PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE Mineralogical and Geochemical Discrimination of the 10.1002/2017GC007060 Occurrence and Genesis of Palygorskite in Eocene Sediments Key Points: on the Northeastern Tibetan Plateau Two different palygorskite types in Eocene fluvial sediments in the Chengcheng Ye1,2, Yibo Yang1,3, Xiaomin Fang1,3 , Hanlie Hong4, Weilin Zhang1,3, Qaidam Basin Rongsheng Yang1,2, Bowen Song5, and Zhiguo Zhang1,2 Well-crystallized palygorskite formed during postdeposition while poorly 1Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of crystallized palygorskite is 2 3 catchment-delivered detritus Sciences, Beijing, China, University of Chinese Academy of Sciences, Beijing, China, CAS Center for Excellence in Tibetan 4 Poorly crystallized palygorskite Plateau Earth Sciences, Beijing, China, Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei, China, suggests a less humid climate after 5Institute of Geological Survey, China University of Geosciences, Wuhan, Hubei, China the Early Eocene Climate Optimum Supporting Information: Abstract Palygorskite is a widely used indicator of semiarid to arid environments in paleoclimate studies. Supporting Information S1 In this study, we present detailed mineralogical and geochemical investigations exploring the genesis of Table S1 palygorskite found in Eocene fluvial sediment in the northern Qaidam Basin on the northeastern Tibetan Plateau. The presence of two types of palygorskite is revealed, based on their crystallinity characteristics Correspondence to: and distinctive rare earth element (REE) patterns in the coexisting clay fraction. Well-crystallized palygorskite X. Fang, [email protected] samples are characterized by remarkably negative Ce anomalies and obvious middle rare earth element enrichment. Poorly crystallized palygorskite samples generally exhibit positive Ce anomalies and less Citation: pronounced middle rare earth element enrichment, which resemble those of nonpalygorskite-bearing clay Ye, C., Yang, Y., Fang, X., Hong, H., samples. Given the presence of an overall oxidized fluvial sedimentary environment, we attribute the well- Zhang, W., Yang, R., et al. (2018). crystallized palygorskite (which has textures comprising long, interwoven fibers) to direct precipitation (i.e., Mineralogical and geochemical discrimination of the occurrence and neoformation) occurring within a reducing environment during early/postdepositional processes while the genesis of palygorskite in Eocene poorly crystallized palygorskite (which is characterized by short, club-shaped single crystals) originates as sediments on the northeastern Tibetan catchment-delivered detritus. These poorly crystallized palygorskites occur mostly in 49.5–47.0 Ma and are Plateau. Geochemistry, Geophysics, Geosystems, 19, 567–581. https://doi. accompanied by decreasing kaolinite content, increasing chlorite content, and abundant xerophytic spore- org/10.1002/2017GC007060 pollen from the Qaidam Basin, and its neighboring Xining Basin. Collectively, these evidences suggest that a less humid climate followed after the Early Eocene Climate Optimum. Received 8 JUN 2017 Accepted 27 JAN 2018 Accepted article online 5 FEB 2018 Published online 1 MAR 2018 1. Introduction The Cenozoic phase of the uplift of the Tibetan Plateau exerted great influence on global climate change (e.g., An et al., 2001; Kutzbach et al., 1989; Raymo & Ruddiman, 1992). To date, the interplay between late Cenozoic uplift and regional climate change is well documented (e.g., An et al., 2001; Harris, 2006; Li et al., 2014; Molnar, 2005; Nie et al., 2017; Yang et al., 2017). However, current knowledge of climate change dur- ing the early Cenozoic in the surrounding area of the Tibetan Plateau is limited. The primary obstacle is the lack of unambiguous climatic archives with precise age controls recorded in frequently interbedded con- glomerates and facies changes. Therefore, to further divulge the relationship between the uplift of the Tibetan Plateau and regional climate change in the early Cenozoic, it is essential to construct a long-term paleoclimate record with precise age control and suitable proxies. The Qaidam Basin, which is located on the northeastern (NE) Tibetan Plateau, is a closed intramontane basin filled with a continuous series of Cenozoic sediments that are up to 12,000 m thick (Fang et al., 2007). The thick Cenozoic sediments received from the surrounding mountains record the interaction of mountain uplift, erosion, and climate change (Fang et al., 2007; Sun et al., 2005; Zhuang et al., 2011). It is therefore an ideal location in which to explore the uplift history of the NE Tibetan Plateau and the regional climate changes that occurred throughout the Cenozoic. Until now, long-term early Cenozoic paleoclimatic archives from the Qaidam Basin were based on data collected from pollen assemblages (Lu et al., 2010; Wang et al., VC 2018. American Geophysical Union. 1999), the carbon, and oxygen isotopic compositions of carbonates (Kent-Corson et al., 2009; Rieser et al., All Rights Reserved. 2009), clay minerals or bulk mineralogies (Wang et al., 2011, 2013), and geochemical proxies (Song et al., 567 Geochemistry, Geophysics, Geosystems 10.1002/2017GC007060 2013). However, these studies have either lacked precise age control (Wang et al., 1999) or suitable proxies with clear paleoclimatic significance in long sedimentary sequences containing frequently interbedded con- glomerates, and facies changes (Song et al., 2013). Therefore, constructing an unambiguous paleoclimate proxy record with well-established age control is crucial for further exploring the interplay between the uplift phases of the NE Tibetan Plateau and the changes in regional climate that occurred during the early Cenozoic. Palygorskite, a Mg-rich phyllosilicate with a fibrous morphological texture, has practical value in industrial production and is of great importance in paleoclimate studies. Palygorskite occurs in diverse geological environments and is geographically distributed within the midlatitude arid zone located between 208N– 408N and 108S–358S in late Neogene terrestrial and marine deposits (Callen, 1984). Therefore, palygorskite is widely accepted as a paleoclimatic indicator of semiarid to arid environments (e.g., Hill et al., 2017; Hong et al., 2007; Knidiri et al., 2014; Singer, 1979; Singer & Galan, 2011; Weaver & Beck, 1977). The occurrence of palygorskite in basin sediment is attributable to detrital or authigenic origins (Singer & Galan, 2011). Detrital palygorskite is generated in soils during catchment weathering and then transported into a subaqueous environment (Bouza et al., 2007; Khademi & Mermut, 1998). Authigenic palygorskite is formed by the trans- formation of other precursor minerals as well as by direct precipitation from Mg-Si-rich solutions (i.e., neo- formation; Monger & Daugherty, 1991; Singer & Norrish, 1974). Palygorskite that originates from transformation undergoes formation from a preexisting structure, in which part or all of the precursor struc- ture is inherited (Singer & Galan, 2011). As this transformation mostly occurs during diagenesis in sedimen- tary environments, palygorskite with a transformation origin could also be defined as diagenetic palygorskite. The potential precursor minerals of diagenetic palygorskite include smectite (Hillier & Phar- ande, 2008; Yaalon & Wieder, 1976), irregular illite/smectite mixed layer clays (I/S) (Patil & Surana, 1992), chlorite (Galan et al., 1975; Hong et al., 2007) or primary silicates (Paquet, 1983). Neoformed palygorskite generally forms a new mineral structure from simple or complex ions without the inheritance of a preexist- ing mineral structure during its crystallization. Neoformation can occur in catchment soil weathering (here we attribute this type of palygorskite as detrital origin as mentioned above), syngenetic (i.e., depositional), and diagenetic environments. Therefore, only detrital and authigenic palygorskite with a syngenetic or penecontemporaneous origin in basin sediment can be used as a paleoclimate indicator, whereas palygor- skite that has formed during a postdepositional process should not be used in paleoclimate reconstructions. Palygorskite is widely distributed in the Cenozoic basin sediments located on the northern Tibetan Plateau (e.g., Hong et al., 2007; Ma & Wu, 1994; Wang et al., 2013; Yin et al., 2010; Zhao, 2003). However, comprehen- sive studies of the origin and genesis of palygorskite are scarce. In this paper, we present detailed mineral- ogical and geochemical investigations to describe the occurrences and possible formation mechanisms of palygorskite in the lowest strata of the Hongliugou (HLG) section in the northern Qaidam Basin. These deposits are precisely dated, using high-resolution magnetostratigraphy (Zhang, 2006), to the 54.0–45.0 Ma, and thus cover the Early Eocene climatic optimum (EECO, ca. 52–50 Ma) as well as the following period of long-term cooling. The EECO and the following long-term cooling has been well documented in marine archives (Zachos et al., 2001); however, this dramatic event is poorly understood in continental records. Since palygorskite is a widely used indicator of semiarid to arid environments, therefore, this area thus pro- vides a rare opportunity to explore the climate shift before/after EECO. 2. Geological Setting The Qaidam Basin located on the NE Tibetan Plateau. It is a closed intramontane