Abstract
Abstract. Asian terrestrial records of the Eocene-Oligocene Transition (EOT) are rare and, when available, often poorly constrained in time, even though they are crucial in understanding the atmospheric impact of this major step in Cenozoic climate deterioration. Here, we present a detailed cyclostratigraphic study of the continuous continental EOT succession deposited between ~35 to 33 Ma in the Xining Basin at the northeastern edge of Tibetan Plateau. Lithology supplemented with high-resolution magnetic susceptibility (MS), median grain size (MGS) and color reflectance (a*) records reveal a prominent ~3.4 m thick basic cyclicity of alternating playa gypsum and dry mudflat red mudstones of latest Eocene age. The magnetostratigraphic age model indicates that this cyclicity was most likely forced by the 41-kyr obliquity cycle driving oscillations of drier and wetter conditions in Asian interior climate from at least 1 million year before the EOT. In addition, our results suggest a duration of ~0.9 Myr for magnetochron C13r that is in accordance with radiometric dates from continental successions in Wyoming, USA, albeit somewhat shorter than in current time scales. Detailed comparison of the EOT interval in the Tashan section with marine records suggest that the most pronounced lithofacies change in the Xining Basin corresponds to the first of two widely recognized steps in oxygen isotopes across the EOT. This first step precedes the major and second step (i.e. the base of Oi-1) and has recently been reported to be mainly related to atmospheric cooling rather than ice volume growth. Coincidence with lithofacies changes in our Chinese record would suggest that the atmospheric impact of the first step was of global significance, while the major ice volume increase of the second step did not significantly affect Asian interior climate.
Highlights
The Eocene-Oligocene Transition (EOT) encompasses the most pronounced cooling event during Cenozoic climate deterioration (Miller et al, 1991; Zachos et al, 1996, 2001; Lear et al, 2000, 2008)
After removing a viscous magnetization component below 250–300 ◦C, a Characteristic Remanent Magnetization (ChRM) component was successfully isolated between 300 ◦C and 620 ◦C with most of the remanence demagnetized at ∼585 ◦C
Astronomical forcing of climate dominated by obliquity is expected at times of low eccentricity values mainly related to the long-period 2.4-Myr eccentricity cycle (Hilgen et al, 2000; Hyland et al, 2009) and at times of strong highlatitude climate change supposedly occurring during phases of incipient continental ice sheets at high latitudes (Palike et al, 2001; Westerhold et al, 2005; Holbourn et al, 2007; Westerhold and Rohl, 2009)
Summary
The Eocene-Oligocene Transition (EOT) encompasses the most pronounced cooling event during Cenozoic climate deterioration (Miller et al, 1991; Zachos et al, 1996, 2001; Lear et al, 2000, 2008). It was accompanied by the expansion of ice sheets on the Antarctic continent Climate modeling results and ocean sediment records have demonstrated that the decline of pCO2 and a peculiar orbital configuration were the primary factors responsible for this transition (Coxall et al, 2005; DeConto and Pollard, 2003; DeConto et al, 2008). Xiao et al.: Asian aridification linked to the first step of the Eocene-Oligocene climate Transition (EOT)
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