman Coulter LS 13320 laser particle analyzer. For this purpose, samples from two different sections with different weathering degrees and sedimentary genesis were measured: (1) the Suohuoduo section on the northeastern fringe of the Tibetan Plateau (China) containing loess and paleosols, and (2) a vibracore from Dusseldorf-Grafenberg (Germany) containing calcareous loess and intercalated interglacial, interstadial and periglacial soils and soil sediments. All samples were pretreated with hydrogen peroxide and sodium pyrophosphate. Subsequently, the samples were prepared with and without the addition of HCl. There is no significant association of the HCl-induced grain size modifications after the HCl treatment with the calcium carbonate content Conversely, a distinct dependence of the modification of grain size distributions on the content of organic matter, the weathering degree of the sediment, and the presence of stable aggregates as well as organo-mineral complexes was observed. Consequently, pretreating post-depositionally modified aeolian sediments with HCl may result in misleadinggrain size distributions and should be avoided in standard analyses of loess-paleosol-sequences. However, the HCl-induced modification of grain size distributions provides an indication of the existence of stable aggregates or organo-mineral complexes. (C) 2015 Elsevier B.V. All rights reserved

Characterisation of transport processes and sedimentary deposits by statistical end-member mixing analysis of terrestrial sediments in the Donggi Cona lake catchment, NE Tibetan Plateau

Unmixing of grain-size distributionswithmultivariate statistical analysis gives indications of themain sediment transport processes and related environmental conditions in an area. We performed end-member mixing analysis (EMMA) of sedimentological data from 912 terrestrial sediment samples collected in the Donggi Cona catchment, north-eastern Tibetan Plateau. Up to the present, this is the largest sedimentological dataset on the Tibetan Plateau. EMMA resulted in the characterisation of three end-members that explain 88% of the variance within the dataset. The end-members all represent aeolian deposits. The first end-member EM 1 represents very fine dune sediments that were deflated from a former lake system. EM 2 represents medium sand deposits that were blown out from playa and alluvial fan sediments. EM 3 represents fine loess(−like) sediments mainly found at higher elevations. Different transformations, adding of a fourth end-member and adding of up to 200 loess samples do not change the composition of the end-members, demonstrating the robustness of themodel. EMMA allows the synchronous interpretation of very large datasets, resulting in a general characterisation of sediment transport in a particular area. Performing EMMA on the dataset demonstrates the importance of aeolian transport in this part of the world

Bulk mineralogy and membership degrees of sediments from the Heihai lake area

Extra-tropical circulation systems impede poleward moisture advection by the Indian Summer Monsoon. In this context, the Himalayan range is believed to insulate the south Asian circulation from extra-tropical influences and to delineate the northern extent of the Indian Summer Monsoon in central Asia. Paleoclimatic evidence, however, suggests increased moisture availability in the Early Holocene north of the Himalayan range which is attributed to an intensification of the Indian Summer Monsoon. Nevertheless, mechanisms leading to a surpassing of the Himalayan range and the northern maximum extent of summer monsoonal influence remain unknown. Here we show that the Kunlun barrier on the northern Tibetan Plateau [~36°N] delimits Indian Summer Monsoon precipitation during the Holocene. The presence of the barrier relocates the insulation effect 1,000 km further north, allowing a continental low intensity branch of the Indian Summer Monsoon which is persistent throughout the Holocene. Precipitation intensities at its northern extent seem to be driven by differentiated solar heating of the Northern Hemisphere indicating dependency on energy-gradients rather than absolute radiation intensities. The identified spatial constraints of monsoonal precipitation will facilitate the prediction of future monsoonal precipitation patterns in Central Asia under varying climatic conditions