Search

The Holocene paleoclimate of the Caucasus region is rather complex and not yet well understood: while existing studies are mainly based on pollen records from high-altitude and humid lowland regions, no records are available from the semi-humid to semi-arid south-eastern Caucasian lowlands. Therefore, this study investigated compound-specific δ2H and δ13C isotopes of leaf wax biomarkers from Holocene floodplain soils in eastern Georgia. Our results show that the leaf wax δ2H signal from the paleosols mostly reflects changes in the moisture source and its isotopic composition. Depleted δ2H values before ~8?cal ka bp change towards enriched values after ~5?cal ka bp and become again depleted after ~1.6?cal ka bp. This trend could be caused by Holocene changes of the isotopic compositions of the Black and eastern Mediterranean Sea, and/or by varying contribution of both moisture sources linked with the North Atlantic Oscillation. The leaf wax δ13C signal from the paleosols directly indicates varying local water availability and drought stress. Depleted δ13C values before ~8 and after ~5?cal ka bp indicate wetter local conditions with higher water availability, whereas more enriched values during the middle Holocene (~8 until at least 5?cal ka bp) indicate drier conditions with increased drought stress. Introduction Materials and methods - Study area and climate setting - Studied section - Analytical procedure Results Discussion - The δ2H signal - The δ13C signal Conclusions

The Western Mediterranean region including the North African desert margin is considered one of the most sensitive areas to future climate changes. In order to refine long-term scenarios for hydrological and environmental responses to future climate changes in this region, it is important to improve our knowledge about past environmental responses to climatic variability at centennial to millennial timescales. During the last two decades, the recovery and compilation of Holocene records from the subtropical North Atlantic and the Mediterranean Sea have improved our knowledge about millennial-scale variability of the Western Mediterranean palaeoclimate. The variabilities appear to affect regional precipitation patterns and environmental systems in the Western Mediterranean, but the timescales, magnitudes and forcing mechanisms remain poorly known. To compare the changes in Holocene climate variability and geomorphological processes across temporal scales, we analysed a 19.63-m long sediment record from Lake Sidi Ali (33°03’ N, 5°00’ W, 2080 m a.s.l.) in the sub-humid Middle Atlas that spans the last 12,000 years (23 pollen-based radiocarbon dates accompanied with 210Pb results). We use calibrated XRF core scanning records with an annual to sub-decadal resolution to disentangle the complex interplay between climate changes and environmental dynamics during the Holocene. Data exploration techniques and time series analysis (Redfit, Wavelet) revealed long-term changes in lake behaviour. Three main proxy groups were identified (temperature proxies: 2ky, 1ky and 0.7ky cycles; sediment dynamic proxies: 3.5ky, 1.5ky cycles; hydrological proxies: 1.5ky, 1.2ky, 0.17ky cycles). For example, redox sensitive elements Fe and Mn show 1ky cycles and higher values in the Early Holocene and 1.5ky cycles and lower values in the Mid- to Late Holocene. All groups show specific periodicities throughout the Holocene, demonstrating their particular climatic and geomorphological dependencies. Furthermore, we discuss these periodicities relating to global and hemispheric drivers, such as the North Atlantic Oscillation (NAO), El-Niño Southern Oscillation (ENSO), Innertropical Convergence Zone variability (ITCZ) and North Atlantic cold relapses (Bond events).

There is an ongoing debate about the main drivers of the Holocene landscape dynamics in Central Europe, i.e. when and where these were dominantly controlled by climatic (e.g. rapid climate changes) or anthropogenic factors. Investigations of colluvial and fluvial deposits are ideal to contribute to this discussion, given that the formation of colluvial deposits in Central Europe is often controlled by human activity, whereas the formation of alluvial deposits might be more influenced by climatic fluctuations.In the forefront of a road construction intercalated colluvial and fluvial sediments with thicknesses up to 4 m were outcropped and studied during archaeological excavations led by the Bavarian State Office for Monument Conservation near Dettenheim in central Bavaria (southern Germany) in 2015. This investigated site is located next to the Fossa Carolina (Karlsgraben), a canal that was built at the end of the 8th century AD to bridge the watershed between Rhine and Danube but that was never finished. We investigated the sediments using sedimentological, archaeological and geochronological (OSL) methods. After a period with fluvial activity during the Early Holocene, the landscape was mostly stable between ca. 10 and 2.5 ka, allowing the formation of soils. Although human activities next to the investigated sites originate from the Hallstatt period, the first interruption of the stable period only occurred during the La Tène Period when fluvial and colluvial deposits were formed. Subsequently, intensive colluviation occurred during the Migration Period/Early Middle Ages, followed by intensive colluvial and fluvial deposition that started since the High Middle Ages and continues until today. Our investigations indicate that human disturbance obviously led to an intensive landscape degradation during the La Tène Period, and the landscape was subsequently much more sensitive towards human and climatic fluctuations than before.

Hydro-sedimentary processes such as soil erosion, sediment transport, deposition, and re-deposition influence the environmental evolution of floodplains, especially in loess-covered catchments. Holocene floodplain deposits are thus a source of information on previous hydro-sedimentary dynamics and land use in the catchment. Resulting from forest clearings in the catchment, the onset of overbank silt-clay deposition is considered as an initial and significant human-induced process affecting Central European floodplain evolution and ecosystems. However, it is difficult to separate climate-related from anthropogenic forces on depositional environments, and the complexity of the hydro-sedimentary responses is part of an ongoing debate in geoscientific, ecological, and archaeological communities. This study focuses on the Central European Weise Elster river system, where humans have been influencing hydro-sedimentary processes since the Early Neolithic due to land-use-induced soil erosion predominantly in the loess-covered sub-basin of the middle course. A catchment-scale XRF element record of fluvial sediment sources combined with the geochemical characterisation of Holocene floodplain deposits aim for a better understanding of the interplay between past soil erosion, overbank deposition in the floodplain, and potential changes in sediment provenances. The Weise Elster floodplain chronosequences show a geochemical differentiation into a lower (Neolithic) and an upper (post-Neolithic) overbank silt-clay deposition. The construction of a sediment source fingerprinting mixing model yields the significant finding that the Neolithic overbank silt-clay deposition reveals a remote provenance signal from the upper catchment and less from the proximal loess-covered sub-catchment. According to a systematic archaeological data survey, the upper catchment was not permanently settled and used for agriculture in the Neolithic period. This contradicts the previous assumption that Neolithic overbank silt-clay deposition primarily originates from forest clearings and subsequent farming-induced soil erosion in the catchment. From a more general perspective, further examination of existing hypotheses concerning overbank silt-clay deposition in Central European floodplains is thus in order.

Leaf wax-derived long-chain n-alkanes are increasingly used for paleovegetation reconstructions, but have hardly been applied to fluvial sediments so far. However, such analyses could significantly enhance knowledge of the former vegetation in many regions devoid of non-fluvial sediment archives. Therefore, this exemplary study investigates potentials and challenges of n-alkane biomarker analyses in a fluvial sediment-paleosol sequence (FSPS) along the upper Alazani River in the central southern Greater Caucasus. Generally, FSPS contain two different biomarker-signals: a catchment signal in fluvial sediments, and a local in-situ signal in intercalated paleosols. This allows paleovegetation reconstructions for both catchment area and sampling site. However, several archive-related challenges exist. Concerning the catchment signal from fluvial sediments, (i) this could be biased by different vegetation zones/land use histories in subcatchments, (ii) long-term pedogenesis on the slopes or intermediate sediment storage in the catchment could have caused a time lag between biomarker formation and sedimentation, (iii) taphonomic processes could have biased the preserved n-alkane pattern, (iv) there could be petrogenically inherited or in-river produced n-alkanes, (v) the signal could be overprinted by post-sedimentary processes. Concerning the local paleosol signal, this could be biased by inherited catchment-derived biomarkers or by taphonomic processes. These factors are thoroughly discussed for the investigated FSPS. With respect to paleovegetation patterns, our investigations gave the following results: (i) Leaf waxes in fluvial sediments deposited prior to/around the Pleistocene/Holocene transition and between ca. 5 and 1.7 cal. ka BP are indicative of more deciduous trees/shrubs in the catchment compared with the other periods that were dominated by grasses/herbs. Considering a possible time lag of ca. 1–4 ka between biomarker formation and deposition, this indicates delayed Holocene reforestation in the catchment compared with western and central Europe, in agreement with regional pollen studies. The subsequent decline of deciduous trees/shrubs could be linked to human activity in the catchment during the late Holocene. (ii) Leaf waxes in floodplain paleosols indicate local grass/herb dominance throughout the Holocene, which is possibly caused by long-lasting local human activity.

Mountain regions such as the Lesser Caucasus are a focus of ongoing environmental changes. To understand their future evolution, information about their former geomorphic and environmental dynamics is required. The former fluvial dynamics derived from fluvial sediment archives can offer such insights. However, the fluvial dynamics of the Lesser Caucasus since the Eemian interglacial have not been systematically investigated so far. Thus, we have studied late Pleistocene and Holocene sediments of several rivers originating from the central Lesser Caucasus. The studied rivers show a mostly coherent record of fluvial dynamics: Minor aggradation occurred during early Marine Isotope Stage (MIS) 5, incision during late MIS 5 or early MIS 4, intensive silty aggradation at least during late MIS 3, incision during early MIS 2, coarse-grained aggradation probably during some millenia until ca. 19 ka, and aggradation ca. 14-13 ka. Following incision around the Pleistocene/Holocene transition, aggradation resumed around 6.0, 3.4 – 2.0 and 0.5 – 0.15 cal. ka BP. Generally, periods of aggradation, incision and stability could be linked with regional climatic or anthropogenic influences on regional landscape stability and water availability. The fluvial dynamics of the central Lesser Caucasus mostly differed even between neighbouring regions, and only in cases of significant hemispheric climatic fluctuations as around 20 ka, during the Pleistocene/Holocene transition or the Little Ice Age, were similar over-regional fluvial patterns observed. This demonstrates the individual character of river systems especially in mountain regions such as the southern Caucasus with strong geoecological gradients. Thus, to understand the former landscape dynamics of mountain landscapes, investigations of fluvial sediment archives on a regional to sub-regional scale are necessary.

The mid-Holocene climate of Northwest Arabia is characterised by a significant increase in aridity which gave rise to changes in water management strategies including sophisticated techniques at later stages. The Rasif site, situated in Northwest Saudi Arabia, reveals a Late Neolithic society with multi-roomed domestic structures (1st phase, 6th millennium BCE, before the current era). At Rasif site the sediments of an endorheic basin (qa) show a minimum in salinity during this 1st phase. The 2nd phase is characterised by a pastoral ‘Chalcolithic’ watering and ‘Chalcolithic’ burial location of the 5th millennium BCE with wells, complex trough systems, and initial, probably coexistent grave structures. During this 2nd phase the qa deposits show already a slight increase in salinity. We have evidence for a sub-surface water-rich sand layer within the qa that was exploited by shallow wells. During the subsequent 3rd phase the number of pastoral ‘Chalcolithic’ D-shaped grave structures within the qa increased, probably co-existing with deepened wells and complex trough systems. At that time the qa is covered by an almost impermeable saline clay layer. The 4th phase is characterised by a culturally yet to identify pastoral post-‘Chalcolithic’ watering location with single small troughs. The final 5th phase represents a culturally unidentified pastoral post-’Chalcolithic’ to sub-recent water harvesting location with complex dam systems, which were probably modified numerous times. The dam systems allowed to flooding the qas for several months, providing (sub-) surface water in the nowadays Northwest Arabian desert.

In order to obtain information about landscape activity and stability during the Late Quaternary in the Transcaucasian region, fluvial sediments of the lower Algeti River in SE-Georgia were studied by means of geomorphologic, sedimentologic and geochronologic methods. These investigations show that sediments aggraded by the Late Pleistocene braided Algeti River were strongly incised around the Pleistocene/Holocene transition, and subsequently a small-scale pattern of diachronous fluvial terraces deposited by the now meandering river developed in the new valley during the Holocene. Prior to ca. 3 cal. ka BP, active phases of river channel aggradation and overbank sedimentation about 6 cal. ka BP and 3.4–3.2 cal. ka BP can be linked with climate-driven vegetation changes, i.e. stronger fluvial dynamics with sedimentation occurred during more arid and colder periods whereas the Caucasian Holocene climate optimum between 6 and 3.4 cal. ka BP was characterized by stable geomorphologic conditions. After 3 cal. ka BP, strongly increased human impact documented by former pollen studies is reflected by the fluvial dynamics as well, i.e. an activity phase of strong overbank sedimentation 2.7–2.1 cal. ka BP occurred during a period when forests in the Lesser Caucasus were burnt down. Two subsequent activity phases with fluvial sedimentation can be linked with historic events that probably both influenced vegetation distribution of the region, i.e. a period of flourishing of Georgia during the Middle Age and a time of large-scale destruction of the country due to a Persian invasion at the end of the 18th century AD, respectively. Thus, whereas no influence of tectonic events or fluctuations of the Caspian Sea level on the fluvial dynamics could be detected, SE-Georgia shows its fragile character with respect to climate- and human driven vegetation-changes similar to other semi-arid landscapes of the Mediterranean region.

In Zauschwitz (Western Saxonian loess area, Central Germany), a ca. 7 m thick loesspalaeosol sequence underlain by fluvial gravels and sands was investigated in order to study regional palaeoenvironmental changes during the late Weichselian Pleniglacial. The lithostratigraphic classification of the loess-palaeosol sequence was combined with polymineral fine grain luminescence dating using the pIRIR290 approach, and correlated with similar loess-palaeosol-sequences from Central Saxony. Doing so, we obtained information about a climatic shift from more humid to more arid conditions during the late Pleniglacial, due to changes in the landscape dynamics of the study area: At ca. 30 ka, braided river floodplain accumulation of the nearby Weisse Elster river was followed by a phase of decreased fluvial activity, allowing initial loess deposition on top of the fluvial sands and gravels. This period was characterized by cold but still quite humid conditions, as indicated by reworked loess and the occurrence of several tundra gley soils. Subsequently, a cold and more arid period of dust accumulation followed after ca. 22 ka. Intensive anthropogenic activity almost totally redeposited the Holocene black soil, demonstrating the attractiveness of the fertile loess area for early human settlement.

The significance of Saharan dust deposits for ecosystems has been demonstrated in several studies, showing its influence on pedogenesis and soil chemical and physical parameters by the addition of nutrient elements. This contribution is especially important for volcanic soils poor in plant-available phosphorous as those of the island of La Palma (Canary Islands, Spain). Thus, this study investigates the spatial distribution of Saharan dust on the island, and determines its influence on soil properties and thus on the ecosystem of the island. Since silt-sized quartz is not autochthonously formed on the island, it served as a reliable indicator for Saharan dust. Quartz was identified and quantified using X-ray diffractometry. The influence of Saharan dust additions on soil properties was assessed by texture analyses and analyses of plant-available phosphorous, an element strongly limiting plant-growth on the island due to its fixation in the structure of amorphous volcanic clay. A luff–lee-effect revealed significant influence on dust distribution, whereas small geomorphologic structures as hollows and mounds hardly influenced the dust distribution pattern. A positive correlation of quartz content with the proportion of the grain size fraction 0.5–10 μm demonstrates an influence on soil physical properties, apparently influencing available water capacity. Furthermore, our data suggest an increasing content of plant-available phosphorous with a rising amount of Saharan dust in the soils. Thus, the input of Saharan dust influences soil fertility of La Palma and is an important factor regarding the fragility of the La Palma ecosystems.

The eastern part of the Republic of Georgia encompasses an intramontane basin situated between the Greater Caucasus in the north and the Lower Caucasus in the south, both showing a high tectonic activity. The climate is continental, separated from the Mediterranean climate by the Surami Mountain Range with a maximal altitude of about 1000 m. Due to its proximity to the “Fertile Crescent”, Neolithic activity started quite early here, beginning at the latest from 7.5 ka. Fluvial sediments outcropped at several terrace levels along different rivers (Kura, Algeti, Khrami, Shulaverichai) in the so-called Marneuli-depression south of Tbilisi are investigated in order to reveal the pattern of fluvial activity during the Late Pleistocene and the Holocene. Sedimentological and geochronological investigations of these sediments are complemented by geomorphic mapping of the distribution of fluvial terrace levels. A comparison of the fluvial dynamics of the rivers in the Marneuli depression with that from other rivers in eastern Georgia to be investigated (e.g. the Alazani river) aims to trace the pattern of fluvial activity in the larger region, in order to unravel climatic, anthropogenic and tectonic influences during the Late Quaternary.