Interglacial, Holocene and recent dust accretion in the Danube Basin and beyond -evidence for uninterrupted dust accumulation in Eurasian dry steppe regions

Abstract

Lithospheric dust plays a key role in the Earth’s system connecting litho-and pedosphere with atmo-, hydro-, cryo-and even biosphere, on the one hand materially via the containing minerals and immaterially via the impact on atmospheric radiation balance andthe elevated energy content of gases loaded with dust. Litho-and pedosphere serve as source and sink of dust, which eventually accumulates in the ocean and on continents as environmental archives. These accumulations and the pristine dust consist mainly of common silicates in silty grain sizes and variable contents of carbonates corresponding geochemically to the average continental crust. Here we discuss spatial and temporal patterns of dust accumulation in the Danube Basin and stratigraphic records of environmental proxy data in recent soils, palaeosols and loess and their implications for palaeoclimate reconstruction, pedogenesis and archaeology in Eurasian steppe environments.Especially widespread on the mid-latitude Eurasian continent, dust accumulations are known as loess generally exhibiting a characteristic feature of stratigraphic intercalations of distinct horizons differing mainly in colour and possibly in grain size and chemical composition. Based on analytical evidence, those horizons are interpreted as the result of dominantly synsedimentary alterations of the pristine minerals not directly controlled by temperature, but indirectly through elevated moisture and the related biome. Such coupled processes, permanently creating a global interfaceon top of the lithosphere, are known as pedogenesis forming the pedosphere, which in turn hosts an essential part of the continental biosphere. In case of dust accumulations, the intensity of pedogenesis acts on a wide range of amplitudes reflecting temporal hydroclimate variabilities and transforming accumulated lithospheric dust into loess (loessification) and sometimes into pedohorizons, called soils, when prevailing pedogenesis exceeds critical thresholds. Soils become eventually buried forming the characteristic feature of loess-palaeosol sequences (LPS), fossil soils (palaeosols)interbedded with loess. Hence, palaeosols are the dominant metronomes of LPS reflecting the temporal hydroclimatic variations of the geological past.At least since the early Pleistocene, the western end of the Eurasian steppe belt, notably the Danube Basin and the steppe regions north of the Black Sea, operated as important dust sinks providing decametre thick records of environmental history. We will present results showing that in those steppe environments, synsedimentary pedogenic transformation dominates over translocation processes, in this way preserving primary depositional sedimentary features. Mainly carbonate, sulphate and soil organic matterare widely affected by translocation, whereas the main component of the lithospheric dust deposits, the silicate mineral skeleton, remains largely unaffected yet recording environmental temporal dynamics. Therefore, records of physical properties of LPS as grain size (GS) or magnetic susceptibility (MS) reflect directly synsedimentary environmental conditions as wind speed (GS) or sediment moisture (MS). Under these conditions, pedogenesis is, albeit providing often colourful marker horizons, a rather secondary process operating in steppe LPS more “bottom up than top down” and thus only slightly affecting the primary environmental signal generated by aeolian depositionand early diagenesis (loessification). Hence, palaeosols in LPS primarily record the temporally changeful environment during interglacials and interstadials via their uninterrupted sedimentary nature, and only secondarily the pedogenic overprinting operating top down. Distinctive patterns of environmental proxy records from palaeosols in LPS provide characteristic fingerprints allowing thereby even correlations to sedimentary marine and ice records.Employing those fingerprints (e.g. MS), we present unambiguous continent-wide correlations of Eurasian LPS giving evidence for a quite similar
Workshop Loess and ArchaeologyRWTH Aachen University, Germany, 27-29 November 201927accumulation and/or loessification history which in turn reflects the on-site environmental conditions changing with time. Dust accumulation rates seem to be quite constant in plateau loess settings varying from c. 3 to c. 20 cm/ka, at most. Interestingly, in low accumulation sites relatively fine silt dominates (median c. 20-25 micron) pointing to a long-range transported dust. Similar to Greenland ice cores and lacustrine records, dust accumulation in Eurasian steppe LPS varies with changing climate but does not cease during climatically favoured intervals such as interstadials or interglacials. High-resolution dating results of sites across the Danube Basin document precisely the accumulation history of dust at the transition from Pleistocene to Holocene and during Holocene times. These chronologies prove the dominantly sedimentary character of the Holocene soil, in which pedogenesis mainly operates “bottom up” generally playing only a subordinate role. The continuous nature of dust accumulation in LPS is also proven by the occurrence of numerous volcanic tephra layers and the excellent preservation of delicate archaeologic find horizons, both providing quite short temporal snapshots. Furthermore, geoarchaeological evidence, direct observations and historic records of dust falls supported by numerical models of recent dust deposition in the Danube Basin suggest a significant contribution of aeolian dust to Holocene soils even outside the loess plateaus. Hence, LPS in dry steppe regions of the Eurasian continent are formed by dust accretion representing a unique archive of palaeoenvironmental and human cultural evolution.

Resources

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Bibliography

Hambach, U., Veres, D., Constantin, D., Zeeden, C., Pötter, S., Baykal, Y., Obreht, I., Laag, C., Bösken, J., Schulte, P., Stevens, T., Marković, S., Lehmkuhl, F., Timar-Gabor, A. (2019): Interglacial, Holocene and recent dust accretion in the Danube Basin and beyond -evidence for uninterrupted dust accumulation in Eurasian dry steppe regions. RWTH Aachen University, Workshop Loess and Archaeology, 27-29 November 2019, Aachen, Germany, DOI: 10.18154/RWTH-2019-10413