Magnetic parameters of surface soils and AB profile in NE Iran

The Earth orbital precession cycle (~21-kyr) is expected to have a sustained impact on global climate; however, precession signals are absent from global benthic δ¹⁸O records—a discrepancy known as the “41-kyr problem”. Here, we present new evidence of dominant ~21-kyr precession cycles in terrestrial climate proxies from western arid Central Asia (ACA) during the Early Pleistocene (~2.24-2.0 Ma), demonstrating the significant influence of precession on mid-latitude Asian climate. We found that the ratio of anhysteretic remanent magnetization susceptibility (χ_ARM) to low-frequency magnetic susceptibility (χ_lf) in loess sediments is a robust proxy for paleoprecipitation. A well-dated, ~600 ka χ_ARM/χ_lf record from loess deposits in western ACA shows clear 21-kyr cycles between ~2.24 and 2.0 Ma, when the regional climate was relatively dry and the pedogenic intensity was weak. Our results elucidate the role of precession as a driver of terrestrial climate dynamics and they also address the origin of the “41-kyr problem” in Milankovitch theory.地球轨道岁差周期（Earth orbital precession cycle，~21千年）预计会对全球气候产生持续影响，但全球底栖δ¹⁸O记录中却缺失岁差信号，这一矛盾被称为“41千年难题”。本研究基于中亚干旱区（arid Central Asia, ACA）西部早更新世（~2.24~2.0 Ma）的陆地气候代用指标，提供了占主导地位的~21千年岁差周期的新证据，证实了岁差对中纬度亚洲气候的显著影响。研究发现，黄土沉积物中非滞后剩磁磁化率（anhysteretic remanent magnetization susceptibility, χ_ARM）与低频磁化率（low-frequency magnetic susceptibility, χ_lf）的比值，是可靠的古降水代用指标。对中亚干旱区西部黄土沉积建立的高精度定年~600 ka的χ_ARM/χ_lf记录显示，在~2.24~2.0 Ma期间，该区域存在清晰的21千年周期，此时区域气候相对干旱，成壤强度较弱。本研究结果阐明了岁差作为陆地气候动力学驱动因子的作用，同时也为米兰科维奇理论（Milankovitch theory）中的“41千年难题”提供了成因解释。