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

Abstract

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.

Keywords

• Arid central Asia
• Environmental magnetism
• Surface soils
• Magnetic enhancement
• pedogenic intensity
• climatic boundary