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Abstract The Eocene-Oligocene climate transition (EOCT) at ~34 Ma is a pronounced environmental reorganization that occurred in the Cenozoic era and has been extensively studied via marine archives. However, to date, there is still a less well-dated continental record across the EOCT, which can mostly be attributed to a lack of suitable proxies used in the varied continental settings in terms of sedimentary facies, provenance and climate, thus resulting in a poor understanding of the continental response to this transition. Located on the northeastern Tibetan Plateau, the Qaidam Basin is filled with continuous and well-exposed Cenozoic fluvio-lacustrine sediments. Cenozoic lacustrine sediment has been proven to be an ideal archive to study the continental climatic response to the EOCT. Here, we present the detailed mineralogical and elemental composition of chlorite, a widespread clay mineral in continental sediments formed by rock weathering, collected from a lacustrine sequence in the northern Qaidam Basin to constrain the onset of the EOCT and the related climatic response. The results show that chlorite abundance, crystallinity indices, intensity ratios of chlorite diagnostic peaks and chemical composition collectively exhibit remarkable changes at ~34.3 Ma. To be more specific, the well-crystallized Fe-Mg-rich chlorites were characterized by a high Fe content before the EOCT and turned into high-Mg-content Mg-chlorites with poor crystallinity. The chlorite chemical composition change is assumed to have been related to regional drying after the EOCT, thus providing new insights into the continental paleoenvironmental change based on this novel proxy. Highlights • The first chlorite-derived record of Eocene-Oligocene (EOCT) climate transition • Chlorite abundance, crystallinity and Fe/Mg ratio change together after the EOCT. • Well-crystallized Fe-Mg chlorites turned into Mg-chlorites with poor crystallinity. • Chlorite chemical composition changes linked with regional drying after the EOCT • Chlorite chemical composition can be used as a novel paleoclimatic proxy. [ABSTRACT FROM AUTHOR]

This study focuses on the concentration of major and trace elements in the diluted acetic acid-soluble fraction of SG-1 core lacustrine sediments from the western Qaidam Basin (Tibetan Plateau) in order to explore the potential of applying bulk carbonate analysis to tracing paleolake evolution and climate change. The SG-1 core sediments are considered highly suited to the exploration of the paleoenvironmental significance of bulk carbonate geochemistry, because most carbonates in the core are authigenic in origin. Based on a principal component analysis of the elemental composition of diluted acetic acid leachate, four major components with distinct characteristics have been extracted, accounting for ~ 80% of the total variance. The first and most important component is associated with a high positive loading of Mn, P, Y, Sc and rare earth elements (REE); the second component is characterized by a high positive loading of Ca, Sr, and U; the third component displays a high negative loading of Mg, Li, and B; and the fourth component is associated with a relatively high positive loading of Fe, V and Cr. After considering the geochemical behavior of these element assemblies during acid leaching, we have concluded that these four components contain information indicative of watershed climate conditions, carbonate precipitation, lake water salinity and iron oxide content, respectively. Reconstructions of watershed climate change and paleolake evolution using the component scores are comparable to those reconstructed from other independent proxy records, indicating a long-term trend of lake shrinkage and climatic drying since ~ 2.7 Ma with abrupt drying events at ~ 2.5 Ma, ~ 1.8 Ma, ~ 1.2 Ma and ~ 0.6 Ma. Our results suggest that lake carbonates bear more information about watershed climate and lake evolution than expected, and that occurrences of some elements, in particular trace elements, in the diluted acetic acid leachate are not solely and necessarily linked to carbonate minerals. [ABSTRACT FROM AUTHOR]

Using co-variations of Sr/Ca and Mg/Ca, we examined the carbonate compositions of various bedrocks (silicate and carbonate rocks) and sediments (eolian and fluvial sediments, sand, and topsoil) found in the NE Tibetan Plateau (TP) region. A combined carbonate composition dataset based on our results and other reported data shows that bedrock carbonate composition on the NE TP displays a much broader range of Sr/Ca and Mg/Ca ratios than restricted source carbonate endmembers reported upon in previous studies. This has clear implications for modern weathering studies in addition to paleo-reconstructions in this tectonically active and climatically variable area during the Late Cenozoic. Bedrock carbonate compositions are characterized by disseminated carbonates with higher Sr/Ca and Mg/Ca ratios, and sedimentary carbonates (mostly marine) with lower Sr/Ca, but variable Mg/Ca, ratios. The mostly authigenic carbonates found in sediments show similar trends, with a gradient ~ 0.97–1.00 in a plot of log (Sr/Ca) versus log (Mg/Ca), suggesting that ‘calcite precipitation’ processes – i.e. the sources of the dissolved cations in the water – control their chemistry. Based on observations and modeling, we conclude that the mixing of authigenic and bedrock carbonate endmembers, plus the incongruent dissolution of bedrock carbonates, accounts for the bulk carbonate composition of sediments ( e.g. loess, sand and topsoil). A comparison of bedrock and sedimentary carbonate composition with reported fluvial water data in the NE TP suggests that weathering of carbonates in terrigenous sediments, rather than in bedrock, is mostly responsible for the changes in fluvial Sr, Mg and Ca compositions. Our study suggests that interactions between carbonates and water occur widely during the exposure, transport and deposition of sediments, significantly modifying regional carbonate compositions and fluvial geochemistry. [ABSTRACT FROM AUTHOR]

• Clay-sized geochemistry reveals the Qaidam Basin (QB) Paleogene weathering history. • QB Paleogene silicate weathering intensity presents a continuously decreasing trend. • Global cooling was the main regulator of Paleogene chemical weathering processes. • Clay-sized geochemical indices aptly reveal weathering histories in coarse sediment. An integrated multiproxy dataset involving both mineralogical and geochemical investigations of the northern Tibetan Plateau helps in assessing regional weathering processes linked to topographic evolution and environmental change. However, changes in landscape and drainage reorganizations caused by active tectonics and remarkable climate change during the Cenozoic characterize the basins in and around the Tibetan Plateau by frequent facies changes and coarse lithologies, which largely limit the reliability of bulk geochemical indicators for revealing the regional weathering history. Here, we present detailed geochemical investigations of clay-sized sediments collected from the Hongliugou section (54–26 Ma) in the northern Qaidam Basin to reconstruct high-resolution chemical weathering records of the northern Tibetan Plateau. To prevent chlorite bias when calculating the chemical index of alteration (CIA) in the clay-sized sediments, we propose a modified CIA formula (CIA Mg+), in which Mg is added as a soluble cation that can be removed during weathering, similar to Na, K and Ca; thus, CIA Mg+ = Al 2 O 3 /(Al 2 O 3 + Na 2 O + K 2 O + CaO* + MgO) × 100. The CIA Mg+ and illite chemistry indices are generally the highest values in the early Eocene Climate Optimum (∼52–49.5 Ma) and a long-term decreasing trend is present from ∼ 49.5–26 Ma; the Mg/Al ratios of the clay-sized sediments and hydrochloric acid leachates are the lowest during the EECO and present a long-term increasing trend in the following 49.5–26 Ma, thus collectively indicating a continuous decrease in the regional chemical weathering intensity. The weakening weathering process during the Paleogene is mainly attributed to the decreasing global temperature and decline in precipitation caused by global cooling. Our study suggests that the proposed geochemical index based on clay-sized sediments can serve as an efficient proxy for regional chemical weathering reconstruction, even in thick sedimentary sequences with frequently changing facies and lithology. [ABSTRACT FROM AUTHOR]

Abstract: Phosphorus (P) provides an important biologically limiting nutrient in lake ecosystems, and its concentrations furnish significant information on lake nutrient status, climatic change, and landscape development within a drainage area. This study explores the potential of calcium-bound P and iron-bound P in calcareous lacustrine sediments to reveal variations of the paleolake nutrient status and its related climatic changes. Data come from the nearly 1000-m long core (SG-1) from the western Qaidam Basin (NE Tibetan Plateau), which has been well-dated by paleomagnetism to between 2.77 and 0.1 Ma. Calcium-bound P and iron-bound P make up the two most frequent phases of inorganic phosphorus pools in the calcareous sediment. This study compares the variations of calcium-bound P and iron-bound P with the independent proxy for biologic production proxy Total Organic Carbon (TOC) as well as a climatic proxy for lake level fluctuation from the borehole. Those results indicate that calcium-bound P responds sensitively to paleolake nutrients and climatic changes, while the iron-bound P responds to more complex factors, more specific, to the content of iron oxides. Variations of the calcium-bound P in the core SG-1 show a long-term, stepwise decrease, with shifts at ∼2.5 Ma, 1.8 Ma, 1.2 Ma, and 0.6 Ma, indicating a long-term stepwise decrease of the paleolake nutrient supply in the drainage. This suggests that the ecologic systems experienced a long-term deterioration, and the climate in central Asia became increasingly arid since after the late Pliocene. [Copyright &y& Elsevier]

Abstract: Manganese (Mn) in lake sediments reacts strongly to changes of redox conditions. This study analyzed Mn concentrations in oxides, carbonates, and bulk phases of the calcareous lacustrine sediments of a 938.5-m-long core (SG-1) taken from the western Qaidam Basin, well dated from 2.77Ma to 0.1Ma. Comparisons of extractions from diluted hydrochloric acid, acetic acid and citrate–bicarbonate–dithionite demonstrate that variations of Mn concentrations from acetic acid leaching (MnHOAc) are mostly responsible for Mn (II) fluctuations in the carbonate phase. Taking into account the relevant processes during weathering, transportation, deposition and post-deposition of Mn-bearing rocks, we conclude that Mn input from catchment weathering and paleolake redox condition provide the primary controls on variations in the Mn records of carbonate and oxide phases. We propose MnHOAc as a new sensitive indicator of paleolake redox evolution and catchment-scale climate change. The MnHOAc variations show a long-term upward decreasing trend, indicating a long-term decrease of Mn input from catchment weathering associated with increasing oxygen content in the paleolake bottom water. The great similarities of the MnHOAc record with other regional and global records suggest that paleolake redox changes and climatic drying in the Qaidam Basin may be largely related to global cooling. [Copyright &y& Elsevier]

The paleoenvironment and paleoclimate of Asia dramatically changed during the late Cenozoic, such as enhanced Central Asian aridification and East Asian monsoon development. The uplift of the Tibetan Plateau in the late Cenozoic is considered one of the major drivers forcing the Asian paleoenvironmental changes. However, the complex interaction between tectonic and climate change is still unclear. This is largely due to 1) the combined effect of aridification of the continent by global cooling, oceanic closure and uplift of the plateau, and 2) the lack of temporally and spatially matching tectonic and climatic records in a critical zone. The Xining Basin lies in the transitional zone between the arid Asian interior and the East Asian monsoon region. Here, we present detailed paleomagnetic analysis of a fluvial-lacustrine sequence in the Xining Basin and the first record of clay mineralogy and elemental geochemistry. High-resolution paleomagnetism revealed 16 normal and 16 reversed zones that correlate well with chrons 3n to 5Ar.1r of the Geomagnetic Polarity Time Scale, constraining the section to ~12.7–4.8 Ma. The changes in lithofacies from floodplain to braided river at ~8.6 Ma with predominantly southerly paleocurrent directions occur simultaneously with an increase in the sedimentation rates, representing a rapid uplift in the eastern Qilian Shan to the north. Geochemical provenance proxies (Th/Sc, Zr/Sc and Cr/Zr) in the <2 μm fraction show a significant provenance change at ~8.8 Ma. Silicate-based weathering indexes (CIA, CIW and PIA) displayed coeval changes with provenance but discrepant changes with regional climate. Since the clay mineralogy exhibit significant change at ~7.8 Ma uncorrelated with modifications in provenance, it can be employed to reveal regional climate change. The rise in illite, and associated decrease in the sum of smectite and illite/smectite mixed-layers reflect gradual and slow aridification since ~12.7 Ma with intensified drying since ~7.8 Ma until approaching the modern climate status. Our results, together with other regional climatic and tectonic records, clearly illustrate that accelerated uplift of the northeastern TP since ~8-9 Ma has mainly modulated the regional erosion, weathering, transportation and sedimentation, and amplified the global cooling and drying trend towards the regional climate of modern conditions. Our study suggests that in the tectonically active northeastern TP, a comprehensive mineralogical and geochemical investigation of the fine-grained fraction of the basin sediments could retrieve the interactions between tectonics and climate behind the complex change in exhumed lithology and sedimentary routing systems. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Detailed 15.3–1.8 Ma clay elemental geochemistry records for the northeastern Qaidam Basin are presented. • Long-term and persistent weakening of the chemical weathering intensity has occurred since the Middle Miocene. • Global cooling-related drying played a key role in the chemical weathering intensity. • Miocene uplift of the northern Tibetan Plateau might exert some influence on the weakened chemical weathering intensity at 12–8 Ma. Abstract Cenozoic basin sediments around the Tibetan Plateau are valuable archives that can be used to reveal long-term interactions among tectonics, climate and weathering. However, the reconstruction of silicate weathering histories in this tectonically active region is consistently hindered by the presence of frequently embedded conglomerate and sandstone in the long sedimentary sequences. Here, we present detailed geochemical compositions of the clay components of the sediments in the Huaitoutala section (ca. 15.3–1.8 Ma) of the northeastern Qaidam Basin to reconstruct the chemical weathering history of the catchment since the Middle Miocene. Major element ratio proxies reveal evidence of four stages of change: from ca. 15.3 to 12.6 Ma, in which the clay components are characterized by relatively high Chemical Index of Alteration (CIA) values (75–88, average = 81) but low mobile/immobile oxide ratios (e.g., K 2 O/Al 2 O 3 , Na 2 O/Al 2 O 3 and CaO/Al 2 O 3), thus indicating the occurrence of intense chemical weathering conditions; from ca. 12.6 to 6.6 Ma, in which decreasing CIA values, together with increasing mobile/immobile oxide ratios, suggesting that a long-term decrease in silicate weathering intensity occurred in the source area; from 6.6 to 2.6 Ma, in which the clay components in the sediments exhibit persistently low CIA values (66–81, average = 76.8) and high oxide ratios, which are indicative of mild to moderate chemical weathering; and since 2.6 Ma, in which the CIA values (64–80, average = 75.5) have continued to decrease with increasing oxide ratios, thus reflecting mild chemical weathering conditions. The geochemical proxy records demonstrate that the intensity of chemical weathering has continually decreased and that the Qaidam Basin has experienced continuous aridification since ca. 12.6 Ma. We suggest that Middle-Miocene global cooling-related drying has exerted a significant influence on the trend of decreasing chemical weathering intensity since ca. 12.6 Ma. In addition, the Late Cenozoic rapid uplift and exhumation of the NE Tibetan Plateau could have limited the time that the silicate rocks were exposed in the weathering profile prior to the deposition, which may have led to their weakened intensity of chemical weathering. [ABSTRACT FROM AUTHOR]

The Quaternary strata in North China primarily consist of fluvial sediments, which react strongly to East Asian Monsoon circulation. However, the records of the Quaternary climate change history in terms of these fluvial sedimentations were limited which impeded understanding the East Asian Monsoon evolution in North China. Thus, a 150 m deep drill core was collected from Changping in the Beijing plain to reconstruct the Quaternary palaeoenvironmental changes. The core was chronologically well constrained using palaeomagnetic and thermoluminescence dating. The magnetostratigraphic result indicated that the Brunhes/Matuyama boundary is at 134.2 m (0.78 Ma) and the age at the bottom of the core is 1.07 Ma. The thermoluminescence (TL) dating of the upper sand at ∼2 m was 10.4 ka. The chemical weathering history since the middle Pleistocene in Beijing plain was reconstructed by using geochemical proxies including selected major element pairs, chemical index of alteration and Al 2 O 3 –CaO*–(Na 2 O + K 2 O) diagrams. The CIA value of the Changping core ranged from 50.3 to 69.4 (average 59.2), which indicated that the sediment has experienced incipient to intermediate chemical weathering during the middle Pleistocene. The result of A–CN–K figure shows a high plagioclase to K-feldspar ratio in the source, which would be indicative of a granodiorite (Gd) source from Jundu Mountains north of the Beijing plain. All the geochemical results revealed a long-term gradual decrease of weathering since the middle Pleistocene, which implied a long-term increasing aridification/cooling in the Beijing plain. [ABSTRACT FROM AUTHOR]

• The late Cretaceous–early Eocene paleoclimate of the Jianghan Basin was studied. • The salinity of the paleolake was mainly controlled by climate change. • The temporal differentiation of the evaporite deposits resulted from tectonics. • The widespread evaporites indicate an extremely arid climate during the Paleocene. • The climate became semi-humid during the early Eocene. The Jianghan Basin, Central China, contains a thick, continuous sequence of early Cenozoic evaporite-bearing deposits that provide a wealth of information about early Cenozoic climate change and evaporites accumulation in East Asia. However, the paleoclimatic history of the basin and its effect on the evaporites are largely unknown. We obtained a deep drill core (SKD1, 2346.5 m long) from the late Cretaceous to early Eocene evaporite-bearing deposits in the Jianghan Basin. Detailed analyses of the lithofacies, mineralogy and geochemistry of the core indicate that salinization and deepening of the lake occurred (corresponding to the Yuyang Formation to the lower Xingouzui Formation), followed by desalting and shallowing (corresponding to the upper Xingouzui Formation). Reconstruction of the spatiotemporal evolution of the evaporites in the Jiangling depression suggests a transition from semi-arid conditions in the late Cretaceous to extremely arid conditions in the initial stage of the Eocene. The climate then became semi-humid during the deposition of the upper Xingouzui Formation, indicating a monsoon or monsoon-like climate in East Asia during the early Eocene. The southward migration of the depocenter of the Jiangling depression, coupled with a warm and extremely arid climate, resulted in the precipitation of evaporites during the accumulation of the upper Shashi Formation and the lower Xingouzui Formation. These results indicate great potential for identifying potash resources within the subsag in the southern Jiangling depression. [ABSTRACT FROM AUTHOR]