| 1. |
- Bradák, Balázs, et al.
(författare)
-
Magnetic susceptibility in the European Loess Belt : New and existing models of magnetic enhancement in loess
- 2021
-
Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier. - 0031-0182 .- 1872-616X. ; 569
-
Tidskriftsartikel (refereegranskat)abstract
- Magnetic susceptibility measurements play a key role in Quaternary studies. Magnetic proxies, such as low field and frequency-dependent magnetic susceptibility, are widely applied in the reconstruction of terrestrial paleoclimate, e.g., in the study of loess-paleosol successions. In general, the interpretation of loess magnetic susceptibility signals is based on two commonly accepted models: the pedogenic magnetic enhancement and wind-vigour models. However, there are an increasing number of cases where such models cannot be used. These cases show unusual relationships between the two common loess magnetic susceptibility proxies: low field and frequency-dependent magnetic susceptibility. Such relationships have been attributed to various phenomena including the dissolution of fine-grain minerals and the formation of ultrafine magnetic rims on the surface of coarser grains by weathering. Despite the growing number of these exceptional cases of magnetic enhancement, our knowledge about the occurrence and potential causes of the unusual behaviour of magnetic susceptibility parameters is still limited. This, in turn, hinders the wider application of magnetic susceptibility parameters in loess. To fill this knowledge gap, magnetic susceptibility data of various profiles from the European Loess Belt were collected and compared to reveal various enhancement trends in loess. Along with the analysis of magnetic susceptibility parameters, combined scanning electron microscopy (SEM) and rock magnetic analyses were applied to samples from the Paks loess sequence in Hungary to describe some of the irregular cases, notably the cause of increasing frequency-dependent susceptibility in non-altered sediments. Analysis of loess, paleosol and common mineral samples separated from loess (e.g., muscovite) revealed that various features may be responsible for these increasing frequency-dependent susceptibility values: i) surface weathering (maghemitization) of coarser detrital grains, ii) nanofragmentation by physical weathering and iii) the appearance of significant amounts of ultrafine magnetic inclusions in micas. These special modes of magnetic enhancement of loess do not undermine the importance of the basic theories suggested above, but rather provide three mechanisms that account for some of the increasing number of unusual cases. To aid in the wider and more accurate use of magnetic susceptibility parameters in loess, we review the current magnetic enhancement models with special emphasis on the identification of unusual trends in magnetic enhancement and understanding their drivers.
|
|
| 2. |
- Bradak, Balazs, et al.
(författare)
-
Searching for potential multi-hazard events during the last 1.5 million years of the Pleistocene epoch
- 2022
-
Ingår i: Forum Geografi. - : Muhammadiyah University Press. - 0852-0682 .- 2460-3945. ; 36:1, s. 39-53
-
Tidskriftsartikel (refereegranskat)abstract
- Increasing attention has been paid to multi-hazards in environmental disaster studies produced during thelast decade. Multi-hazard studies focus on the occurrence, interaction and effect of several natural hazardsin the same region. Despite the increasing number of multi-hazard studies, few investigations have focusedon global-scale multi-hazard events. With the aim of closing this gap, our study focuses on the identificationof periods during the last 1.5 million years of the Pleistocene epoch, with the quasi-parallel appearance ofnatural hazards (e.g., asteroid impacts and large volcanic eruptions with a Volcanic Explosivity Index (VEI)of 8 and 7) amplifying their individual effects and thus causing long-term, global-scale changes. Of the sevenidentified potential multi-hazard events, three were considered as possible global-scale events with a longerterm environmental (paleoclimatic) impact; dated to c.a., 1.4 Ma (marine isotope stage – MIS45), 1.0 Ma(MIS 27), and 100 ka (MIS 5c), respectively. Two additional periods (around 50 and 20 ka) were identifiedas being associated with more restricted scale multi-hazard events, which might cause a “Little Ice Age-like”climatic episode in the history of the Pleistocene Period. In addition, we present a hypothesis about thecomplex climatic response to a global-scale multi-hazard event consisting of a series of asteroid impacts andvolcanic eruption linked to a geomagnetic polarity change, namely the Matuyama-Brunhes Boundary, whichmight be accompanied by global cooling and result in the final step of the Early Middle Pleistocene Transition.
|
|
| 3. |
- Költringer, Chiara, et al.
(författare)
-
Enviromagnetic study of Late Quaternary environmental evolution in Lower Volga loess sequences, Russia
- 2021
-
Ingår i: Quaternary Research. - : Cambridge University Press. - 0033-5894 .- 1096-0287. ; 103, s. 49-73
-
Tidskriftsartikel (refereegranskat)abstract
- The late Quaternary development of the Lower Volga region of Russia is characterized by an alternating influence of marine and continental environments resulting from fluctuations in Caspian Sea level during the last glaciation. However, sediments deposited under continental conditions have received very little research attention compared to the under- and overlying marine deposits, such that even their origin is still in debate. Detailed magnetic mineralogical analyses presented here show clear similarities to loess. The results suggest that climate during the time of loess deposition, the Atelian regression (27-80 ka, MIS 4-3), was dry and cool, similar to the modern-day Northern Caspian lowland. The magnetic properties recorded in the loess-paleosol sequences of the Lower Volga also point to short episodes of potentially more humid and warmer climate during the late Atelian. The new findings in regard to the local Caspian climate and environmental evolution support decreased river discharge from the Russian Plain and Siberian Plain as the dominant factor causing the low Caspian sea level stand during the Atelian, although local-regional climate changes might have had an additional influence.
|
|
| 4. |
- Költringer, Chiara, et al.
(författare)
-
Palaeoenvironmental implications from Lower Volga loess - Joint magnetic fabric and multi-proxy analyses
- 2021
-
Ingår i: Quaternary Science Reviews. - : Elsevier. - 0277-3791 .- 1873-457X. ; 267, s. 107057-
-
Tidskriftsartikel (refereegranskat)abstract
- Magnetic fabric (MF) investigations complemented by geochemical and grain surface analyses of the understudied and controversial marine isotope stage (MIS) 5 b, 4 and 3 loess deposits in the Lower Volga region, Russia show that the material has been transported and deposited by wind and to a large extent experienced post-depositional reworking. Grain surface features suggest that the material was glacially ground and fluvially transported prior to final aeolian deposition as loess. Secondary magnetic fabrics in the loess reveal pedogenic and cryogenic processes and a generally cold environment with brief shifts to warmer climate during late MIS 5 and MIS 3. Palaeowind reconstructions derived from preserved primary aeolian MF, indicate locally influenced westerly and north-westerly flow as part of a wider scale westerly wind pattern, similar to modern day winter conditions. We suggest that the climate of the last glacial in the Northern Caspian Lowland was cold and dry, with higher windspeeds and less variability during MIS 4 compared to MIS 3.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Stevens, Thomas, 1979-, et al.
(författare)
-
Abrupt last glacial dust fall over southeast England associated with dynamics of the British-Irish ice sheet
- 2020
-
Ingår i: Quaternary Science Reviews. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0277-3791 .- 1873-457X. ; 250
-
Tidskriftsartikel (refereegranskat)abstract
- Loess deposits in southern Britain contain a record of dust, climate and landscape dynamics over the last glacial, yet their age and accumulation rate remain poorly known. Furthermore, the environmental controls on the loess-soil stratigraphy shown in the thickest deposits in southeast England are still debated. Here we apply the first high sampling resolution quartz optically stimulated luminescence study of dust accumulation and loess formation in Britain at the Pegwell Bay site in east Kent. We couple this to mineral magnetic, particle size and geochemical analyses to understand climate, environment and post depositional modification of the loess. The luminescence ages and Bayesian age modelling results suggest two phases of greatly enhanced dust accumulation at the site. Loess began to accumulate around c. 25-23.5 ka, coinciding with Heinrich event 2, and after subsequent lower accumulation rates, a second enhanced phase of deposition occurred at around 20-19 ka. We propose a model where the dynamics of the British-Irish and Fennoscandian Ice Sheets, associated glacial lake drainage, and linked reorganisations of atmospheric circulation, all controlled loess accumulation in southern Britain. Accumulation in the first phase was triggered by increased sediment supply from initial retreat of the North Sea ice lobe, and drainage of Dogger Lake. Loess accumulation during this phase was enhanced by easterly winds from Atlantic depressions tracking to the south of Britain, caused by the maximum extent of the Irish Sea Ice Stream at c. 25-24 ka. The subsequent retreat of the western part of the British Irish Ice Sheet then allowed storm tracking further north, which reduced effectiveness of dust transporting winds across southern Britain, while sediment supply and availability was reduced in North Sea source areas. A second retreat of the maximum extent of the North Sea Lobe of ice after c. 21-20 ka would have led to another abrupt input of sediment-rich ice dammed lake and meltwater from eastern England and the North Sea into the exposed southern North Sea area. This would have again dramatically increased sediment availability for transport and deposition as loess in SE England, resulting in the second dust accumulation phase. We also propose that the abrupt stratigraphic change from calcareous to non-calcareous loess up section at Pegwell Bay was driven, not by these changes in dust input, but rather deepening of the permafrost active layer after c. 21 ka. This deepening was associated with warmer and wetter conditions driven by Atlantic storms tracking further north following the regression of the Irish Sea Ice Stream and overall ice sheet retreat. As such, last glacial dust dynamics and loess accumulation in Britain is highly influenced by the interaction of the British Irish Ice Sheet the Fennoscandian Ice sheet, Atlantic storm tracks, and the topography and drainage of the exposed North Sea.
|
|
