Loess Paleosol-Sequences Along a Climatic Gradient in Northern Iran
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149-173 Eiszeitalter und Gegenwart 55 Hannover 2005 4 Abb., 5 Tab. Loess paleosol-sequences along a climatic gradient in Northern Iran M K, R S, H A, M F A S*) Keywords: loess, paleosol, luminescence, past cli- 7 or older interglacials. Th e pedocomplexes at Neka mate change, Pleistocene, Iran and Now Deh indicate polycyclic soil genesis includ- ing soil formation, truncation of the upper soil hori- Abstract: In Northern Iran, loess is found in diff er- zons, loess deposition and again soil formation. ent geomorphological settings along a climatic gradi- In the loess hills near Agh Band, 40 m thick homog- ent ranging from subhumid to semiarid conditions. enous loess covers a brown paleosol (Bw(t)), possibly Loess-paleosol sequences were investigated in detail correlating with the last interglacial soil. Th e loess at in three key sections located on the northern foot- Agh Band section has a high percentage of fi ne sand hills of Alborz mountains (sections at Neka and at and coarse silt and contains signifi cant amounts of Now Deh) and in the loess hills north of Gonbad-e gypsum. Kavus (section at Agh Band). Th e loess-paleosol sequences indicate pronounced At the section at Neka, two pedocomplexes consist- climate changes from dry and cool to moist and ing of moderately to strongly developed Bwk, Bt or warm conditions with loess deposition and soil AhBt horizons are intercalated in fi ne textured loess. formation, respectively. Th ey are excellent terrestrial Luminescence age estimates indicate that the upper archives of Quaternary climate and environment paleosol was formed during OIS 5a or 5c, whereas change in Northern Iran. the lower pedocomplex might represent OIS 5e or older interglacial periods. [Löss-Paläobodensequenzen entlang eines Kli- Nine light or dark brown paleosols (CBk, Bwk and magradienten in Nordiran] Btk horizons) are intercalated in the loess at the sec- tion at Now Deh indicating diff erent weathering intensities most likely during interglacial and inters- tadials periods of the Middle and Upper Pleistocene. Zusammenfassung Th e upper Bt horizon of Now Deh likely correlates with OIS 5e. Th e lower Bt horizons join to form a Im südlichen Kaspischen Tiefl and und seinen strong pedocomplex possibly correlating with OIS umliegenden Gebieten treten Lösse in verschie- denen geomorphologischen Positionen entlang * Anschrift der Verfasser: Dr. M. K und Prof. eines rezenten Klimagradienten von subhumi- Dr. A. S, INRES-Soil Sciences, Univer- den zu semiariden Verhältnissen auf. Drei Löss- sity of Bonn, Nußallee 13, 53115 Bonn, Germany, Paläobodenabfolgen werden beschrieben, die Email: [email protected], Prof. Dr. R. S, im nördlichen Vorgebirge des Alborz (Profi le Department of Geography, Shahid Beheshti Univer- sity, Tehran, Iran, Prof. Dr. H. A, College of Neka und Now Deh) und im Lösshügelland Natural Resources, University of Tehran, Karaj, Iran, nördlich Gondbad-e Kavus (Profi l Agh Band) Prof. Dr. M. F, Leibniz Institute for Applied aufgeschlossen sind. Geosciences (GGA-Institut), Geochronology and Das Profi l Neka besteht aus feinkörnigem Löss, Isotope Hydrology, Stilleweg 2, 30655 Hannover, der von zwei kräftig entwickelten Paläoboden- Germany komplexen aus Bwk-, Bt- und AhBt-Horizon- 150 M K, R S, H A, M F A S ten durchzogen wird. Erste Lumineszenzda- ern foothills of the Alborz mountains between tierungen weisen darauf hin, dass der obere Sari and Minoodasht (N I O Pedokomplex während der Sauerstoff -Isotopen- C 1978; G S Stadien (OIS) 5a und/oder 5c gebildet wurde, M E I) and locally während der untere das OIS 5e oder ein älteres occurs, for instance, on fl uvial terraces of Se- Interglazial repräsentieren könnte. fi d-Rud and Chalus River (N I Neun hell braune oder dunkel braune bis röt- O C 1978; E 1971; P & lich-braune Paläoböden (CBk-, Bwk- und Bt- D 1980) or in the Gharatikan watershed Horizonte) des Lösses bei Now Deh spiegeln (O & A, 2001). Th ough T unterschiedliche Verwitterungsintensitäten von (1877), S (1923) and B (1937) al- Interglazialen und Interstadialen des Mittleren ready mentioned loess deposits in Northern bis Oberen Pleistozäns wider. Während der obe- Iran, little information about the nature, origin re Bt-Horizont von Now Deh wahrscheinlich and chronology of the loess has ever been pub- mit dem OIS 5e korreliert, bilden die unteren lished. Bt-Horizonte einen Pedokomplex, der das In- B (1960) gave a short description of terglazial des OIS 7 repräsentiert. Die Pedo- loess in the Sefi d-Rud valley and correlated komplexe in Neka und Now Deh weisen jeweils these deposits with the Holocene. Following auf polyzyklische Bodenentwicklung hin, die this view, E (1971) suggested that brown Bodenbildung, Bodenabtrag, Lössaufwehung paleosol horizons intercalated in the loess from und erneute Bodenbildung umfasste. Sefi d Rud and also in loess deposits near Neka In den Lösshügeln bei Agh Band bedeckt ein 40 formed during moist periods of the Holocene, m mächtiges, weitgehend homogenes, gipshal- whereas loess deposition took place during dry tiges und feinsand- sowie grobschluff reiches periods. L (1988) described loess deposits Lösspaket einen braunen Paläoboden (Bw(t)), with intercalated brown paleosols near Now der vermutlich in das letzte Interglazial zu stel- Deh, which are likely identical to the “folded len ist. loess” described by R (1966). L Die beschriebenen Löss-Paläobodenabfolgen (1988) supposed that the brown paleosols dokumentieren den mehrfachen Wechsel von represent the last interglacial period and that trocken-kalten zu feucht-warmen Klimaverhält- loess deposition took place during glacial times. nissen mit Lössablagerung bzw. Bodenbildung. Paleosols as markers of interglacial periods have Sie stellen ausgezeichnete terrestrische Archive widely been used in the pedostratigraphical des quartären Klima- und Umweltwandels correlation of loess-paleosol sequences with the Nordirans dar. global climatic record (e.g., B, 2002). Soil formation requires moist and warm climat- ic conditions during interglacial or interstadial times. A vegetation cover is needed to supply 1 Introduction organic matter for accumulation of soil humus and to protect the soil against erosion. In con- In Northern Iran, loess is found in hilly areas trast, the deposition of loess took place dur- along the rivers Aras in East Azerbaijan prov- ing periods of low temperature and increased ince (Fig. 1), bordering the Gorgan and Atrek aridity, when production of silt-sized grains rivers in Golestan province and west of the Hari was intensifi ed by frost-shattering and glacial river in Khorassan province (B, G activity (e.g., W 2001). Mainly silt-sized & S 1990). Loess also covers the north- sediments were defl ated from the fl ood plains Loess paleosol-sequences along a climatic gradient in Northern Iran 151 and accumulated again by trapping through available including radiocarbon, luminescence shrub or grass vegetation. In Southern Iran, dating, amino acid racemisation and paleo- loess and loess-like sediments were deposited in magnetic measurements. Th e infrared optically the Basin of Persepolis located in the Southern stimulated luminescence method (IRSL) has Zagros mountains. Th ere, the loess formation been shown to be highly suitable for the dat- pathway is explained by intensifi ed production ing of loess in Tajikistan and to determine loess of silt-sized material during the Last Glacial mass accumulation rates in Europe (F maximum, fl uvial transport to the alluvial & D 1998; F, O & K- plains, defl ation, aeolian deposition and fl uvial 2003). displacement (K et al. 2005). In this paper we give fi rst detailed descriptions Cyclical climate changes like those of the Qua- of three key sections of Northern Iranian loess ternary can result in the formation of stacked and its paleosols. From these sequences eight loess-paleosol sequences as observed in the samples were taken for a fi rst luminescence study area. Th e grey, brown, reddish-brown dating approach to investigate the suitability of or mottled paleosol horizons of loess-paleosol loess from Northern Iran and to set up a more sequences likely correlate with diff erent weath- reliable chronological framework. Th e high po- ering intensities depending on temperature tential of Iranian loess as terrestrial archives of and precipitation during the period of forma- climate and environment change is indicated. tion (e.g., B, W & H 1998). Furthermore, several other physical properties of loess-paleosol sequences are suit- able as indirect proxy measures of past climate. 2 Materials and methods Th ese so-called climate proxies include grain 2.1 The study area size, colour, mineral assemblages, the content of organic carbon, CaCO3 or iron fractions, Th e loess-paleosol sequences are located at Neka micromorphological features, major and trace and Now Deh on the northern foothills of the element composition, isotope signatures or Alborz mountains east of Sari and east of Gor- magnetic susceptibility (e.g., T et al. gan, respectively, and at Agh Band in the loess 1985; B & H 1989; B, hills north of the city of Gonbad-e Kavus (Fig. W & S 1998; D, K 1). Th e loess hills near Agh Band are part of the M 1997; H et al. 2001; D et al. so-called Iranian loess plateau. Th e coordinates 2002). Furthermore, biological indicators like and altitudes of the sections of interest are given pollen or mollusc assemblages proved very use- in table 1. ful as climate proxies (e.g., K & S Modern mean annual precipitation ranges from 1995). ~750 mm/a to ~350 mm/a in the vicinity of the Absolute age determinations are required to sections at Neka and Agh Band, respectively. correlate loess deposits with those from diff er- Th is gradient refl ects the decline in precipita- ent areas and with the global marine or ice core tion from west to east along the southern coast record (oxygen isotope stages, OIS) in order to of the Caspian Sea.