487 research outputs found
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Loess-Palaeosol Sequences in the Kashmir Valley, NW Himalayas: A Review
Loess deposits and intercalated palaeosols are widespread in the Quaternary record, and these have been extensively used to gain insights into continental paleoclimatic and paleo-environmental conditions and changes. Especially over Eurasia, loess geoarchives play an important role for our understanding of past changes. Loess covers almost 500 km2 of the Kashmir Valley in north-western India, it occurs dominantly in plateau positions, but also on terraces and sometimes forms slope deposits with thicknesses ranging from several to more than 20 m. For the time being, however, the timing of the initiation of the loess accumulation, the provenance, the grain size composition and also the paleo-environment have not been studied systematically and yet only little quantitative data is available. Yet it is clear that deposition rates are at least comparable to European loess, and that the presence of multiple palaeosols intercalated in the loess can provide valuable information on the history of the region. The limited available data hinders regional and continental correlation, and tapping its value as archive of past environmental changes in this sensitive region with influence from Westerlies and the Indian Monsoon. However, several characteristic palaeosol patterns can be traced throughout the Kashmir valley, which provide stratigraphic information. Several studies investigate physical and chemical properties of the loess-paleosol sequences and conclude to its aeolian origin and recording of past climates. The intensity of soil formation phases is traced through various proxies in low resolution and yet without conclusive age control. Here we review the exiting literature, available data, and interpretations from loess-palaeosol sequences in the Kashmir Valley. These are placed in the context of our own observations and loess from the Indian subcontinent. © Copyright © 2020 Dar and Zeeden
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An astronomical age-depth model and reconstruction of moisture availability in the sediments of Lake Chalco, central Mexico, using borehole logging data
Understanding the moisture history of low latitudes from the most recent glacial period of the latest Pleistocene to post-glacial warmth in continental tropical regions is hampered by the lack of continuous time series. We conducted downhole spectral gamma (Îł) ray and magnetic susceptibility logs over 300 m of lacustrine deposits of Lake Chalco (Mexico City) to reconstruct an age-depth model using an astronomical and correlative approach, and to reconstruct long-term moisture availability. Our results suggest that the Lake Chalco sediments contain several rhythmic alternations with a quasi-cyclic pattern comparable to the Pleistocene benthic stack. This allows us to calculate a time span of about 500,000 years for this sediment deposition. We developed proxies for moisture, detrital input, and salinity, all based on the physical properties of Îł-ray spectroscopy and magnetic susceptibility. Our results indicate that Lake Chalco formed during Marine Isotope Stage 13 (MIS13) and the lake level gradually increased over time until the interglacial MIS9. Moisture content is generally higher during interglacials than during glacials. However, two periods, namely MIS6 and MIS4, have higher moisture contents. We developed a model by comparing the obtained moisture proxy with climatic drivers, to understand how different climate systems drove effective moisture availability in the Chalco sub-basin over the past 500,000 years. Carbon dioxide, eccentricity, and precession are all key drivers of the moisture content of Lake Chalco over the past 500,000 years
A chronological and palaeoenvironmental reâevaluation of two loessâpalaeosol records in the northern Harz foreland, Germany, based on innovative modelling tools
The continuing development of analytical methods for investigating sedimentary records calls for iterative reâexamination of existing data sets obtained on loessâpalaeosol sequences (LPS) as archives of palaeoenvironmental change. Here, we reâinvestigate two LPS (Hecklingen, Zilly) in the northern Harz foreland, Germany, being of interest due to their proximity to the Scandinavian Ice Sheet (SIS) and the position between oceanic climatic influence further west and continental influence towards the east. First, we established new quartz OSL and polymineral IRSL chronologies. Both methods show concordant ages in the upper part of the Hecklingen profile (~20â40Â ka), but in the lower part IRSL underestimates OSL ages by up to ~15Â ka for the period 40â60Â ka. Interpretations hence refer to the OSL data set. Second, we applied Bayesian ageâdepth modelling to data sets from Hecklingen to resolve inversions in the original ages, also reducing averaged 1Ï uncertainty by ~19% (OSL) and ~12% (IRSL). Modelled chronologies point out phases of increased (MIS 2, early MIS 3) and reduced (middle and late MIS 3) sedimentation, but interpretation of numerical rates is problematic because of intense erosion and slope wash particularly during MIS 3. Finally, previously obtained grainâsize data were reâinvestigated by end member modelling analyses. Three fundamental grainâsize distributions (loadings) explain the measured data sets and offer information on intensity and â combined with modelled OSL ages â timing of geomorphic processes. We interpret the loadings to represent (i) primary loess accumulation, (ii) postdepositional pedogenesis and/or input of aeolian fine fractions, and (iii) input of coarse aeolian material and/or slope wash. The applied modelling tools facilitate detailed understanding of siteâformation through time, allowing us to correlate a strong peak in mean grain size at ~26â24Â ka to the maximum extent of the SIS and increased influence of easterly winds.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/50110000165
Investigating the loessâpalaeosol sequence of Bahlingen-Schönenberg (Kaiserstuhl), southwestern Germany, using a multi-methodological approach
Loessâpalaeosol sequences (LPSs) are key archives for the reconstruction of Quaternary environmental conditions, but there is a lack of investigated records from the southern Upper Rhine Graben (southwestern Germany). To close this gap, a LPS at Bahlingen-Schönenberg was investigated at high resolution using a multi-method approach. Infrared stimulated luminescence screening reveals a major hiatus in the lower part of the LPS that according to luminescence dating is older than marine isotope stage (MIS) 4. The section above the hiatus formed by quasi-continuous loess sedimentation between ca. 34 and 27âka, interrupted by phases of weak reductive pedogenesis. The fact that this pedogenesis is much weaker compared to corresponding horizons in the more northerly part of the Upper Rhine Graben could be due to regionally drier conditions caused by a different atmospheric circulation pattern at the time of deposition. Our results reinforce earlier notions that the major environmental shifts leading into the Last Glacial Maximum (LGM) of southern Central Europe significantly predate the transition of MIS 3 to 2 (ca. 29âka). In particular, the last massive phase of loess accumulation started several thousand years prior to the arrival of glaciers in the foreland of the Alps, which raises questions regarding the source and transport paths of the dust. It is also noted that no loess dating to the LGM or the time thereafter was observed due to either a lack of deposition or later erosion.Löss-PalĂ€oboden Sequenzen (LPS) sind SchlĂŒsselarchive fĂŒr die Rekonstruktion von quartĂ€ren Umweltbedingungen, aber es mangelt an der Untersuchung solcher Abfolgen aus dem sĂŒdlichen Oberrheingraben. Um diese LĂŒcke zu schlieĂen, wurde eine LPS bei Bahlingen-Schönenberg mit einem multimethodischen Ansatz hochauflösend untersucht. Die Untersuchung mit Infrarot Stimulierter Lumineszenz Screening zeigt einen Hiatus im unteren Teil der LPS, der laut Lumineszenzdatierungen Ă€lter ist als das Marine Isotopenstadium (MIS) 4. Der Abschnitt oberhalb des Hiatus bildete sich durch quasi-kontinuierliche Lössablagerung zwischen ca. 34 und 27âka, unterbrochen von Phasen schwacher reduktiver Pedogenese. Da die Pedogenese im Vergleich zu entsprechenden Horizonten im nördlicheren Teil des Oberrheingrabens viel schwĂ€cher ausgeprĂ€gt ist, könnte dies auf regional trockenere Bedingungen zurĂŒckzufĂŒhren sein, verursacht durch ein anderes atmosphĂ€risches Zirkulationsmuster zur Zeit der Ablagerung. Unsere Ergebnisse bestĂ€tigen frĂŒhere Annahmen, dass die groĂen UmweltverĂ€nderungen, die zum letzten glazialen Maximum (LGM) im sĂŒdlichen Mitteleuropa fĂŒhrten, deutlich vor dem Ăbergang von MIS 3 zu 2 (ca. 29âka) lagen. Insbesondere begann die letzte massive Phase der Lössakkumulation mehrere tausend Jahre vor der Ankunft der Gletscher im Alpenvorland, was Fragen zu den Quellen und Transportwegen des Staubs aufwirft. Es ist auch festzustellen, dass kein Löss aus dem LGM oder der Zeit danach gefunden wurde, entweder aufgrund fehlender Ablagerung oder spĂ€terer Erosion
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Rock Magnetic Cyclostratigraphy of Permian Loess in Eastern Equatorial Pangea (Salagou Formation, South-Central France)
We present the findings from analysis and modeling of a stratigraphic series of magnetic susceptibility (MS) data measured with a portable MS meter from the Permian Salagou Formation loessite (south-central France). The results reveal discernible Milankovitch-scale paleoclimatic variability throughout the Salagou Formation, recording astronomically forced climate change in deep-time loessite of eastern equatorial Pangea. Optimal sedimentation rates are estimated to have ranged between 9.4 cm/kyr (lower Salagou Formation) and 13 cm/kyr (mid-upper Salagou Formation). A persistent 10-m-thick cyclicity is present that likely represents orbital eccentricity-scale (âŒ100 kyr) variability through the middle to late Cisuralian (ca. 285â275 Ma). Subordinate, higher frequency cycles with thicknesses of âŒ3.3â3.5 and âŒ1.8 m appear to represent obliquity and precession-scale variability. If the driver of magnetic enhancement is pedogenic, then the âŒ10 m thick cyclicity that is consistent over âŒ1000 m of section may represent the thickness of loessiteâpaleosol couplets in the Salagou Formation. Laboratory rock magnetic data show generally low magnetic enhancement compared to analogous Eurasian Quaternary loess deposits. This is related to the predominance of hematite (substantially weaker signal than magnetite or maghemite) in the Salagou Formation which may be explained by different conditions of formation (e.g., syn depositional processes, more arid, and/or oxidizing climate conditions) than in present Eurasia and/or post depositional oxidation of magnetite and maghemite. © Copyright © 2020 Pfeifer, Hinnov, Zeeden, Rolf, Laag and Soreghan
Challenges of loess formation models for the Carpathian Basin
The general pattern of loess-paleosol sequences for the last several glacial-interglacial cycles in South-eastern Europe is becoming more established through an increasing amount of available data. However, the paleoclimatic mechanisms leading to these patterns are much less understood, especially the deviations from northern hemisphere patterns. Here we use relatively simple models to determine the effect of insolation forcing and global climate, as represented by benthic d18O data, onto loess-paleosol sequence
A postâ IR IRSL chronology and dust mass accumulation rates of the Nosak loessâpalaeosol sequence in northeastern Serbia
A post-IR IRSL chronology and dustmass accumulation rates of the Nosak loess-palaeosol sequence in northeastern Serbia
In the Middle Danube Basin, Quaternary deposits are widely distributed in the Vojvodina region where they cover
about 95% of the area.Major research during the last two decades has been focused on loess deposits in the Vojvodina
region. During this period, loess in the Vojvodina region has become one of the most important Pleistocene European
continental climatic and environmental records.Herewe present the dating results of 15 samples taken from theNosak
loess-palaeosol sequence in northeastern Serbia in order to establish a chronology over the last three glacialâ
interglacial cycles. We use the pIRIR290 signal of the 4â11 lm polymineral grains. The calculated ages are within the
error limits partially consistent with the proposed multi-millennial chronostratigraphy for Serbian loess. The average
mass accumulation rate for the last three glacialâinterglacial cycles is 265 g mïżœ2 aïżœ1
, which is in agreement with the
values of most sites in the Carpathian Basin. Our results indicate a highly variable deposition rate of loess, especially
during the MIS 3 and MIS 6 stages, which is contrary to most studies conducted in Serbia where linear sedimentation
rates were assume
Three climatic cycles recorded in a loess-palaeosol sequence at Semlac (Romania) â implications for dust accumulation in the Carpathian Basin and the northern Hemisphere
Semlac is regarded as a key section for the Carpathian Basin because of the good preservation of the fine silt. The site is situated at the MureĆ River in its lower reaches (Banat region, western Romanian). The more than 10 m thick loess sequence includes four fossil sol-complexes developed in homogenous relatively fine silty loess and dates back to marine isotope stage (MIS) 10. This setting offers possibilities to a) improve the understanding of the type and composition of the lowland loess sequences in the Carpathian Basin for the last interglacial palaeosol complex and beyond, b) reconstruct the temporal evolution of the local loess-palaeosol successions and c) compare the loess of the region to loess-sequences in adjacent areas (Carpathian Basin, Lower Danube) and to dust proxy data in the northern hemisphere. A strikingly sinusoidal course of physical property data in depth and time point to relatively homogenous, quasi-continuous background sedimentation of dust, which are interpreted as resulting from long-range transport. This is in contrast to a commonly observed more glacial-interglacial pattern with sharp boundaries of paleosol
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