Approaches and challenges to the study of loess—Introduction to the LoessFest Special Issue

In September 2016, the annual meeting of the International Union for Quaternary Research's Loess and Pedostratigraphy Focus Group, traditionally referred to as a LoessFest, met in Eau Claire, Wisconsin, USA. The 2016 LoessFest focused on thin loess deposits and loess transportation surfaces. This LoessFest included 75 registered participants from 10 countries. Almost half of the participants were from outside the United States, and 18 of the participants were students. This review is the introduction to the special issue for Quaternary Research that originated from presentations and discussions at the 2016 LoessFest. This introduction highlights current understanding and ongoing work on loess in various regions of the world and provides brief summaries of some of the current approaches/strategies used to study loess deposits

Eolian dust dispersal patterns since the last glacial period in eastern Central Asia: insights from a loess-paleosol sequence in the Ili Basin

The extensive loess deposits of the Eurasian midlatitudes provide important terrestrial archives of Quaternary climatic change. As yet, however, loess records in Central Asia are poorly understood. Here we investigate the grain size and magnetic characteristics of loess from the Nilka (NLK) section in the Ili Basin of eastern Central Asia. Weak pedogenesis suggested by frequency-dependent magnetic susceptibility (chi fd%) and magnetic susceptibility (MS) peaks in primary loess suggest that MS is more strongly influenced by allogenetic magnetic minerals than pedogenesis, and may therefore be used to indicate wind strength. This is supported by the close correlation between variations in MS and proportions of the sand-sized fraction. To further explore the temporal variability in dust transport patterns, we identified three grain size end-members (EM1, mode size 47.5 mu m; EM2, 33.6 mu m; EM3, 18.9 mu m) which represent distinct aerodynamic environments. EM1 and EM2 are inferred to represent grain size fractions transported from proximal sources in short-term, near-surface suspension during dust outbreaks. EM3 appears to represent a continuous background dust fraction under non-dust storm conditions. Of the three end-members, EM1 is most likely the most sensitive recorder of wind strength. We compare our EM1 proportions with mean grain size from the Jingyuan section in the Chinese loess plateau, and assess these in the context of modern and Holocene climate data. Our research suggests that the Siberian High pressure system is the dominant influence on wind dynamics, resulting in loess deposition in the eastern Ili Basin. Six millennial-scale cooling (Heinrich) events can be identified in the NLK loess records. Our grain size data support the hypothesis that the Siberian High acts as teleconnection between the climatic systems of the North Atlantic and East Asia in the high northern latitudes, but not for the mid-latitude westerlies

Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes

Loess is a widespread, wind-transported, silt-dominated deposit that contains geologic archives of atmospheric circulation and paleoclimate on continents. Loess may cover as much as 10% of the Earth’s land surface. It is composed mainly of quartz, feldspars, and clay minerals, with varying amounts of carbonate minerals. The geochemistry of loess differs from region to region, depending on source materials, but all loess is very high in SiO2 with lesser amounts of other major elements. Trends in loess downwind from source areas include systematic decreases in thickness and amounts of sand and coarse silt, and increases in amounts of fine silt and clay. Loess particle size also varies at a given locality over time within individual depositional packages. This variability may be a function of changing wind strengths, different source sediments, or some combination of the two. The classical concept of loess is that it is a product of glacial grinding, with subsequent entrainment by wind from outwash deposits. However, it is now known that other processes contribute to silt particle formation, including frost shattering, salt weathering, fluvial and colluvial comminution, eolian abrasion, and ballistic impact. Much debate has taken place over the concept of “desert” (nonglaciogenic) loess, which is widespread in some regions but of limited distribution elsewhere. Nevertheless, glacial silt production probably exceeds the amount of silt generated by all other processes. Much of the loess in or adjacent to deserts may be inherited silt-sized particles from siltstone, mudstone, shale, and volcanic ash. In many regions, loess is near dune fields or eolian sand sheets. A question that arises from this geographic assocation is whether or not eolian sand and loess should be considered facies of the same depositional unit. There are regions such as China where these deposits are interbedded, which supports the facies concept. In other regions, such as North America, detailed geochemical and isotopic analyses show that the majority of loess particles were derived from a different, and more distant source than eolian sand. Key to understanding loess stratigraphy and interpreting environments of the past is the recognition of buried soils (paleosols). Ancient soils can be recognized by their distinctive morphological features and by vertical changes in particle size, chemistry, and mineralogy. Paleosols represent past periods when loess sedimentation rates decreased to zero or slowed significantly. Thus, loess and their interstratified soils represent end members of a continuum of sedimentary extremes: high rates of sedimentation yield relatively unaltered loess in the stratigraphic record, whereas low or episodic rates of sedimentation commonly leave a record of buried soils. The shift between these sedimentary extremes is preserved in the long-term glacial-interglacial record of the Quaternary. Although it is now known that not all eolian silt is glaciogenic, in almost all loess regions, eolian sedimentation rates were much higher during glacial periods than during interglacial periods. Drier, colder climates, a decreased intensity of the hydrologic cycle, stronger or more-persistent winds, increased sediment supply, decreased vegetation cover, and increased sediment availability all probably contributed to the sedimentary “extreme” of rapid loess accumulation during the last glacial period. The present interglacial period represents an opposite sedimentary “extreme” of minimal loess sedimentation and i

Danube loess stratigraphy - Towards a pan-European loess stratigraphic model

The Danube River drainage basin is the second largest river catchment in Europe and contains a significant and extensive region of thick loess deposits that preserve a record of a wide variety of recent and past environments. Indeed, the Danube River and tributaries may themselves be responsible for the transportation of large volumes of silt that ultimately drive loess formation in the middle and lower reaches of this large catchment. However, this vast loess province lacks a unified stratigraphic scheme. European loess research started in the late 17th century in the Danube Basin with the work of Count Luigi Ferdinand Marsigli. Since that time numerous investigations provided the basis for the pioneering stratigraphic framework proposed initially by Kukla (1970, 1977) in his correlations of loess with deep-sea sediments. Loess-palaeosol sequences in the middle and lower reaches of the Danube River basin were a key part of this framework and contain some of the longest and most complete continental climate records in Europe, covering more than the last million years. However, the very size of the Danube loess belt and the large number of countries it covers presents a major limiting factor in developing a unified approach that enables continental scale analysis of the deposits. Local loess-palaeosol stratigraphic schemes have been defined separately in different countries and the difficulties in correlating such schemes, which often change significantly with advances in age-dating, have limited the number of basin-wide studies. A unified basin-wide stratigraphic model would greatly alleviate these difficulties and facilitate research into the wider significance of these loess records. Therefore we review the existing stratigraphic schemes and define a new Danube Basin wide loess stratigraphy based around a synthetic type section of the Mošorin and Stari Slankamen sites in Serbia. We present a detailed comparison with the sedimentological and palaeoclimatic records preserved in sediments of the Chinese Loess Plateau, with the oxygen isotope records from deep-sea sediments, and with classic European Pleistocene stratigraphic subdivisions. The hierarchy of Danubian stratigraphic units is determined by climatically controlled environmental shifts, in a similar way to the Chinese loess stratigraphic scheme. A new unified Danube loess stratigraphic model has a number of advantages, including preventing confusion resulting from the use of multiple national schemes, a more transparent basis, and the potential to set Pleistocene palaeoenvironmental changes recorded in the Danube catchment area into a global context. The use of a very simple labelling system based on the well-established Chinese loess scheme facilitates interpretation of palaeoenvironmental information reported from the Danube Basin loess sites in a wider more accessible context that can be readily correlated world-wide. This stratigraphic approach also provides, for the first time, an appropriate framework for the development of an integrated, pan-European and potentially pan-Eurasian loess stratigraphic scheme. © 2015 Elsevier B.V

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 (∼100 kyr) variability through the middle to late Cisuralian (ca. 285—275 Ma). Subordinate, higher frequency cycles with thicknesses of ∼3.3–3.5 and ∼1.8 m appear to represent obliquity and precession-scale variability. If the driver of magnetic enhancement is pedogenic, then the ∼10 m thick cyclicity that is consistent over ∼1000 m of section may represent the thickness of loessite–paleosol 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

Initial results of the colorimetric indices of the oldest exposed pedocomplex (Titel loess plateau, Serbia)

In this study we present an in-depth description of the colorimetric values for the lowest section of the Dukatar Loess Palaeosol Sequence (LPS) pedocomplex S5. Formed during the Marine Isotope Stage (MIS) 13-15, it represents the oldest pedocomplex exposed at the base of the Titel loess plateau (TLP), near the confluence of the Tisa and Danube rivers in Vojvodina (northern Serbia). The results of low-field magnetic susceptibility measurements (χlf) were compared to colour properties (obtained by conventional methods as well as instrumental measuring) and quantified Soil Development Indices (SDI). Of these measurements we found that the Redness Index (RI1) yielded the most useful results, as this index appears most sensitive to lithological changes and soil development intensity. It was also observed that a high level of correlation existed between χlf, and a* chromaticity. The initial results of this study highlight the utility of colorimetric methods as an interdisciplinary tool when evaluating the presence of ferromagnetics, and the application of rock magnetism to the Middle and Upper Pleistocene LPS of the Middle Danube Basin. The presented approach can be used to observe the evolution of climatic and ecological conditions in the given study area, and for establishing correlations between sites extending over the Eurasian LPS provinces

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

Late Miocene magnetostratigraphy of Jianzha Basin in the northeastern margin of the Tibetan Plateau and changes in the East Asian summer monsoon

Jianzha Basin is located in the northeastern Tibetan Plateau (NETP) and contains a thick sequence of Cenozoic sediments that are an archive of information about the growth of the Tibetan Plateau and the evolution of the arid environment of the interior of Asia. Here, we present magnetostratigraphic and palaeoenvironmental records from a 361-m-thick sequence of Late Cenozoic eolian Red Clay and intercalated fluviolacustrine deposits in the Jianzha Basin. The magnetostratigraphic results show that the sediments have recorded a continuous geomagnetic polarity sequence from C5r.3r to C3r, spanning the interval from 11.8 to 5.8Ma in the Late Miocene. There are two intervals of rapidly fluctuating sedimentation rates between similar to 10 and similar to 6Ma, which we interpret as a response to a series of uplifts and expansions to the north and to the east in the NETP. The fluctuations in Rb/Sr ratio and magnetic susceptibility before similar to 8.57Ma reflect intensified East Asian summer monsoon (EASM) precipitation which resulted from the growth of the NETP. From similar to 8.57 to similar to 7.21Ma, the EASM was impacted by global cooling and ice build-up in the Northern Hemisphere in addition to the uplift of the Tibetan Plateau (TP) in the Late Miocene. From similar to 8.57 to similar to 7.21Ma, there is a lack of coherency between the fluctuations in MS and Rb/Sr ratio; however, subsequently, there is significant coherency between the Rb/Sr ratio and the deep-sea oxygen isotope record present. This suggests that from similar to 8.57Ma, the eolian Red Clay sediments in the Jianzha Basin were significantly affected by the addition of dust derived from the deforming and uplifting areas of the TP

Loess Deposits, Origins and Properties

Loess is an eolian (windblown) sediment that is an important archive of Quaternary climate changes. It may provide one of the most complete terrestrial records of interglacial–glacial cycles. Loess is unusual as a record of Quaternary climate change because it is one of the few sediments that is deposited directly from the atmosphere. Thus, it is a geologic deposit that contains a record of atmospheric circulation and can be used to reconstruct synoptic-scale paleoclimatology. Loess is also unusual in that it can be dated directly using ‘trapped electron’ or luminescence methods that require only the sediment itself. Commonly, loess deposits are not homogenous sediments, but most contain buried soils, or paleosols. It is the combination of both unaltered loess deposits and intercalated paleosols that gives this sedimentary record much of its richness as a Quaternary paleoclimate record