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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 Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO2 consumption process. Here, we performed a first-order estimate of the CO2 consumption induced by silicate chemical weathering and organic carbon burial in subtropical China related to this climatic reorganization. Our results show that an increase in long-term CO2 consumption by silicate weathering varies from 0.06×1012 to 0.87×1012 mol yr−1 depending on erosion flux reconstructions, with an ~50% contribution of Mg-silicate weathering since the late Oligocene. The organic carbon burial flux is approximately 25% of the contemporary CO2 consumption by silicate weathering. The results highlight the significant role of weathering of the Mg-rich upper continental crust in East China, which would contribute to the rapid decline in atmospheric CO2 during the late Oligocene and the Neogene rise in the seawater Mg content. If this climatic reorganization was mainly induced by the Tibetan Plateau uplift, our study suggests that the growth of the Himalayan-Tibetan Plateau can lead to indirect modification of the global carbon and magnesium cycles by changing the regional hydrological cycle in areas of East Asia that are tectonically less active. [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]

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]