Astronomical forcing of terrestrial climate recorded in the Pleistocene of the western Tarim Basin, NW China

摘要

Astronomically forced variations of climate are well documented in lacustrine and marine settings. However, few studies have examined astronomical forced cycles within Quaternary fluvial-fan successions. To explore whether Milankovitch cyclicity might also be present in fluvial-fan successions, we carried out high-resolution gamma ray (GR) from downhole logging, magnetic susceptibility (MS), Rb/Sr and total organic carbon (TOC) analysis of a core from an 800-m borehole (KM) from the Kashgar region in the western Tarim Basin, China. Spectral analyses of the GR, MS and Rb/Sr data reveal cycles with similar to 70 m, similar to 30 m and similar to 14 m wavelengths. These pertain to relative variations in grain size. Electron spin resonance (ESR) dating and magnetostratigraphy imply that the cored interval spans the past 1.1 Myr. This age-span constraint, coupled with the comparison of the ratios of these cycle wavelengths to expected ratios of Milankovitch frequencies, suggests that these cycles represent similar to 100-kyr short-eccentricity, similar to 40-kyr obliquity and similar to 20-kyr precession frequencies, respectively. We constructed an astronomical time scale (ATS) spanning the past 1.13 Myr using the similar to 100-kyr short-eccentricity period. The average accumulation rate is about 70 cm/kyr for this 800-m core succession, and the evolutive spectrum emphasizes that long-term sedimentation rates were stable. We also applied correlation coefficient (COCO) analysis to identify the astronomical forcing. Thus, we conclude that orbitally forced climate change was the main driver of stratigraphic alternations between sand- and clay-rich intervals in the Kashgar region. During maxima of the 100-kyr short-eccentricity cycles, which correspond to interglacial intervals, more chemical weathering and increased seasonal runoff delivered a larger amount of relatively coarser-grained sediments to the fluvial fan in the Kashgar region, and vice versa. The Mid-Pleistocene