δ18O and δ13C records from a Cenozoic sedimentary sequence in the Lanzhou Basin, Northwestern China: Implications for palaeoenvironmental and palaeoecological changes

Abstract

In northwestern China, carbonate δ 18 O variation has been closely associated with evaporation and precipitation, whereas the variation of carbonate δ 13 C generally reflects patterns of palaeovegetation. Located within the transitional zone between the Chinese Loess Plateau and the Tibetan Plateau, the Lanzhou Basin has developed a continuous sequence of Cenozoic sediments which have been subjected to detailed sedimentological and high-resolution magnetostratigraphic analyses. In the present study, pedogenic carbonate O and C isotopic analyses were obtained throughout the entire Cenozoic sequence. The δ 18 O record exhibits a general positive trend with several abrupt changes. A dramatic positive shift in the δ 18 O record at ~33 Ma indicates the initiation of the aridification process within the basin, which was likely associated with the late Eocene westward retreat of the Tethys Sea and global cooling. Two significant positive shifts in the δ 18 O record at ~22 Ma and ~3.5 Ma are synchronous with major increases in aeolian dust deposition on the Chinese Loess Plateau and in the North Pacific Ocean, suggesting the intensified aridity of the Asian interior, which is likely related to the stepwise uplift of the Tibetan Plateau via the blocking of water vapour pathways. The δ 13 C values exhibit a weak positive trend with a remarkable shift at ~3.5 Ma. This trend is likely related to a decrease in vegetation density in response to the ongoing Cenozoic aridification, whereas the shift at ~3.5 Ma may reflect the large-scale expansion of C 4 plants.

Geochemistry and mineralogy of the Oligo-Miocene sediments of the Valley of Lakes, Mongolia

“…2), which can be assigned to a distinct early Oligocene aridification in Central Asia, a feature confirmed by the δ 18 O data. This aridification step has to be differentiated from the well-known Eocene–Olicocene Transition aridification event at ~33 Ma (Meng and McKenna 1998; Dupont-Nivet et al 2007; Kraatz and Geisler 2010; Abels et al 2011; Wang et al 2012; Bosboom et al 2014; Caves et al 2014; Li et al 2016) as it post-dated it clearly. This shift at ~31 Ma correspond to the boundary between Biozone A and B.…”

“…δ 18 O values of pedogenic carbonates are mainly determined by the δ 18 O composition of the soil water, which in turn is strongly related to the δ 18 O composition of local meteoric water, being affected by soil temperature and evaporation rate (Cerling 1984; Quade et al 1989; Li et al 2016). It has been shown that in semi-arid and arid areas, the ratio soil water evaporation/amount of precipitation has the most significant effect (Quade and Cerling 1995; Li et al 2016).…”

“…It has been shown that in semi-arid and arid areas, the ratio soil water evaporation/amount of precipitation has the most significant effect (Quade and Cerling 1995; Li et al 2016). …”

“…The δ 13 C peak is not seen at the transition between the Tibetan and the Chinese Loess Plateaus (Li et al 2016). However, an increase in δ 18 O could be seen in the Lanzhou basin (Li et al 2016) and in some section of the south Tarim Basin and of the Qaidam Basin (Kent-Corson et al 2009). This discrepancy could be explained by changes in air-masses circulation, which have only localy an impact on soil productivity.…”

“…It is well recorded in δ 18 O profiles established on the basis of deep-sea benthic foraminifers (Zachos et al 2001; Coxall et al 2005). In Central Asia, this transitional phase started in the late Eocene and is marked by a sudden acceleration of the aridification (Dupont-Nivet et al 2007; Xiao et al 2010; Abels et al 2011; Bosboom et al 2014; Li et al 2016) and by a dramatic faunal turnover, the “Mongolian Remodeling” (Meng and McKenna 1998; Kraatz and Geisler 2010; Harzhauser et al 2016). The EOT aridification has been assigned to coupled global cooling, stepwise retreat of the proto-Paratethys Sea (Abels et al 2011; Bosboom et al 2014; Caves et al 2015) and intensified uplift of the Tibetan and Mongolian Plateaus (Wang et al 2012; Caves et al 2014).…”