Late Miocene to Pleistocene Environmental Changes in the Western Qaidam Basin (NE Tibetan Plateau) Revealed By Magnetic Properties in Lacustrine Sediments

Abstract:

The advantage of studying magnetic properties of the lacustrine sediments deposited in the Qaidam paleolake to reconstruct past environmental changes was shown in detail for the 940‑m‑long drill core SG-1 (2.69–0.08 Ma) recovered from the western Qaidam Basin. Especially magnetic susceptibility (χ) turned out to be a useful proxy indicating past humidity changes in the study region. The causes of χ variation could be related to weathering (low-temperature oxidation) in the catchment area and a changing catchment area, respectively. To benefit further from magnetic properties as high-resolution paleoclimate proxies in the Qaidam paleolake, we expand our magnetic record to the late Miocene by investigating magnetic properties of the 723‑m deep drilling SG-1b. While SG-1 was drilled in the flat lying strata of the Chahansilatu sub-basin, SG-1b was recovered from the adjacent Jianshan anticline (in ~20 km distance from SG-1). Magnetostratigraphy of core SG-1b detects a time span ranging from 7.3 to 1.6 Ma, missing the younger sequence as a consequence of the anticline structure. We analyze climatic implications of the magnetic record of SG-1b, especially by observing 750 hysteresis loops and 20 FORC diagrams, and check for small-scale variations of the studied sediments by comparing χ of bulk samples (in the order of 10 g) and samples used for hysteresis measurements (in the order of 0.01 g). Spectral analysis based on the time scale provided by magnetostratigraphy reveals variations of χ in the order of Milankovitch cycles (precession, obliquity, and eccentricity) indicating insolation changes as one important driving factor of the magnetic concentration signal. The magnetic records of SG-1 and SG-1b are also used to check implications of the anticline structure on magnetic signatures by comparing the overlapping interval of both cores (2.69–1.6 Ma).