Critical Altitudinal Shift From Detrital to Pedogenic Origin of the Magnetic Properties of Surface Soils in the Western Pamir Plateau, Tajikistan

Abstract

Sedimentary archives of the westerlies‐influenced Pamir Plateau in Tajikistan, a climatically sensitive region in arid central Asia, provide information on past climatic changes and the tectonic evolution of the northern Tibetan Plateau. However, due to the scarcity of meteorological stations, knowledge of the recent precipitation history of the region is limited. We conducted a detailed rock magnetic investigation of 34 surface loess samples from the western Pamir Plateau. The results show that on the windward side, below an altitude of 2,000–2,200 m above sea level, the concentration of magnetic minerals initially decreases and then begins to increase with increasing altitude. Below 2,000–2,200 m above sea level, the surface soils are enriched in multidomain magnetite particles, suggesting a detrital origin of the magnetic minerals, whereas above this altitude the samples are enriched in fine superparamagnetic magnetite/maghemite grains of pedogenic origin. The inferred variations in pedogenic intensity, combined with analysis of the available meteorological data and the content of organic matter and clay, support the occurrence of an altitudinal threshold separating a predominantly detrital control on surface soil magnetic mineral assemblages from a predominantly pedogenic control. Within the plateau interior, the surface loess samples exhibit low and uniform values of magnetic concentration parameters and organic matter and clay content, which is attributed to the decreased supply of moisture from westerly sources because of the rain shadow effect. Our results indicate that rock magnetic investigations can be used to define a critical climatic boundary in arid central Asia.

A Combined Rock Magnetic and Meteorological Investigation of the Precipitation Boundary Across the Tibetan Plateau

“…Our combined analysis of rock magnetic and meteorological data demonstrates that the magnetic properties of surface soils in the SETP and the central TP are closely related to the spatial distributions of precipitation. This observation is consistent with the results of rock magnetic investigations of surface soils in the northern TP (Jia et al, 2013;Kang et al, 2020;Zan et al, 2020), confirming that rock magnetic methods can be used to determine critical precipitation boundaries. We now present a synthesis of new and published rock magnetic data from surface soil samples from the entire TP and its adjacent areas (Figure 4a) in order to provide a detailed picture of the precipitation boundaries in the region and their climatic significance.…”

“…Reference to Figure 3c reveals a well-defined inverse relationship between MAT and altitude, which is consistent with the theoretical lapse rate 0.65 °C/100 m. This observation provides further support for the view that, compared to precipitation, temperature plays a minor role in determining the magnetic variations of surface soils across the Russian steppe, North Africa and East Asia (Han et al, 1996;Kang et al, 2020;Lyons et al, 2010;Maher et al, 2002Maher et al, , 2003Nie et al, 2010;Song et al, 2014). Moreover, the positive relationship between pedogenic magnetite and/or maghemite content (measured by fd E  ) and MAP can offer a convenient and effective method to determine the precipitation distribution across the TP, especially for the areas lacking meteorological stations.…”

“…For example, increases in the magnetic susceptibility of surface soils with increasing mean annual precipitation (MAP) have been observed in East Asia, the Russian steppe and North Africa, where pedogenesis results in the magnetic enhancement of surface soils (Han et al, 1996;Lyons et al, 2010;Maher et al, 2002;Nie et al, 2010;Song et al, 2014). Based on detailed investigations of the spatial and altitudinal variations of the magnetic properties of surface soil samples, several recent studies have delineated the precipitation boundary between sub-humid to semi-arid and arid regions in the northern TP (Kang et al, 2020;Zan et al, 2020). However, comparable rock magnetic studies have not yet been extended to the central and southeastern parts of the TP, hindering our understanding of the spatial distribution of precipitation across the entire TP and its adjacent areas.…”