Early‐Middle Eocene Hydroclimate Variations Recorded by Environmental Magnetism in the Linxia Basin, NE Tibetan Plateau

Abstract

Although Neogene climate change reconstruction in central Asia is important for understanding global cooling, Asian monsoon evolution, and the Tibetan Plateau stepwise uplift during the Cenozoic, the paleoenvironmental evolution during the Paleogene in this region is still ambiguous due to a lack of corresponding continuous and complete terrestrial records with precise age constraints. Recently, a new fluvial sedimentary sequence with magnetostratigraphic ages spanning from ∼53 to 40 Ma was found in the Linxia Basin, NE Tibetan Plateau, thereby providing a window to understand Eocene climate change in the Asian interior. From this fluvial sedimentary sequence, we present early‐middle Eocene hydroclimate variations based on a combined method of rock magnetism and diffuse reflectance spectroscopy. Our results show magnetic enhancement of fluvial sediments at 51.7 Ma resulted from abundant single‐domain magnetite inputs from the surrounding mountains through strong erosion and transport in a relatively wet environment. Subsequently, at 47.6 Ma, magnetic weakening of floodplain sediments corresponds to an increase in hematite concentrations produced via low‐temperature oxidation in a prolonged dry environment. Comparisons of paleoclimatic proxies, such as organic geochemistry and rock magnetism from neighboring basins, tectonic deformation of the NE Tibetan Plateau, and sea level change in the Paratethys Sea suggest that the hydroclimate variation from relatively wet to dry climate in the early‐middle Eocene in this area was mainly controlled by global climatic change and probably superimposed by the uplift of the Tibetan Plateau.

Early Eocene magnetostratigraphy and tectonic evolution of the Xining Basin, NE Tibet

“…Map of the north‐eastern Tibetan Plateau showing tectonic events during the early Eocene: (1) Li et al (2018); (2) Jolivet et al (2001); (3) He et al (2017); (4) He et al (2018); (5) He et al (2020); (6) Li et al (2020); (7) Ji et al (2017); (8) Staisch et al (2020); (9) He et al (2021) and this study; (10) Wang et al (2016); (11) Feng et al (2021); (12) Clark et al (2010); (13) Liu, Zhang, Pan, et al (2013); and (14) Chen et al (2021). Modern day topographic base map is from the GeoMapApp software (Ryan et al, 2009) available at http://www.geomapapp.org/.…”

“…Another increase in single domain magnetite was observed at 51.7 Ma in the neighboring Linxia Basin coeval with a shift in detrital zircons and linked to uplift of the Western Qinling Shan (Fig. 10; Feng et al, 2021;2022). A similar change in provenance may explain the appearance of magnetite in the Xining Basin.…”

“…A similar shift has been recognized in the Miocene strata of the Xining Basin where an increase in detrital magnetite was derived from plutonic and volcanic rocks from the uplifting Laji Shan and was coupled to unstable sedimentation rates and an unconformity (Xiao et al, 2012). Another increase in single‐domain magnetite was observed at 51.7 Ma in the neighbouring Linxia Basin coeval with a shift in detrital zircons and linked to the uplift of the Western Qinling Shan (Figure 10; Feng et al, 2021, 2022). A similar change in provenance may explain the appearance of magnetite in the Xining Basin.…”