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Terrestrial silicate weathering records from the tectonically active Tibetan Plateau are crucial to explore the interactions among climate change, tectonic uplift and chemical weathering during the Cenozoic. However, the complex sedimentary facies changes and frequent occurrence of coarse lithologies related to intense tectonic uplift of the Tibetan Plateau have largely limited the reliability of bulk chemical weathering indexes to reveal the silicate weathering history. Here, we present detailed records of clay minerals and geochemical compositions from clay-sized (< 2 μm) sediments collected from the well-dated Huaitoutala section (ca. 15.3–1.8 Ma) in the NE Qaidam Basin to reconstruct the chemical weathering history of the NE Tibetan Plateau during the late Cenozoic. The relatively high (illite–smectite mixed layers + smectite)/(chlorite + illite) ratio indicates a strong silicate weathering intensity from 15.3 to 12.6. Ma, while a continuous decrease in this ratio since 12.6 Ma reflects a long-term decreasing intensity of silicate weathering. By comparing the clay mineral records with the climatic reconstruction and the tectonic events in the NE Tibetan Plateau, the results reveal that middle Miocene global cooling-related drying exerted a significant influence on the decreasing weathering intensity since ca. 12.6 Ma. Furthermore, the late Cenozoic episodic and persistent uplift of the NE Tibetan Plateau may have exerted an additional influence on the long-term decrease in weathering intensity. Specifically, the dramatic fluctuation in the silicate weathering intensity during ∼11–9.6 Ma is mainly due to uplift-induced provenance changes. Moreover, the fluctuating increase in the silicate weathering intensity during ∼8–5 Ma may have been caused by intensified East Asian summer monsoon precipitation. Our study suggests that both climate change and tectonic deformation controlled the chemical weathering intensity in the NE Tibetan Plateau during the late Cenozoic. • Detailed 15.3–1.8 Ma clay minerals records from the northeastern Qaidam Basin. • Long-term continuous weakening of silicate weathering intensity since 12.6 Ma. • Global cooling-related drying mainly caused weakening of silicate weathering. • Uplift-induced provenance change influenced weathering proxy during 11–9.6 Ma. • Increase in silicate weathering intensity during ∼8–5 Ma caused by monsoon rainfall. [ABSTRACT FROM AUTHOR]

The uplift of the northern Tibetan Plateau (TP) has long been regarded as one of major factors accounting for both the enhanced process of aridification recorded in central Asia and the shifts in the Asian monsoonal circulations that are known to have occurred at approximately 8 Ma. Until now, there have been few sedimentary records reported on which can directly, and in an integrated fashion, shed light on the region's tectonic and climate regimes. This can be largely attributed to the fact that the influence exerted by the presence of source materials and subsequent sedimentary sorting in this tectonically active region would be likely to bias significantly any paleoclimatic interpretation of silicate-derived proxies. Based on detailed investigations of strontium (Sr) distributions revealed by multi-step leaching in a fluvial sequence from the Linxia Basin which includes paleosols, we reconstructed the long-term late Miocene regional climatic and environmental changes likely to have occurred on the northeastern TP from 12.1 to 5.2 Ma. We investigated Sr-related proxies in two typical paleosol profiles to constrain Sr mobility in paleosol weathering, and, thus, to further prompt the interpretation of Sr-related proxies along the whole section. We would suggest that carbonate-derived Sr concentrations and Sr/Ca ratios are suitable proxies for tracing regional climate change associated with weathering and pedogenesis; together, they exhibit weakened chemical weathering intensity and pedogenesis at ~ 8.6 Ma. The concomitant appearance of immature sediments through poor sedimentary sorting revealed by silicate-derived Rb/Sr, 87 Sr/ 86 Sr and Eu anomaly (Eu/Eu*) alongside a sharp increase in sedimentation rates noted at this time suggests that it was indeed tectonic uplift that principally controlled the dramatic climatic and environmental changes. This multi-proxy record of concomitant tectonic uplift and climate change in the Linxia Basin provides direct geological evidence for uplift driven climate change in the northeastern TP region. [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]

Late Cenozoic changes in chemical weathering and the dust cycle in response to the regional aridification and global cooling in the tectonically active northern Tibetan Plateau is an intriguing issue, although it remains poorly understood. The change in the 87Sr/86Sr ratio of basin water is linked to variations in the overall input of various Sr sources with different 87Sr/86Sr ratios, and analyses of such changes can be a useful tool for revealing changes in regional lithology, weathering regimes and the dust cycle linked to tectonics and climate. In this study, we collected fossil shells of ostracods and gastropods from the Miocene Dahonggou section to reconstruct the paleolake water 87Sr/86Sr ratio from 14.8 Ma to 11.1 Ma in the northern Qaidam Basin on the northeastern Tibetan Plateau. The reconstructed paleolake water 87Sr/86Sr ratios based on fossil shells and bulk carbonates from sediments display a coincident and stable 87Sr/86Sr ratio of approximately 0.7115, which did not show any detectable change in response to the enhanced aridification since ~13 Ma as inferred by the ostracod assemblage and its stable carbon and oxygen isotopes. The results indicate that the Sr influx regime from various sources did not change significantly during the aridification processes ~13 Ma, which may have been caused by a less detectable change in proportion of silicate versus carbonate weathering, and/or an overwhelming Sr source (e.g., extrabasinal eolian dust) that occurred from 14.8 Ma to 11.1 Ma. Further, the 87Sr/86Sr ratios of modern lake water and river sand carbonates in the study region are significantly higher, approximately ≥0.001, than those of the Miocene lake water. This observation can be explained by two possible mechanisms that need to be tested in the future: 1) given a Miocene Sr input regime that is similar to today, a large contribution from some extrabasinal Sr source (e.g., eolian dust) with a much lower 87Sr/86Sr ratio is expected to have prevailed from 14.8 Ma to 11.1 Ma but is absent in the modern setting or 2) given a Miocene Sr input regime that could yield an overall lower lake water 87Sr/86Sr ratio of approximately 0.7115, a significant increase in the 87Sr/86Sr ratio of catchment-scale Sr source may have occurred after 11 Ma in response to the uplift of the Qilian Mountains, which would have enhanced hydrothermal activity, metamorphism, glaciation and landslide processes, thereby resulting in more-radiogenic Sr source. Our study suggests that reconstructions of river/lake water 87Sr/86Sr ratios can place powerful constraints on the complex interactions among tectonics, climate and landscape evolution in the northeastern Tibetan Plateau based on a new perspective. • 87Sr/86Sr ratios of paleolake water at 14.8–11.1 Ma in the northern Qaidam Basin. • Ostracod shell and bulk carbonates show stable 87Sr/86Sr ratios at ~0.7115. • Paleolake water 87Sr/86Sr ratio is lower than modern lake water value of ~0.7128. • The paleo and modern lake 87Sr/86Sr contrast links to eolian input or Qilian uplift. [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 relationship between tectonic uplift, climate change and continental silicate weathering has long been debated because deconstructing the tectonic and climatic impacts on continental silicate weathering is challenging. To address this issue, we present long-term climatic and silicate weathering records since 7.3 Ma on tectonically active NE Tibet retrieved from the SG-1 and SG-1b lake sediment drill cores in the western Qaidam Basin. We find distinct correlations between silicate weathering and regional climate on long-term (>106 years) and short-term (105–106 years) time scales. On long-term scales, silicate weathering intensity exhibits a consistent evolution with regional climate, both of which are dominantly controlled by global climate change. This evolution resulted in a stable weathering intensity during 7.3–~3.6 Ma and a weakening of weathering intensity since ~3.6 Ma, followed by a continuous decreasing trend after 2.6 Ma. However, on short-term scales, silicate weathering intensity displays out-of-phase changes with the regional climate at 6.9–3.7 Ma and 0.2–0.1 Ma and in-phase changes at 7.3–6.9 Ma and 3.7–0.2 Ma. We attribute this long-term consistency between the weathering intensity and climate to aridification in inland Asia, but the short-term inconsistencies at 6.9–3.7 Ma and 0.2–0.1 Ma to the change in the sediment routing system impacted by the regional climate within a tectonically active setting. Such a change in the sediment routing system along with sediment recycling may bias weathering indication on orbital time scales, and our study therefore suggests a more complex response of silicate weathering to tectonic and climatic forcings. [Display omitted] [ABSTRACT FROM AUTHOR]

In order to better understand how stable metal isotope signals in large rivers can be used to constrain present and past weathering, the seasonal riverine Mg Sr isotopic pattern in the middle Yellow River was systematically investigated based upon weekly collected samples for the whole year of 2013. The results demonstrate that Mg is mainly transported in the dissolved form (65%) in this river system and that 45% of the total dissolved Mg is transported during the monsoon seasons, with 2% exported over 4 days during a single storm event. Dissolved Mg in the middle Yellow River is dominantly derived from both silicate and carbonate (82–89%) in this semi-arid region, with limited evaporite contribution (∼7%). Lithological mixing is the first order control on riverine dissolved Mg and Sr isotopes, with a contribution from ∼40% carbonate dissolution and ∼ 60% from silicate dissolution in the dry seasons, and ∼ 50% carbonate and ∼ 50% silicate during the monsoon seasons according to δ26Mg signals. Furthermore, a significant role of prior calcite precipitation (PCP) can be quantified, which fractionates Mg isotopes by about 0.17‰ to 0.39‰ positively depending on the choice of elemental and isotope partition of Mg in secondary carbonates. Clay formation following the PCP further fractionates riverine Mg isotopes to the negative side. An ∼0.2‰ decrease of riverine Mg isotopes is attributable to (1) a single storm event causing carbonate dissolution and (2) delayed delivery of depleted waters to rivers (∼3 months after the storm event) because of subsurface hydrological circulation. Annually, the weighted average riverine δ26Mg (−1.05‰) in the middle Yellow River is identical to the global average (−1.09‰). Despite the significant impact of lithology on the riverine dissolved Mg isotope signature, the mixing proportions of different Mg sources remain virtually constant, even when there are huge contrast of temperature, hydrology, and precipitation seasonally along the year, providing a basis for dissolved δ26Mg response to climatic forcing on the continental scale. This means that significant changes in the sedimentary Mg isotope records would reflect extreme conditions in deep time. [ABSTRACT FROM AUTHOR]

• The hominoid distribution is consistent with advance-retreat of South Asian Monsoon. • Humidity is a controlling factor on the Miocene hominoid migration in East-South Asia. • Clay mineral and δ18O records suggest a hot-wet climate for hominoid-bearing deposits. The discovery of hominoids dated to the Middle–Late Miocene in the southeastern Tibetan Plateau (SE TP) has attracted widespread attention and highlighted the significance of the SE TP as an important refuge for hominoids. However, research on the living environment of hominoids in the region has primarily focused on the Late Miocene, and there are solely few earlier records on earlier migrations. A recently investigated sedimentary sequence within the Xiaolongtan Basin in the SE TP, which contains two hominoid-bearing levels dated respectively to 12.5 Ma and 11.7 Ma, provides a unique window to characterize and understand the living environment that attracted hominoids to the region. The clay mineral, geochemical and sedimentary analyses of the sequence, characterized by the highest kaolinite content and chemical weathering intensity and the coeval occurrence of thick lignite layers during ∼ 12.6–11.6 Ma, suggest that the climate when hominoids occupied the area was hot with higher humidity compared to the preceding and succeeding intervals. A spatiotemporal analysis of the hominoid distribution in the SE TP and the adjacent Indo-China Peninsula indicates a north–south–north pattern of movement, with their ages of ∼ 13–12.5 Ma, ∼11 to 8–7 Ma, and after ∼ 8–7 Ma. These movements accord roughly with the timing of the advance and retreat of the South Asian monsoon (SAM). We therefore infer that, against the overall cooling climatic trend of the Middle–Late Miocene, the increased precipitation and flourishing vegetation promoted by the SAM expansion, was the principal factor that attracted the hominoids to the region. Overall, our results provide new insights into the relationship between the paleoenvironment and hominoid migration/settlement. [ABSTRACT FROM AUTHOR]

Continental weathering is a crucial factor to maintain the balance of the carbon cycle and thus the livability of the Earth. The Tibetan Plateau (TP), known as the Third Pole of the Earth, has undergone strong uplift since the Cenozoic, thus making it an ideal area to study continental weathering processes and associated driving factors. However, early Cenozoic weathering records are rare in this region. Here, we report high quality mineralogical and geochemical records of clay fraction (< 2 μm) of a middle to late Eocene sequence retrieved from the Qujing Basin, southeastern TP, to track the early Cenozoic chemical weathering history in this area. The results show that the main clay mineral assemblages contain illite-smectite mixed layers, illite and palygorskite. The chemical weathering intensity indicators (e.g., clay mineral assemblages and the chemical index of alteration (CIA)) collectively indicate a clear long-term modulation of global cooling on regional chemical weathering processes during middle to late Eocene, as indicated by high correlation between global temperatures and chemical weathering indicators and the similarity in their periodicity characteristics. Moreover, a sudden decrease of the chemical weathering intensity with large fluctuations was identified at 42–39.9 Ma, which may be linked to rapid transport of clay minerals caused by tectonic activity and/or an increased precipitation in the southeastern TP. This study indicates a control of global cooling on chemical weathering of the southeastern TP area during middle to late Eocene, while tectonic activity or other factors might exert catalytic influences at some certain periods. • Middle-late Eocene chemical weathering records in southeastern Tibetan Plateau (TP) • Control of global cooling on early Cenozoic chemical weathering in southeastern TP • Transition of chemical weathering intensity at 42–39.9 Ma [ABSTRACT FROM AUTHOR]