Quartz OSL dating of loess deposits since the late glacial in the Southeast of Caspian Sea

Abstract

Here we present loess deposits since the late glacial dated by OSL technique in the southeast of the Caspian Sea, bordering continents of Asia and Europe. Chronology of Holocene loess deposits are rarely reported in Eastern Europe and Western Asia, especially for their spacious mid-latitude adjacent region, between Balkans to the west and Pamirs to the east. In this study, an OSL-based chronology was developed for loess deposits in northern Iranian Plateau, based on the YBD loess profile which locates close to center of hilly Iranian Loess Plateau in the southeast of the Caspian Sea and on the northern slope of the Alborz Mountains. The OSL characteristics and their reliability were investigated on coarse-grained quartz (63–90 μm) of the profile, a preheat plateau test and dose recovery test of which reveal that the lower preheat is suitable for loess equivalent dose determination in the area. In addition, the SAR-OSL is an appropriate methodology for dating loess deposited since the late glacial in situ. With preservation of a complete record of loess accumulation since the late glacial, YBD section contains a potentially valuable paleoclimatic record for Caspian region and hence the vast transitional region between Asia and Europe.

Introduction

Optical dating has been widely applied to terrestrial loess deposits worldwide (Forman and Pierson, 2002; Berger, 2003; Buylaert et al., 2007; Miao et al., 2010; Frechen, 2011; Stevens et al., 2013; Lu et al., 2007; Li et al., 2014); thus, it has become an indispensable part of late Quaternary geochronology. The abundant quartz and feldspar grains in aeolian sediments are an ideal material for optical dating (Roberts, 2008). Quartz contributes the major component of loess deposits (Liu, 1966; Pye, 1987) and it is chemically stable and resistant to weathering. Quartz OSL signals can be easily bleached under natural conditions (light or heat) compared to feldspar IRSL signals. Feldspar has a higher sensitivity and higher saturation level (Huntley and Lamothe, 2001; Reimann and Tsukamoto, 2012), which makes it more suitable for dating older samples. However, the hard-to-bleach feldspar residual signals need to be discussed. Therefore, for young samples within 150 Gy, quartz OSL dating method is more suitable for age establishment.

Loess deposits are precious and irreplaceable in serving as paleoclimatic and environmental archives in both North and South hemispheres (Liu et al., 2007; Carter-Stiglitz et al., 2006; Paleari et al., 2019; Chen et al., 2016). Loess deposited in Chinese Loess Plateau, mid-latitude Asia of North hemispheres, provides paleo-records spanning Quaternary or even up to Cenozoic era in connection with results of pelagic sediments mostly from distant ocean and ice cores from either high latitudes or high mountains (Guo et al., 2002; Porter et al., 2019). Not only that, loess deposits could also be employed as high resolution sequences ranging from Holocene, late glacial to Last Glacial Maximum (LGM) during past decades in e.g. Chinese Loess Plateau, central Asia (Wang et al., 2014; Li et al., 2016; Chen et al., 2016), which significantly enriched understandings of regional climatic evolutions; however, compared to extensively used OSL dating applications in the CLP, which have provided convinced chronological frames of high-resolution (e.g., Lu et al., 2006; Buylaert et al., 2007; Lu et al., 2007; Stevens et al., 2008; Sun et al., 2012), the development of the dating on loess deposits of westerly-dominated arid central Asia commenced relatively late and are reported limitedly. Among the limited amount of paleoclimatic researches conducted in arid central Asia, OSL dating has been mainly applied to loess deposits of the Tianshan Mountains; e.g., in the eastern (Li et al., 2016) and central (Machalett et al., 2006; Chongyi et al., 2012; Yang et al., 2014; Song et al., 2015; Kang et al., 2015) parts. For further westward regions, K-feldspar IRSL and pIRIR have been mainly applied to loess deposits in the western Pamirs and the northern Iran Plateau (Frechen and Dodonov, 1998; Frechen et al., 2009; Karimi et al., 2011; Lauer et al., 2017a, b). Notably, however, studies of the chronology of continuous loess deposits since the late glacial are rarely reported likewise, especially in the vast mid-latitude Eurasia transitional region, between Balkans to the west and Pamirs to the east. This may reflect the view that Holocene strata are heavily eroded or bioturbated in these areas (Frechen and Dodonov, 1998; Frechen et al., 2009; Karimi et al., 2011; Lauer et al., 2017a, b), which makes the Holocene loess preserved in situ even rare and precious in terms of potential valuable paleoclimatic recording.

In the present study, an OSL-based chronology was developed for loess deposits since the late glacial developed close to center of hilly Iranian Loess Plateau in the southeast of Caspian Sea. The OSL characteristics and their reliability were investigated in detail and used to establish a robust regional chronological framework.