Authors:	Dave, Aditi Krishna
Title:	Understanding Quaternary aeolian landscape-climate interaction in the piedmonts of Central Asia using luminescence and electron spin resonance techniques
Online publication date:	8-Mar-2022
Year of first publication:	2022
Language:	english
Abstract:	Deposits of wind-blown dust, or loess, are excellent terrestrial archives of past climate change. These deposits form stacked sequences of primary loess and buried soils – commonly known as loess-palaeosol sequences - that are largely hypothesised to reflect colder, drier, more windy phases, and more humid, warmer, less windy periods, respectively. The extensive loess deposits that drape the piedmonts of the Asian high mountains in Arid Central Asia (ACA) are one of the major loess deposits in the world. They lie at the core of the Eurasian loess belt and, thus provide a key link between European and East Asian terrestrial paleoenvironmental records. The ACA loess deposits are influenced by, and therefore record the imprint of, two major drivers of the Northern hemisphere climate system: the mid-latitude Westerlies and the Siberian High. The location of the ACA loess thus makes these deposits invaluable for understanding the relationship between loess accumulation and climate. Yet, relatively little is known about the role of ACA in global climate dynamics past and present. Emerging datasets from Central Asian loess records suggest that (i) the prevailing assumptions linking increased loess accumulation with cold glacial conditions do not entirely hold for this region, (ii) the ‘rate of accumulation’ and the ‘timing of accumulation peaks’ in loess vary considerably between sites, (iii) we need to reassess the role of deserts as dust sources, as posited by early models for loess formation, and that (iv) we need to better understand the link between past wind regimes and dust transport pathways in the region. This thesis aims to address these issues in two ways, by reconstructing the timing and provenance of loess deposits across ACA. In order to do so, this thesis explores different applications of trapped charge techniques of Luminescence and Electron Spin Resonance (ESR), namely as classical dating methods and as a provenance tool. These techniques are applied to two of the most ubiquitous mineral types found within loess, quartz and feldspar. Firstly, this work addresses the question of how loess accumulation responds to climate in ACA. Towards this goal, luminescence dating of both quartz and feldspar is used to generate a high-resolution chronological framework for five new loess sites located along a c. 200 km transect of the understudied piedmonts in the Ili Basin of southeast Kazakhstan. Bayesian age modelling of the age-depth profiles is performed in order to optimise the precision of the datasets and to facilitate the calculation of mass accumulation rates (MARs) through time. The new chronology shows that loess accumulation along the piedmont spans the mid-Holocene to beyond the last interglacial, and reveals spatio-temporal inhomogeneity in loess accumulation. To understand the nature and possible drivers of this non-uniformity in sedimentation patterns, the spatial coverage of this investigation was extended by calculating MARs from published luminescence ages from 30 additional sites across the Ili basin, from neighbouring basins in ACA and from across the Chinese loess plateau. This synthesis suggests that loess deposits represent a complex response to the interaction of local wind regimes, topography, sediment supply and availability, indicating that interpretations of loess packages as climate archives made hitherto are oversimplified and need to be reassessed. This thesis proposes that the response of loess MAR’s can be viewed as a composite of two parameters: ‘net accumulation’ and ‘timing of peaks in accumulation’. The net sedimentation rate at a given site responds both to local topographic context and sediment availability, and to climate. By contrast, the ‘timing of peaks in accumulation’ largely represents a response to large scale climate dynamics in a region only if derived from an aggregate dataset from multiple sites. An aggregate of MARs from multiple sites removes site-specific bias and provides a more robust tool for understanding past climate dynamics across a region. Secondly, this thesis deals with the question of loess provenance. Identifying the source rock or location of loess-forming sediments, as well as changes in source through time, can provide insight into dust transport pathways, and consequently facilitate reconstruction of past atmospheric circulation in the region. Ultimately this also helps us to understand the processes of loess genesis, transport, and deposition. This thesis provides the first crucial step towards this goal by developing a new proxy for identifying the provenance of quartz. This new technique utilises the intensities of two paramagnetic centres in quartz, the natural E’ and peroxy centres, that are measured using electron spin resonance (ESR). It is based on the premise that these centres arise from Schottky-Frenkel defect pairs, which increase with the age of the quartz-bearing host rock. Following experimental observations that confirm the physical basis of the method, this new approach was applied to a suite of 114 fine-grained loessic quartz samples from two sedimentary basins in Kazakhstan and Tajikistan respectively. Loess within the two basins are known to have different dust source regions and derive from source rocks of very different ages. The results yield a positive correlation between natural E’ and peroxy intensities and demonstrate that quartz derived from older rocks (Kazakhstan) yields stronger signals than those sourced from younger rocks (Tajikistan), thus reaffirming the basis of the new method. This new provenance tool was then applied to successfully identify source changes down a long loess-palaeosol sequence in Tajikistan. The natural E’-peroxy centre provenance approach represents a simplified measurement protocol in comparison to other existing ESR-based techniques, is highly applicable to aeolian records, and holds great potential for application to other sedimentary systems. The application of this new technique was extended further by combining it with an investigation of optically stimulated luminescence (OSL) and thermoluminescence (TL) characteristics of quartz. The intensity of E’ defect centre in quartz represents a single defect, while luminescence sensitivity represents an aggregate of charge traps influenced both by the thermal history of the quartz-bearing source rocks and the sedimentary history of the grain. Variation in OSL and TL sensitivity of quartz down long loess-palaeosol sequences have been linked to climate-driven shifts in provenance. Nevertheless, the processes responsible for the observed variations in luminescence sensitivity of quartz, including its link to the original rock source and/or its sedimentary history, remains a topic of contention. This thesis investigates both paired E’-peroxy intensities and luminescence sensitivity characteristics of quartz in parallel down a long loess sequence from southern Tajikistan, to gain a more nuanced insight into how these different trapped-charge indicators reflect source change through time. OSL and TL sensitivity of quartz within buried soil units (palaeosol) show a distinct increase compared to primary loess units. Using natural E’ and peroxy intensities from quartz as an indicator of provenance, the observed increase in OSL and TL sensitivity of loessic quartz is attributed largely to repeated cycles of natural irradiation and bleaching, facilitated by low deposition rates during pedogenesis. The applications and method development explored in this thesis have important implications not only for the study of Central Asian loess deposits, but also more generally for reconstructing past climatic changes from terrestrial sediment archives. The comprehensive assessment of the response of loess MAR’s to climate, in context of geomorphic and topographic constraints, provides a more realistic framework for the interpretation of loess sites as land-based climate records. The development of a new proxy for the provenance of quartz using ESR opens exciting new avenues for application of this technique to other sedimentary systems.
DDC:	300 Sozialwissenschaften 300 Social sciences 500 Naturwissenschaften 500 Natural sciences and mathematics 550 Geowissenschaften 550 Earth sciences 900 Geschichte 900 Geography and history
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-6387
URN:	urn:nbn:de:hebis:77-openscience-6c939c0d-2d9c-42ea-9d35-339aa183405b8
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	http://rightsstatements.org/vocab/InC/1.0/
Extent:	a-g, vi, 211 Seiten, Illustrationen, Diagramme, Karten
Appears in collections:	JGU-Publikationen