Paleoclimate Variations Spanning the Past 29–4 Ma Inferred From Lipid Biomarkers and Carbon Isotopes in the Linxia Basin, Northeast Tibetan Plateau

Thick and continuous deposits in the Linxia Basin, located in the landing area of westerlies and monsoons, offer a good opportunity for understanding the climatic evolution and tectonic activities. However, detailed paleoclimate reconstruction based on lipid biomarkers was rare, which limited our further knowledge, even though there were some relevant reports regarding pollen assemblages, microbial communities. In the present study, we conducted systematic analyses on the lipid biomarkers and carbon isotope values of the sediments, in an effort to reconstruct the evolution history of paleoclimate and figure out the potential driving mechanism. Our results showed that the organic matter was from mixed sources including lower organisms and terrestrial higher plants. The organic matter sources varied in response to the change of paleoclimate conditions as revealed by the lipid-derived proxies and organic carbon isotopes. Significant climatic events like late Oligocene Warming, Mid-Miocene Climatic Optimum and aridification at ∼8.5 Ma were observed throughout the sequence. Our results further indicated that the paleoclimate conditions in the study area primarily followed the pace of long-term global cooling, and the aridification at ∼8.5 Ma was associated with the uplift of the Tibetan Plateau.

Introduction

The increasing research hotspots of global environment and climate change has recently received a great attraction. Especially since the late Cenozoic, a series of major geological events have taken place on the earth, resulting in great changes in global climate and environment, such as global cooling (Miller et al., 1987; Miller et al., 1991; Zachos et al., 2001; Molnar et al., 2010; Haider et al., 2013; Zhuang et al., 2014; Kern et al., 2016), the aridity in north America and Asia (Ruddiman and Kutzbach, 1989; Manabe and Broccoli, 1990; Kutzbach et al., 1993; Rea et al., 1998; Wu et al., 2007; Wan et al., 2010; Tang et al., 2011; Kita et al., 2014; Jia et al., 2015), formation of the Asian monsoon (Quade et al., 1989; Li, 1995; An et al., 2001; Liu et al., 2008; Miao et al., 2011; Miao et al., 2012; Miao et al., 2013b; Liu et al., 2013; Li et al., 2014; Sun et al., 2014; Liu et al., 2015). Among these geological events, the aridity in North America and Asia is a major scientific problem, which has the most profound and direct impact on human living environment. As the largest temperate arid zone in the world, the onset time, evolution sequence, formation process and driving mechanism of aridity in inland Asia are the research hotspots of environmental change in the Northern Hemisphere of Cenozoic era.

In terms of the development trend of inland aridity in Asian, the key scientific problem to be resolved is to reveal the onset time, evolution sequence, formation process of inland aridity in Asian (Sun and Wang, 2005), so as to explore its relationship with the driving mechanism (Boos and Kuang, 2010; Shi et al., 2011; Miao et al., 2012; Li et al., 2014). There have been different views on the driving mechanism of the evolution of Asian inland aridity in the late Cenozoic. At present, it is mainly reflected in three aspects: the uplift of the Tibet Plateau, the global cooling of the Cenozoic and the retreat of the Paratethys Sea. Previous studies have shown that the uplift of the Tibetan Plateau, the global cooling of the Cenozoic (the expansion of the arctic ice cap) and the evolution and retreat of the Paratethys Sea have undoubtedly played an important role in the formation and intensification of inland aridity in Asia (Sun et al., 2008; Bosbom et al., 2011; Qiang et al., 2011; Miao et al., 2012; Chi et al., 2013; Haider et al., 2013; Bosbom et al., 2014a; Bosbom et al., 2014b; Bosbom et al., 2014c; Li et al., 2014; Sun et al., 2014; Zhang et al., 2015). However, owing to the different sensitivity of different climatic indicators and the sparse of complete and continuous records, the primary driving mechanism for the evolution of inland aridity in Asian is still under debate.

The use of fossil molecules, such as n-alkanes, alkanoic acids, alkenones, GDGTs and those derived from specific biological sources, and δ¹³C values of TOC to reconstruct paleoclimate has become an important part of molecular stratigraphy (Xie et al., 2003; Fan et al., 2007; Weijers et al., 2007; Wang et al., 2012; Wu et al., 2018; Tian et al., 2019; Wang et al., 2021a; Wang et al., 2021b). Wang et al. (2012) analyzed the n-alkanes and n-alkan-2-ones of the sediments collected from the Linxia Basin, and proposed that these biomarkers was in agreement with the palynofloras showing significant variations in response to the climate changes and uplift of the Tibetan Plateau at ∼8 Ma. Furthermore, Wu et al. (2018) applied the organic carbon isotopes (δ¹³C_TOC) to reveal the ecological response to the Eocene/Oligocene transition in the Lanzhou Basin, and found that the variations of δ¹³C_TOC responded to the global cooling. All these indicators displayed great potential in paleoclimate reconstruction in inland Asia.

Northwest China is mainly made up of large inland basins, with extra-thick Cenozoic sediments accumulated. The Linxia Basin is one of them and rich in lipids with weak diagenesis, therefore, is an ideal recorder of arid climatic changes. Previous studies have carried out relevant analyses in the Linxia Basin to establish the general framework of paleoclimate evolution, including low-resolution lipid biomarkers (Wang et al., 2012), microbial communities (He et al., 2020), pollen assemblages (Ma et al., 1998). Nevertheless, detailed paleoclimatic characteristics were limited due to the sample resolution. In the present study, on the basis of previous results, we carried out a systematically high-resolution analyses of lipid biomarkers and δ¹³C values of TOC of the Maogou sediments in the Linxia Basin, in hope of revealing the process of arid environmental changes and understanding the driving mechanism of the evolution of Asian inland aridity in the late Cenozoic.

Geological Setting and Chronological Framework

The Linxia Basin is located on the northeastern edge of the Tibetan Plateau (102°30′–104°E, 35°10′–35°51′N, Figure 1). Sedimentary rocks of the Linxia Basin are mainly composed of mudstone and sandstone of fluvial and lacustrine origin (Fang et al., 2003; Zhang et al., 2019). Climatically, the Linxia Basin is now dominated by an arid/semiarid continental climate with mean annual temperature (MAT) of 6.3°C, mean annual precipitation of 537 mm, and mean annual evaporation of 1,198–1745 mm. Rainfall mainly occurs between June and August, generally following a typical monsoon pattern.

FIGURE 1

FIGURE 1. (A) Location of the Linxia Basin (red circle) and the arrow indicating the present main climate controls of China, including southwest summer monsoon, east Asian summer monsoon and winter monsoon (modified from Shao et al. (2006)) (B) Simplified geological map of the Linxia Basin. Red pentacle representing the Maogou section (cited from Fang et al., 2003) (C) outcrop photograph of the Dongxiang Formation of the Maogou section.

The Maogou section in this research is in the central part of the Linxia Basin (Figure 1), which is representative of the tertiary red bed. The Maogou section is characterized by continuous sedimentary strata with a total thickness of 443 m. Li (1995) and Fang et al. (2003) have provided a high-resolution paleomagnetic analysis of the Maogou section to establish the chronological framework. Basically, from the bottom to the top of Maogou section, identified as Tala Formation (0–91 m, ∼29–21.4 Ma), Zhongzhuang Formation (92–188 m, 21.4–14.7 Ma), Shangzhuang Formation (189–230 m, 14.7–13.1 Ma), Dongxiang Formation (231–313 m, 13.1–7.8 Ma), Liushu Formation (314–376 m, 7.8–6 Ma) and Hewangjia Formation (377–443 m, 6–4.3 Ma), the magnetostratigraphy results indicated that the sediments from the Maogou section deposited continuously from 29 to 4.3 Ma, spanning from the upper Oligocene to the Pliocene. It is also should be pointed out that there were abundant mammalian fossils which can serve as clear indications of chronology (Deng et al., 2004; Deng et al., 2013). Consequently, the chronological framework was established reasonably and precisely.

Sampling and Experiments

Sampling

Actually, our sediment samples were collected from the same section as Li (1995) and Fang et al. (2003) proposed. In order to get the fresh and uncontaminated sediments, an exploratory trench (0.5 m deep and wide) was dug. And then a total of 229 unweathered sediment samples were collected from the trench from the bottom to the top of the Maogou section, from the Tala Formation to the Hewangjia Formation, generally with the sampling interval of 2 m. Most of the samples were composed of silt sandstones, mudstones. All the sediment samples were marked with thickness and stored at -20 C for further analyses.

Sample Pretreatment and GC-MS Analysis

The samples (150 g each) were powdered (80 mesh) and extracted with dichloromethane/methanol (93:7) in a Soxhlet extractor for 70 h, and the solvent was removed by distillation. The extracts were condensed and weighed. Because the extracted material was only 1–5 mg, fractionation using chromatography on silica gel or alumina was avoided to prevent the potential loss of components such as the trace alkanes and the oxygen compound. After natural air-drying and dilution using chloroform, the total extracts were analyzed directly using GC-MS. The blank samples were run under similar conditions. The GC-MS spectra of blank samples showed that the lipid molecules to be discussed in this paper were not found.

GC-MS analysis was performed using a HP 5973 MSD interfaced to a HP 6890 gas chromatograph fitted with a 30 m × 0.25 mm i. d. fused silica capillary column coated with a film (0.25 μm) of 5% phenyl-methyl-DB-5. For routine GC analysis, the oven was programmed from 80 to 300°C at 3°C/min with an initial and final hold time of 1 and 30 min, respectively. Helium was used as carrier gas at a linear velocity of 32 cm/s, with the injector operating at a constant flow of 0.9 ml/min. The MS was operated with an ionisation energy of 70 eV, a source temperature of 230°C and an electron multiplier voltage of 1900 V over a range of 35–550 Da.

Carbon Isotopes of Organic Matter (δ¹³C_org)

An aliquot of each sample was acidified with HCl to remove carbonates before analysis. Then, samples were washed with deionized water until a neutral pH value was reached. Next, samples were dried in an oven at 90°C. Pretreated samples were analyzed using a Flash 2000-MAT 253 system. The Flash 2000 was fitted with an oxidation-reduction tube filled with Cr₂O₃, Cu and silver-bearing CoO. Treated samples were oxided at 960°C with flowing oxygen. Helium was used as carrier gas. IAEA-600 (caffeine) was used as standard sample (Coplen et al., 2006). Each sample was analyzed twice, and final averaged results were expressed as ‰ relative to the VPDB (Vienna Peedee Belemnite) standard. During the analysis, 15 standard samples were determined to monitor the accuracy of the instrument. Reproducibility, as determined from the replicate standard samples, were better than 0.05‰ for δ¹³C_org of organic carbon.

Results

Distribution Patterns of n-alkanes, n-Alkane-Derived Proxies and Carbon Isotopes of Bulk Organic Matter

Basically, the n-alkanes ranges from C₁₆ to C₃₂ with an obvious maximum carbon number at C₁₈ throughout the entire section, characterized by unimodal distribution patterns. Moreover, some samples show a significant bimodal distribution, with C₁₈ and C₂₉ as the main peaks. A distinct odd-over-even predominance of the long-chain n-alkanes is observed throughout the profile. Representative relative abundance of n-alkanes from the Maogou section sediments are shown in Figure 2.

FIGURE 2

FIGURE 2. Representative chromatographs of n-alkanes (m/z 85) in the Maogou sediments.

In terms of the n-alkane derived proxies, the ratio of C₂₇/C₃₁ and (C₁₇-C₂₁)/(C₂₇-C₃₁) values were calculated in this study. Specifically, the ratio of (C₁₇-C₂₁)/(C₂₇-C₃₁) can be calculated using the following equations:

$\left({\text{C}}_{17}{-\text{C}}_{21}\right)/\left({\text{C}}_{27}{-\text{C}}_{31}\right)=\left({\text{C}}_{17}{+\text{C}}_{18}{+\text{C}}_{19}{+\text{C}}_{20}{+\text{C}}_{21}\right)/\left({\text{C}}_{27}{+\text{C}}_{28}{+\text{C}}_{29}{+\text{C}}_{30}{+\text{C}}_{31}\right)$

Besides, the δ¹³C_org values of the Maogou section sediments varies between -31.0‰ and -22.0‰, with an average value of -26.0‰ (Figure 3C).

FIGURE 3

FIGURE 3. Comparison of different proxies from the Maogou sediments with other indicators, including the ratio of (C₁₇-C₂₁)/(C₂₇-C₃₁) (a), the C₂₇/C₃₁ ratio (b), the δ¹³C_org values of organic matter from Maogou sediments (c), the percentage of tree pollen from the Yanwan section, Tianshui Basin (d) (Hui et al., 2011), the PC1 scores from the Jingou River section in northern Tian Shan, with higher PC1 scores denoting wetter conditions (e) (Tang et al., 2011), the atmospheric CO2 concentrations reconstructed based on the carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifer (f) (Pagani et al., 1999), and the compiled δ¹⁸O records (g) (Westerhold et al., 2020). Further comparison containing the sedimentary facies, lithology, age-depth model modified from He et al. (2020), and vegetation evolution based on pollen records from Ma et al. (1998).

Discussion

Inputs of Organic Matter

Typically, n-alkanes are widely distributed in sedimentary organic matter derived from organisms living in the study area and their catchments, and different compositions of n-alkanes can directly reflect different biota (Pearson et al., 2007). Normally, the ratio of (C₁₇-C₂₁)/(C₂₇-C₃₁) of n-alkanes can indicate the relative contribution from lower organisms including algae, cyanbacteria, fungi and microbes relative to that from terrestrial higher plants, and thus the corresponding climate conditions (Eglinton and Hamilton, 1967; Cranwell et al., 1987; Rielley et al., 1991; Meyers and Ishiwatari, 1993; Ficken et al., 2000). Besides, the ratio of C₂₇/C₃₁ is usually applied to indicate the relative abundance of trees versus grasses, as trees produce high abundance of C₂₇ or C₂₉ n-alkane, while grasses typically generate high C₃₁ n-alkane. Consequently, a high C₂₇/C₃₁ ratio indicates a thrive of trees at the expense of grasses, and thus more contribution from trees to the organic matter (Cranwell et al., 1987).

In our study, abundant n-alkanes were detected in all the 229 samples from the Maogou section. Generally, the n-alkanes displayed bimodal or unimodal distribution patterns, suggesting that the organic matter were contributed by mixed sources, including lower organisms and terrestrial higher plants. Generally, the ratio of (C₁₇-C₂₁)/(C₂₇-C₃₁) and C₂₇/C₃₁ displayed a similar variation trend throughout the sequence (Figures 3A,B), indicating the variations of organic matter sources. From 29 to 21.8 Ma, the ratio of (C₁₇-C₂₁)/(C₂₇-C₃₁) and C₂₇/C₃₁ remained relatively low-value stage except some fluctuations, suggesting a high proportion of grasses contributed to the organic matter. During the period of 21.8–18 Ma, both the (C₁₇-C₂₁)/(C₂₇-C₃₁) and C₂₇/C₃₁ values increased, implying the source of organic matter changed and lower organisms and trees flourished during this interval. From 18 to 14 Ma, the lipid-derived proxies showed a decreasing trend, indicating the sources of trees and lower organisms were limited at this time. Between 14 and 4.3 Ma, our records indicated that the contribution from lower organisms constantly remained constrained, and the abundance of trees slightly increased. According to these observations, we may infer that different organic matter sources at different timescales corresponded to the varying climate conditions.

Paleoclimate Conditions Inferred From δ¹³C_org and Lipid-Derived Proxies

Terrestrial high plants can normally be divided into three categories, C₃ plants, C₄ plants and CAM plants based on their photosynthesis characteristics and carbon atomicity. C₃ and C₄ plants can be identified according to their carbon isotope values, as C₃ plants are characterized by an δ¹³C value ranging from -40‰ to -20‰ with an average of -27‰, and C₄ plants have a distinct δ¹³C value between -19‰ and -9‰ with an average value of -12‰, respectively (Smith and Epstein, 1971; Oleary, 1981). It is also generally acknowledged that C₄ plants adapt to an environment which is typical of high temperature, high aridity and low atmospheric CO₂ concentration (Wang and Greenberg, 2007). Particularly, C₄ plants have greater water-use efficiency than C₃ plants (Collatz et al., 1998). In the present study, there is no exact relationship between the δ¹³C_org values from the Maogou sediments and the reconstructed atmospheric CO₂ concentrations based on the carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifer (Pagani et al., 1999), suggesting the concentration of CO₂ may not be the main factor influencing the vegetation in the Linxia Basin (Figure 3). Additionally, previous study indicated that terrestrial higher plants contributed significantly to the organic matter in the sediments in the Linxia Basin (Fan et al., 2007). Consequently, the δ¹³C_org values may reflect the information of the vegetation communities and its associated climate conditions, such as temperature and/or precipitation (Wang and Deng, 2005; Kohn, 2010; Wu et al., 2018). Overall, the δ¹³C_org values of the sediments throughout the ∼29 to 4.3 Ma varied from -31.0‰ to -22.0‰ (Figure 3C), indicating that C₄ grasses were possibly not significant in the Linxia region prior to 4.3 Ma, which is consistent with the conclusion proposed by Fan et al. (2007).

In terms of this background, we attributed the obvious fluctuations of δ¹³C_org values to the climate variations somewhat arbitrarily, with positive δ¹³C_org values corresponding to a relatively high abundance of C₄ plants. In the present study, the records of δ¹³C_org values seemed more sensitive to climate variations as it revealed more stages of climate shifts. From 29 to 21.8 Ma, the δ¹³C_org values exhibited an overall increasing trend, indicating a relatively high proportion of C₄ plants. Coincidently, the C₂₇/C₃₁ ratio displayed a decreasing trend, suggesting an expansion of grasses. As previous study has confirmed that the grasses usually applied the C₄ photosynthesis (Wang and Greenberg, 2007), our results indicated that the vegetation evolution from trees to grasses reflect the vegetation type variations from C₃ to C₄ plants under the relatively warm and dry climate condition (Ao et al., 2021). The pollen assemblages also indicated an open woodland-steppe vegetation mainly composed of Chenopodipollis, Polygonaceae, Compositae for the herbs and Quercoidites, Betulaepollenites, Fraxinoipollenites and Cupressaceae for the woody plants under a dry condition during this time (Ma et al., 1998). While between 21.8 and 18 Ma, the C₄ grasses shrank while the C₃ trees flourished in this interval, implying a gradual increase in humidity, as indicated by the decreasing δ¹³C_org values and high C₂₇/C₃₁ ratios, corresponding to a general coniferous forest (Ma et al., 1998). During this interval, the high (C₁₇-C₂₁)/(C₂₇-C₃₁) ratio possibly indicated an increasing contribution from lower organisms to the organic matter. Normally, the δ¹³C_org of the typical lake algae in fresh water is about −28‰ (Meyers, 1994; Meyers, 2003), thus enhanced inputs of lower organisms may result in negative δ¹³C_org values in our records. In addition, the sedimentary face was identified as floodplain-shallow lake (Fang et al., 2016), furthering indicating an increasing humidity. Thereafter, a sharp increase in δ¹³C_org values demonstrated that C₄ grasses increased at the expense of C₃ trees from 18 to 14 Ma, indicating the climate was turning to warm and dry conditions gradually, possibly corresponding to the MMCO (Figure 3D) (Zachos et al., 2001; Zan et al., 2015). During this interval, the C₂₇/C₃₁ and (C₁₇-C₂₁)/(C₂₇-C₃₁) ratio decreased, suggesting an enhanced contribution from terrestrial higher plants such as grasses and limited inputs of lower organisms, which was in good accordance with the increasing δ¹³C_org values. The warm and dry climate conditions was beneficial to the growth of grasses. Although this period was recognized as coniferous forest, the percentage of herbs was increasing (Ma et al., 1998). Specifically, the initial stage of the MMCO was characterized by a relatively humid condition as reflected by the proxies, which was also recorded by the pollen assemblages from the northern Tian Shan (Tang et al., 2011) and Linxia Basin (Ma et al., 1998). After the MMCO, the climate was generally dry from 14 to 9.1 Ma, although the humidity showed a slight increase, generally consistent with the vegetation type as coniferous and broad-leaved mixed forest. A rapid increase in δ¹³C_org values and a sharp decrease in C₂₇/C₃₁ and (C₁₇-C₂₁)/(C₂₇-C₃₁) ratio within only less than 1 Ma from 9.1 to 8.5 Ma were observed, suggesting a significantly strengthened aridity at this time under the background of long-term global cooling in the late Cenozoic (Zachos et al., 2001; Westerhold et al., 2020), in accordance with the steppe forest expansion at ∼8.5 Ma (Ma et al., 1998). This aridity sustained to ∼6.4 Ma and then the humidity displayed a slight increase afterwards. Coincidently, the vegetation evolved towards coniferous and broad-leaved mixed forest, implying the climate became relatively wet (Ma et al., 1998). Considering the refined age-depth model and sample resolution, our records generally agreed with the pollen analysis.

It also should be pointed out that throughout the sequence, the variations of δ¹³C_org values corresponded well with the changes of lithology basically. This interesting phenomenon was primarily observed in the Dongxiang Formation sediments. The high δ¹³C_org values occurred when the gray-green clay deposited, and the red bed was characterized by low δ¹³C_org values. Further studies are necessary to figure out the potential driving mechanisms regarding the frequent fluctuation of the δ¹³C_org values.

Comparison With Regional and Over-Regional Paleoclimate Records

As mentioned above, our records based on the n-alkane-derived proxies and δ¹³C_org values are overall in good accordance with the pollen records in the Linxia Basin (Ma et al., 1998). Furthermore, the sporopollen records from the Tianshui Basin in the NE Tibetan Plateau demonstrated a temperate, warm-temperate broad-leaved forest between 17.1 and 14.7 Ma, forest or forest-steppe between 14.7 and 11.7 Ma, broad-leaved forest during 11.7–8.5 Ma and steppe vegetation from 8.5 to 6.1 Ma (Hui et al., 2011). Similarly, the sporopollen reports in the Jiuxi Basin revealed a semi-moist climate from 13 to 11.2 Ma, warm and moist climate between 11.2 and 8.6 Ma, warm and semi-moist climate during 8.6–5.6 Ma, semi-arid climate during 5.4–4.9 Ma and arid climate condition in the interval of 4.9–2.2 Ma (Ma et al., 2005). Also, Tang et al. (2011) reported that the climate was wet during the late Oligocene and shifted to dry conditions between 23.8–17.3 Ma, and subsequently ameliorated to a relatively wet stage to 16.2 Ma, but then turned to dry condition until 4.2 Ma and the aridity reached a peak at 13.5 Ma. Despite the lack of long-term comparison of paleoclimate records, our results are in good concert with the periodical reports from previous studies.

Moreover, our reconstructed paleoclimate conditions based on the n-alkane-derived proxies and the δ¹³C_org values exhibited significant variations especially during the typical climate events, which behaved synchronously with previous studies. During the period of ∼26 to 25 Ma, our results suggested the grasses expanded, and contributions from trees and lower organisms declined, roughly in association with the late Oligocene Warming (Zachos et al., 2001). Sporopollen records from the Nima Basin indicated that broad-leaved trees increased obviously after 25.6 Ma, in response to the late Oligocene warming, further suggesting the development of south Asian monsoon and accompany of global warming (Wu et al., 2019). At ∼23 Ma, the increase of the (C₁₇-C₂₁)/(C₂₇-C₃₁) and C₂₇/C₃₁ ratio implied that trees and lower organisms characterized by negative the δ¹³C_org values increased, leading to the decrease of δ¹³C_org values from the sediments. This most apparent explanation for the change of organic matter source at ∼23 Ma is the Mi-1 Glaciation as revealed by the compiled oxygen isotope records (Zachos et al., 2001). Palynological results from the Xining Basin showed the cold-tolerant conifers shrived while thermophilic plants declined during the Mi-1 Glaciation at ∼23 Ma (Miao et al., 2013a). While during the interval of ∼18 to 14 Ma, the (C₁₇-C₂₁)/(C₂₇-C₃₁) ratio and C₂₇/C₃₁ value were decreasing, while the δ¹³C_org values exhibited an opposite variation, which suggested that C₄ grasses gradually flourished when the warm and wet climate shifted to warm and dry conditions during the MMCO (Zachos et al., 2001; Westerhold et al., 2020).

The pollen records from both the Tianshui Basin and northern Tian Shan suggested a warm and humid early stage of MMCO turned to warm and dry conditions in the latter periods (Hui et al., 2011; Tang et al., 2011). Moreover, the timing and duration of the MMCO was reported from nearby records as 18–14 Ma in the Qaidam Basin (Miao et al., 2011; Miao et al., 2016) and 17–14 Ma in the Xining Basin (Zan et al., 2015).

Our records demonstrated another significant climate shift at ∼9 to 8 Ma. During this period, the (C₁₇-C₂₁)/(C₂₇-C₃₁) and C₂₇/C₃₁ value decreased dramatically, and the δ¹³C_org values of sediments showed a sudden increase within less than 1 Ma, suggesting a prominent plant variation from C₃ to C₄ plants, which was ascribed to the enhanced aridification. Our previous work on n-alkan-2-ones and microbial communities from the Maogou section sediments also identified this severe climatic variation (Wang et al., 2012; He et al., 2020). Besides, the pollen assemblages also indicated an obvious aridification event happened at ∼8.5 Ma (Ma et al., 1998). The sporopollen data from the Tianshui Basin recorded a rapid development of steppe at about 8.5 Ma, which suggested a permanent drying of the Asian interior at this time (Hui et al., 2011). Yang et al. (2016) reported that carbonate-derived Sr concentrations and Sr/Ca ratios on the northeastern Tibetan Plateau exhibited weakened chemical weathering intensity and pedogenesis at ∼8.6 Ma corresponding to a cold and dry climate linked with the enhanced aridification. Furthermore, stable isotope evidence including the δ¹³C and δ¹⁸O values of carbonates and δ¹³C_org values from the Linxia Basin indicated that the most severe aridity occurred from 9.6 to 8.5 Ma (Fan et al., 2007).

In summary, based on these observations, the reconstructed paleoclimate conditions in the Linxia Basin generally followed the global climate variation trend. Specifically, our records indicated an overall warm and dry period of late Oligocene warming and MMCO, which was in favor of the growth of C₄ grasses, corresponding well with the global climate as revealed by the oxygen isotope records (Zachos et al., 2001; Westerhold et al., 2020). And the dry climate conditions spanning 14–4.3 Ma generally coincided with the global cooling since the expansion of polar ice-sheets at 14 Ma (Zachos et al., 2001). Under this circumstance, the expansion of C₄ grasses at 9–8 Ma cannot be sufficiently explained by the global climate trends. Thus, other possible factors need to be considered. The uplift of the Tibetan Plateau may be responsible for the aridification, as extensive evidence corroborated that the tectonic uplift of the Tibetan Plateau occurred approximately at ∼8 Ma (An et al., 2001; Lease et al., 2007; Miao et al., 2012; Yang et al., 2016). The uplift of the Tibetan Plateau resulted in a geographic barrier which blocked moisture transportation from neighboring oceans, leading to an arid continental interior due to the rain shadow effect (Boos and Kuang, 2010), and therefore an enhanced aridification. Moreover, Miao et al. (2012) proposed that the uplift of the TP resulted in the changes of topography, which strongly influenced the moisture pattern in Central Asia during Miocene times, particularly the precipitation rates in the Linxia Basin were significantly affected by the tectonic uplift of the surrounding mountains.

Conclusion

In the present study, lipid biomarkers and carbon isotopes of organic matter were analyzed to reconstruct the paleoclimate conditions based on the sediments from the Maogou section, Linxia Basin, northeast TP. Generally, the distribution patterns of n-alkanes and its derived proxies indicated that the organic matter of the Maogou section sediments were contributed by mixed sources, including lower organisms like algae, fungi, cyanbacteria and microbes, and terrestrial higher plants. The variations of organic matter sources and evolution of higher plants behaved in accordance with the carbon isotope values δ¹³C_org, serving as good indicators for paleoclimate conditions. Our results clearly demonstrated some typical climatic events, such as the late Oligocene Warming event, MMCO and aridification at ∼8.5 Ma. Generally, the paleoclimate conditions in the Linxia Basin were primarily controlled by the global climate variations, but the uplift of the TP was responsible for the aridification event at ∼8.5 Ma, which was superimposed on the long-term global cooling.

Data Availability Statement

The original contributions generated for this study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author Contributions

GW, YW and ZW contributed conception and design of the study. GW, TZ and XY carried out chemical analysis. WH, XM, TZ and XY contributed to the field work. GW wrote the first draft of the manuscript. All authors contributed to the article and approved the submitted version.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 41831176, 41902028, and 41972030), the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (Grant Nos. 2019QZKK0707), the Chinese Academy of Sciences Key Project (Grant Nos. XDB26020302), the CAS ‘Light of West China’ Program and the Youth Innovation Promotion Association CAS (No. 2021425).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

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