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Reconstructing climate from the Dead Sea sediment record using high-resolution micro-facies analyses
(2015)
The sedimentary record of the Dead Sea is a key archive for reconstructing climate in the eastern Mediterranean region, as it stores the environmental and tectonic history of the Levant for the entire Quaternary. Moreover, the lake is located at the boundary between Mediterranean sub-humid to semi-arid and Saharo-Arabian hyper-arid climates, so that even small shifts in atmospheric circulation are sensitively recorded in the sediments. This DFG-funded doctoral project was carried out within the ICDP Dead Sea Deep Drilling Project (DSDDP) that intended to gain the first long, continuous and high-resolution sediment core from the deep Dead Sea basin. The drilling campaign was performed in winter 2010-11 and more than 700 m of sediments were recovered. The main aim of this thesis was (1) to establish the lithostratigraphic framework for the ~455 m long sediment core from the deep Dead Sea basin and (2) to apply high-resolution micro-facies analyses for reconstructing and better understanding climate variability from the Dead Sea sediments.
Addressing the first aim, the sedimentary facies of the ~455 m long deep-basin core 5017-1 were described in great detail and characterised through continuous overview-XRF element scanning and magnetic susceptibility measurements. Three facies groups were classified: (1) the marl facies group, (2) the halite facies group and (3) a group involving different expressions of massive, graded and slumped deposits including coarse clastic detritus. Core 5017-1 encompasses a succession of four main lithological units. Based on first radiocarbon and U-Th ages and correlation of these units to on-shore stratigraphic sections, the record comprises the last ca 220 ka, i.e. the upper part of the Amora Formation (parts of or entire penultimate interglacial and glacial), the last interglacial Samra Fm. (~135-75 ka), the last glacial Lisan Fm. (~75-14 ka) and the Holocene Ze’elim Formation. A major advancement of this record is that, for the first time, also transitional intervals were recovered that are missing in the exposed formations and that can now be studied in great detail.
Micro-facies analyses involve a combination of high-resolution microscopic thin section analysis and µXRF element scanning supported by magnetic susceptibility measurements. This approach allows identifying and characterising micro-facies types, detecting event layers and reconstructing past climate variability with up to seasonal resolution, given that the analysed sediments are annually laminated. Within this thesis, micro-facies analyses, supported by further sedimentological and geochemical analyses (grain size, X-ray diffraction, total organic carbon and calcium carbonate contents) and palynology, were applied for two time intervals:
(1) The early last glacial period ~117-75 ka was investigated focusing on millennial-scale hydroclimatic variations and lake level changes recorded in the sediments. Thereby, distinguishing six different micro-facies types with distinct geochemical and sedimentological characteristics allowed estimating relative lake level and water balance changes of the lake. Comparison of the results to other records in the Mediterranean region suggests a close link of the hydroclimate in the Levant to North Atlantic and Mediterranean climates during the time of the build-up of Northern hemisphere ice sheets during the early last glacial period.
(2) A mostly annually laminated late Holocene section (~3700-1700 cal yr BP) was analysed in unprecedented detail through a multi-proxy, inter-site correlation approach of a shallow-water core (DSEn) and its deep-basin counterpart (5017-1). Within this study, a ca 1500 years comprising time series of erosion and dust deposition events was established and anchored to the absolute time-scale through 14C dating and age modelling. A particular focus of this study was the characterisation of two dry periods, from ~3500 to 3300 and from ~3000 to 2400 cal yr BP, respectively. Thereby, a major outcome was the coincidence of the latter dry period with a period of moist and cold climate in Europe related to a Grand Solar Minimum around 2800 cal yr BP and an increase in flood events despite overall dry conditions in the Dead Sea region during that time. These contrasting climate signatures in Europe and at the Dead Sea were likely linked through complex teleconnections of atmospheric circulation, causing a change in synoptic weather patterns in the eastern Mediterranean.
In summary, within this doctorate the lithostratigraphic framework of a unique long sediment core from the deep Dead Sea basin is established, which serves as a base for any further high-resolution investigations on this core. It is demonstrated in two case studies that micro-facies analyses are an invaluable tool to understand the depositional processes in the Dead Sea and to decipher past climate variability in the Levant on millennial to seasonal time-scales. Hence, this work adds important knowledge helping to establish the deep Dead Sea record as a key climate archive of supra-regional significance.
The non-linear behaviour of the atmospheric dynamics is not well understood and makes the evaluation and usage of regional climate models (RCMs) difficult. Due to these non-linearities, chaos and internal variability (IV) within the RCMs are induced, leading to a sensitivity of RCMs to their initial conditions (IC). The IV is the ability of RCMs to realise different solutions of simulations that differ in their IC, but have the same lower and lateral boundary conditions (LBC), hence can be defined as the across-member spread between the ensemble members.
For the investigation of the IV and the dynamical and diabatic contributions generating the IV four ensembles of RCM simulations are performed with the atmospheric regional model HIRHAM5. The integration area is the Arctic and each ensemble consists of 20 members. The ensembles cover the time period from July to September for the years 2006, 2007, 2009 and 2012. The ensemble members have the same LBC and differ in their IC only. The different IC are arranged by an initialisation time that shifts successively by six hours. Within each ensemble the first simulation starts on 1st July at 00 UTC and the last simulation starts on 5th July at 18 UTC and each simulation runs until 30th September. The analysed time period ranges from 6th July to 30th September, the time period that is covered by all ensemble members. The model runs without any nudging to allow a free development of each simulation to get the full internal variability within the HIRHAM5.
As a measure of the model generated IV, the across-member standard deviation and the across-member variance is used and the dynamical and diabatic processes influencing the IV are estimated by applying a diagnostic budget study for the IV tendency of the potential temperature developed by Nikiema and Laprise [2010] and Nikiema and Laprise [2011]. The diagnostic budget study is based on the first law of thermodynamics for potential temperature and the mass-continuity equation. The resulting budget equation reveals seven contributions to the potential temperature IV tendency.
As a first study, this work analyses the IV within the HIRHAM5. Therefore, atmospheric circulation parameters and the potential temperature for all four ensemble years are investigated. Similar to previous studies, the IV fluctuates strongly in time. Further, due to the fact that all ensemble members are forced with the same LBC, the IV depends on the vertical level within the troposphere, with high values in the lower troposphere and at 500 hPa and low values in the upper troposphere and at the surface. By the same reason, the spatial distribution shows low values of IV at the boundaries of the model domain.
The diagnostic budget study for the IV tendency of potential temperature reveals that the seven contributions fluctuate in time like the IV. However, the individual terms reach different absolute magnitudes. The budget study identifies the horizontal and vertical ‘baroclinic’ terms as the main contributors to the IV tendency, with the horizontal ‘baroclinic’ term producing and the vertical ‘baroclinic’ term reducing the IV. The other terms fluctuate around zero, because they are small in general or are balanced due to the domain average.
The comparison of the results obtained for the four different ensembles (summers 2006, 2007, 2009 and 2012) reveals that on average the findings for each ensemble are quite similar concerning the magnitude and the general pattern of IV and its contributions. However, near the surface a weaker IV is produced with decreasing sea ice extent. This is caused by a smaller impact of the horizontal 'baroclinic' term over some regions and by the changing diabatic processes, particularly a more intense reducing tendency of the IV due to condensative heating. However, it has to be emphasised that the behaviour of the IV and its dynamical and diabatic contributions are influenced mainly by complex atmospheric feedbacks and large-scale processes and not by the sea ice distribution.
Additionally, a comparison with a second RCM covering the Arctic and using the same LBCs and IC is performed. For both models very similar results concerning the IV and its dynamical and diabatic contributions are found. Hence, this investigation leads to the conclusion that the IV is a natural phenomenon and is independent from the applied RCM.
The main goal of this cumulative thesis is the derivation of surface emissivity data in the infrared from radiance measurements of Venus. Since these data are diagnostic of the chemical composition and grain size of the surface material, they can help to improve knowledge of the planet’s geology. Spectrally resolved images of nightside emissions in the range 1.0-5.1 μm were recently acquired by the InfraRed Mapping channel of the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M-IR) aboard ESA’s Venus EXpress (VEX). Surface and deep atmospheric thermal emissions in this spectral range are strongly obscured by the extremely opaque atmosphere, but three narrow spectral windows at 1.02, 1.10, and 1.18 μm allow the sounding of the surface. Additional windows between 1.3 and 2.6 μm provide information on atmospheric parameters that is required to interpret the surface signals. Quantitative data on surface and atmosphere can be retrieved from the measured spectra by comparing them to simulated spectra. A numerical radiative transfer model is used in this work to simulate the observable radiation as a function of atmospheric, surface, and instrumental parameters. It is a line-by-line model taking into account thermal emissions by surface and atmosphere as well as absorption and multiple scattering by gases and clouds. The VIRTIS-M-IR measurements are first preprocessed to obtain an optimal data basis for the subsequent steps. In this process, a detailed detector responsivity analysis enables the optimization of the data consistency. The measurement data have a relatively low spectral information content, and different parameter vectors can describe the same measured spectrum equally well. A usual method to regularize the retrieval of the wanted parameters from a measured spectrum is to take into account a priori mean values and standard deviations of the parameters to be retrieved. This decreases the probability to obtain unreasonable parameter values. The multi-spectrum retrieval algorithm MSR is developed to additionally consider physically realistic spatial and temporal a priori correlations between retrieval parameters describing different measurements. Neglecting geologic activity, MSR also allows the retrieval of an emissivity map as a parameter vector that is common to several spectrally resolved images that cover the same surface target. Even applying MSR, it is difficult to obtain reliable emissivity maps in absolute values. A detailed retrieval error analysis based on synthetic spectra reveals that this is mainly due to interferences from parameters that cannot be derived from the spectra themselves, but that have to be set to assumed values to enable the radiative transfer simulations. The MSR retrieval of emissivity maps relative to a fixed emissivity is shown to effectively avoid most emissivity retrieval errors. Relative emissivity maps at 1.02, 1.10, and 1.18 μm are finally derived from many VIRTIS-M-IR measurements that cover a surface target at Themis Regio. They are interpreted as spatial variations relative to an assumed emissivity mean of the target. It is verified that the maps are largely independent of the choice of many interfering parameters as well as the utilized measurement data set. These are the first Venus IR emissivity data maps based on a consistent application of a full radiative transfer simulation and a retrieval algorithm that respects a priori information. The maps are sufficiently reliable for future geologic interpretations.