African Linguistics in Central and Eastern Europe, and in the Nordic Countries

Non peer reviewe

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

Hard-water dynamics and their reservoir effects on radiocarbon dating of Lake Heihai sediments (NE Tibetan Plateau, Qinghai, China)

Age determination of lake sediments with radiocarbon dating can always entail a perturbation with hard water. Atmospheric carbon (expressing the "real" ages) can be mixed with older carbon from allochthonous input (e.g. marl or limestone), causing an overestimation of 14C ages. The usual approach to eliminate this effect is to date living plants or shells to determine the modern offset in age. Subsequently, this offset is subtracted from 14C ages of a sediment core to attain hard water corrected ages. However, this approach assumes a constant hard water effect over the entire period under consideration, which generally is unlikely. Here we present a highly variable hard water effect through time determined from a combined chronology of two long sediment cores from Lake Heihai (NE Tibetan Plateau). The chronology is based on 20 14C AMS dates of Potamogeton spec. Based on the relation between 14C ages and the input of allochthonous carbonates as well as calculated sedimentation rates, we developed an age-depth-model that estimates the actual ages of the sediments and allows the quantification of hard water effect through time. As a result this model suggests a fluctuating hard water effect varying between 10.2 to 10.3 ka. Ages in the lower 3 meter of the core, which corresponds to late glacial times, strongly correlate with the input of dolomite (CaMg(CO3)2). The correlation suggests a strong linkage between the allochthonous input of old carbon and the variations in dating results. In this section, the estimated hard water effect shows its highest values. Results of XRD, grain size and pollen data confirm a shallow lake with high rates of detrital input. The Late Glacial - Holocene transition to warmer and wetter conditions is marked by prominent changes in the mineralogy of lacustrine carbonates and the composition of pollen taxa. During this time the lake constantly rose and increasingly buffered the influence of allochthonous carbonates. The episode is characterized by a straight and steady rise in the 14C ages, hence the hard water effect can be assumed as constant. The stability lasted for about 5 ka, while Lake Heihai probably reached its highest stand of +6 m above recent lake level. The following period was dominated by colder and dryer conditions, causing a drop in lake level and the exposure of the formerly deposited lake sediments. Periglacial conditions caused freezing of the exposed lake sediments and the formation of segregated ice in the pores and between the strata layers. The increase in volume and permafrost heave caused further uplift of the sediments. The subsequent exposition to fluvial and littoral wave activity eroded these fossil lake sediments and thus contaminated younger sediments with older organic particles. Seasonal thawing of the frozen ground leached the sediments and mixed dissolved inorganic carbon with the lake water. Both, dissolved old and modern carbon was incorporated in plants through metabolism and led to thealteration of their 14C ages

Sedimentological discontinuties as chances for enhancing process-based palaeo-environmental reconstruction

Morpho-stratigraphic approaches in the field of palaeoenvironmental reconstruction are time consuming, have a low temporal resolution and the problem of equifinality in terms of processes or simply the reputation of fuzziness. Nevertheless, investigations of seemingly continuous archives such as lake sediments are more effective if climate driven catchment signals are considered. Lake level data sets provide valuable indices for modelling the palaeoclimatic history and feedback-mechanisms on regional, supra-regional or even hemispherical scales. However, they are frequently used without considering the role of sediment trapping, signal buffering and random events (tectonic, mass-movements, etc.) along related sediment cascades. Moreover, dating inversions, record gaps and unlikely high or low SARs are considered as archive-internal disturbances or simply measurement errors which have to be smoothed by increasingly sophisticated tools of statistics or simply eliminated. This poster examines the value of such discontinuities and random events within continuous lake record for deciphering catchment-wide feedbacks/responses and their related processes. Our first example shows the influence of lake level changes and permafrost uplift on the reservoir effect of Lake Heihai, Northern Tibetan Plateau. A drop in lake level induced the reworking of sediment sequences, which is not identifiable by disturbances of the stratigraphy. This problem only becomes apparent, if several on-shore sediment records and at least two lake cores are compared. Several lake records from Hala Lake, Qilian Mountains, northeastern Tibetan Plateau confirm highly diverse stratigraphies and sediment properties, thus underpinning the necessity of data comparison from different locations (within the lake) for a reliable reconstruction of climate-driven hydrological variations within a lake-catchment system. A third example from a large endorheic foreland basin of the Tibetan Plateau (Ejina Basin also known as Gaxun Nur B.) shows that only a large set of well dated sediment records with overlapping time frame lead to an understanding of the underlying sedimentation processes. The information of sedimentation variance derived from a cored sediment record is punctual. Only the spatial relation and geomorphological context provide insights into a larger set of interrelated processes and thus ensure spatially reliable reconstructions of climate-induced hydrography. Finally we show that a single record without considering the geomorphological process ensemble will provide less resolution or greater fuzziness than a geomorphological archive. Only the combination of more than one core from a “final” sink, an unbiased analyses of different proxy sets and at least a well dated morphostratigraphy may lead to a reliable process-based reconstruction of palaeo-environmental (-climate) variance

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

A common Holocene attracting state for the elemental composition of allochthonous lake sediments across the Tibetan Plateau

Sedimentary archives on the Tibetan Plateau are a reliable source of information on palaeoenvironmental change in central Asia. Reconstructing environmental changes offers the opportunity to gain information on the dynamics of major circulation systems of the Earth´s atmosphere which influence the climatic conditions in central Asia. However, supra-regional comparisons of proxy records inferred from sedimentary archives reveal individual responses of single archives to superimposed environmental change throughout the Holocene (Wischnewski et al., 2011). This observation gave rise to an ongoing debate whether the individual response of sedimentary archives results from high age uncertainties resulting from radiocarbon based dating techniques (earlier proposed by Mischke and Zhang, 2010) or from a non-linear response of sedimentary systems to superimposed environmental change (Wischnewski et al., 2011). Here we present new insights into the dynamics of allogenic sediment supply to lake systems under changing environmental conditions. To reconstruct sediment dynamics across the Tibetan Plateau, we analyzed the elemental composition via non-destructive X-Ray Fluorescence measurements of lacustrine sediment cores retrieved from three lake systems situated on the southern and northern Tibetan Plateau. By applying a factor analysis on the bulk elemental composition of lacustrine sediments we found a common factor as expressed by similar loadings of the elements Ti, Rb, Fe and Zr. Their link to heavy mineral compounds suggests an allogenic origin and hence a coupling of factor scores to sediment routing systems within each catchment. To test a non-linear response of individual sedimentary systems to environmental change we reconstructed phase spaces from time series data in form of factor scores via a time-lag embedding method. A phase space is an abstract space which maps the entire temporal succession of evolutionary states (called trajectory) in a multidimensional Euclidean space. This so-called phase space reconstruction offers the opportunity to identify and characterize the underlying dynamics by analyzing the geometry of trajectories within the reconstructed space. The results suggest that all lake systems share a similar long-term trend in the dynamics of detrital input. Thereby, lake trajectories tend to converge towards a common attracting state in the Late Holocene. After reaching the basin of attraction, all trajectories remain in a cyclic orbit around the attracting state with a frequency spectrum similar to major variations in solar irradiance and the North Atlantic overturning circulation. These results shed new light on the comparability of low dimensional proxy-data and the environmental signal processing of complex sedimentary systems