Universität zu Köln

Vlaminck, Stefan (2018). Northeastern Iranian loess and its palaeoclimatic implications. PhD thesis, Universität zu Köln.

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Abstract

Loess-palaeosol sequences (LPS) are valuable terrestrial archives for the characterization and reconstruction of late Pleistocene climate dynamics over wide areas. Within the Eurasian loess belt most scientific attention was attributed to the loess successions of southeastern Europe and Central Asia. Thus, considerable knowledge gaps remain to be closed for other regions. The loess deposits and LPS of northern Iran represent an important link between these two areas. However, previous investigations were based on coarse-meshed sampling and analyses and therefore provided only limited insights into the lithological properties of loess deposits, intercalated palaeosols and the resulting pedostratigraphy. This applies also to the provenance of loess, the degree of weathering and the genetic interpretation of palaeosols. Consequently, insights into palaeoclimate of northeastern Iran and its last interglacial-glacial landscape evolution are limited, impeding regional as well as supraregional correlations with palaeoclimatic archives and LPS from western and central Asia. The present dissertation aims at elaborating new records of lithological and palaeopedological properties of northeastern Iranian loess. Relative wind velocities during the periods of loess formation and the distance from the dust source are inferred from highly resolved grain-size analyses. The dust source itself is tentatively localized by means of immobile major elements. Moreover, palaeosols are to be identified and characterized in order to establish a regional pedostratigraphy of northeastern Iran. One focus is put on the detection of weakly developed palaeosols, as little is known about their existence and potential correlations with interstadials of MIS 3. A second focus is put on the thorough characterization of the MIS 5 pedo-complex. These foci provide evidence in order to distinguish between syngenetic and post-sedimentary palaeosols. Furthermore, marker horizons are identified and supplemented by luminescence dating results to establish a chronostratigraphic scheme and regional correlation of the LPS. Based on the results of the present dissertation the northeastern Iranian LPS are compared with palaeoclimatic archives from western and central Asia. This requires improved stratigraphical investigations and high-resolution sampling of three newly discovered loess exposures. The studied loess deposits are found along a modern precipitation gradient, since the profiles of Neka-Abelou and Toshan are located in the subhumid foreland of the Alborz Mountain range, whereas the Agh Band (Agh 1/2) profile is located in the so called Iranian loess plateau (ILP). The considerable number of sediment samples (~4500) was used to provide new records on grain-size (laser-diffractometry), soil color (VIS spectrometry), carbonate content (MIRS) and rock magnetics (magnetic susceptibility) which are combined with micromorphological and geochemical findings. The correlation of the northeastern Iranian LPS with marine isotope stages is based on a discussion of the luminescence dating result from these sections. Accordingly, the LPS of Neka-Abelou and Toshan are structured by numerous weakly to strongly developed palaeosols. These sections may be well correlated with other LPS along the northern declivity of the Alborz Mountains, which are known from the scientific literature. The MIS 5 pedocomplex, composed of three strongly developed palaeosols (Bt[k] and Bw[k]-horizons) has proven to be highly suited for correlation. The uppermost of these palaeosols is overlain by weakly weathered MIS 4 loess, which may be well recognized in numerous exposures of the region. The MIS 5 pedocomplex and the MIS 4-loess thus provide excellent stratigraphic marker horizons. Moreover, a tephra layer could be detected at the base of the MIS 4-loess, which might yield independent age control. Additionally, last glacial loess deposits are structured by several syngenetic palaeosols (CB and Bw-horizons), which have formed during the interstadial periods of MIS 3 and 2. The boundaries between these incipient palaeosols and underlying and overlying loess strata are marked by gradual transitions in texture, soil color and magnetic susceptibility. The comparatively fine texture of these soils indicates decreasing wind velocities, while magnetic susceptibility signals weakly enhanced weathering intensities. The latter, however, resulted merely in partial decalcification of the deposits. Hence, the interstadial palaeosols likely developed under relatively dry and dusty climatic conditions, corresponding with increased wind energy and proximal sediment supply. It is assumed that these interstadial palaeosols are the result of millennial scale fluctuations, which are covered by the standard deviation of luminescence dating and thus impede any correlation with the marine record. Conversely, the palaeosol of the MIS 5 pedo-complex formed postgenetically on glacial loess under reduced and distal or absent dust accumulation. These soils are completely decalcified and exhibit abrupt lower boundaries in the form of distinct carbonate enrichment horizons. The transitions into subsequent stadials are again characterized by intervals of syngenetic soil formation, indicating a gradual aridization. From MIS 4 to 2 a distinct coarsening trend of loess texture is apparent, indicating steeper atmospheric pressure gradients and proximal sediment supply. The major phase of loess formation presumably coincides with MIS 3. The ratio of immobile major elements Al2O3/TiO2 reveals a different provenance of loess along the northern declivity of the Alborz Mountains as compared to loess in the ILP. It is assumed that dust, forming the loess deposits of these areas originates from the alluvial plains of the respective rivers Gorgan and Atrek. The palaeosols of the Agh1/2 profile are composed of strongly developed Bwyk-horizons, which could be correlated with MIS 7. Previous findings suggesting a correlation with MIS 5 may therefore be rejected. Moreover, no incipient palaeosols of the last pleniglacial could be detected in Agh Band. Hence the correlation of the Agh Band profile (Agh 1/2) with LPS of Neka-Abelou and Toshan is afflicted with considerable uncertainties. Based on its highly detailed stratigraphy and the exceptional amount of available data, the Toshan section is defined as northern Iranian key sequence. The MIS 5 pedo-complex may be well correlated with loess sections from southern Europe, Armenia and central Asia. This palaeosol triple also reflects palaeoclimatic fluctuations of MIS which are recorded in other climatic archives from (e.g.) the Mediterranean, Lake Van and Lake Urmia. Furthermore, northern Iranian loess and its six interstadial palaeosols seem to offer more differentiated insights into the last glacial than the loess deposits from the other above mentioned regions.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Vlaminck, Stefan vlamincs@uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-79967
Date:	January 2018
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Geographisches Institut
Subjects:	Natural sciences and mathematics Earth sciences Geography and travel
Uncontrolled Keywords:	Keywords Language Iran, loess, palaeoclimate, paleoclimate, soil formation, pedogenesis UNSPECIFIED
Date of oral exam:	5 May 2017
Referee:	Name Academic Title Kehl, Martin PD Dr. Schäbitz, Frank Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/7996