Late Pleistocene Climate and Dust Source From the Mobarakabad Loess–Paleosol Sequence, Northern Foothills of the Alborz Mountains, Northern Iran

Amin Ghafarpour,Xianqiang Meng,Farhad Khormali,Hossein Tazikeh,Thomas Stevens

doi:10.3389/feart.2021.795826

Abstract

Paleoclimatic investigation of loess-paleosol sequences from northern Iran is important for understanding past changes in a region highly sensitive to shifts in precipitation, and along potential routes of past human migration. Here, we present carbon and oxygen isotopic compositions of bulk carbonate (δ13Cbc and δ18Obc, respectively) coupled with particle size distributions of samples from the Mobarakabad section, northern Iran, to study past wind dynamics and hydroclimate. We also present new initial clay-sized Hf-Nd isotope results from key horizons in order to assess general dust sources. Variations of δ13Cbc and δ18Obc values of modern soils compared to paleosols allow reconstruction of late Pleistocene–Holocene climate change in the area. Our results show severe drought during a major eolian deposition phase (EDP) after 34 ka. The thickness and PSD of the C horizon of unit 5 suggest significant shifts in loess sources and depositional environments during this EDP after 34 ka. Indeed, based on our new clay-sized Hf-Nd data, we hypothesize that the loess unit 5 might originate from the young crustal source of the Alborz and Kopet Dagh mountains. In general, the PSD of C horizons in the section is bimodal in the silt fraction and the very small, very fine clay fraction, with a mode at c. 1 μm in the modern soil and paleosols possibly produced by weathering and pedogenic processes. There also appears to be a good correlation between δ13Cbc and δ18Obc values, differentiating phases of loess accumulation and paleosol formation and hence providing quantitative data for reconstructing paleoclimatic conditions in the study area.

Highlights

Loess sediment in the mid-latitude of Eurasia provides an excellent sedimentary archive for understanding past climate and environmental changes in the continental interior (e.g., Rousseau et al, 2017, 2020; Fenn et al, 2020; Fitzsimmons et al, 2020; Költringer et al, 2020; Song et al, 2021)
In northern Iran, loess deposits are found in the so-called Northern Iranian Loess Plateau (NILP) and along the northern foothills of the Alborz Mountain range (NFAM)
The most important result from particle size trends and isotopic signatures of bulk carbonates at the Mobarakabad is that the eolian deposition phase after 34 ka reflects rapid loess accumulation during severe drought associated with windy conditions and potentially activation of close by sediment sources

Summary

Introduction

Loess sediment in the mid-latitude of Eurasia provides an excellent sedimentary archive for understanding past climate and environmental changes in the continental interior (e.g., Rousseau et al, 2017, 2020; Fenn et al, 2020; Fitzsimmons et al, 2020; Költringer et al, 2020; Song et al, 2021). The loess units in the area are separated by paleosols showing various development degrees, providing valuable sedimentary archives of loess accumulation, soil, and paleosol formation within the loess units (Kehl et al, 2005; Frechen et al, 2009; Kehl, 2010; Khormali and Kehl, 2011; Ghafarpour et al, 2016, 2021; Vlaminck et al, 2016, 2018; Lauer et al, 2017a; Lauer et al, 2017b; Shahriari et al, 2017; Pourmasoumi et al, 2019; Sharifigarmdareh et al, 2020; Kehl et al, 2021) These previous studies of loess–paleosol sequences in northern Iran mainly focused on dating of the deposits, particle size distributions, and paleosol formation in response to paleoprecipitation and paleopedogenic process, with little emphasis on phases of eolian deposition, meaning that additional detailed analyses of the loess units in particular are essential. There is still considerable uncertainty about the potential source(s) area of the northern Iranian loess, due to the lack of previous provenance analyses

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