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      Chemical weathering and erosion responses to changing monsoon climate in the Late Miocene of Southwest Asia

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          Abstract

          The late Miocene is a time of strong environmental change in SW Asia. Himalayan foreland stable isotope data show a shift in the dominant vegetation of the flood plains away from trees and shrubs towards more C4 grasslands at a time when oceanic upwelling increased along the Oman margin. We present integrated geochemical and colour spectral records from International Ocean Discovery Program Site U1456 in the eastern Arabian Sea to reconstruct changing chemical weathering and erosion, as well as relative humidity during this climatic transition. Increasing hematite/goethite ratios derived from spectral data are consistent with long-term drying after c. 7.7 Ma. Times of dry conditions are largely associated with weaker chemical alteration measured by K/Rb and reduced coarse clastic flux, constrained by Si/Al and Zr/Al. A temporary phase of increased humidity from 6.3 to 5.95 Ma shows a reversal to stronger weathering and erosion. Wetter conditions can result in both more and less alteration due to the nonlinear relationship between weathering rates, precipitation and sediment transport times. Trends in relative aridity do not follow existing palaeoceanographic records and are not apparently linked to changes in Tibetan or Himalayan elevation, but more closely correlate with global cooling. An apparent opposing trend in the humidity evolution in the Indus compared to southern China, as tracked by spectrally estimated hematite/goethite, likely reflects differences in the topography in the Indus compared to the Pearl River drainage basins, as well as the generally wetter climate in southern China.

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          Trends, rhythms, and aberrations in global climate 65 Ma to present.

          Since 65 million years ago (Ma), Earth's climate has undergone a significant and complex evolution, the finer details of which are now coming to light through investigations of deep-sea sediment cores. This evolution includes gradual trends of warming and cooling driven by tectonic processes on time scales of 10(5) to 10(7) years, rhythmic or periodic cycles driven by orbital processes with 10(4)- to 10(6)-year cyclicity, and rare rapid aberrant shifts and extreme climate transients with durations of 10(3) to 10(5) years. Here, recent progress in defining the evolution of global climate over the Cenozoic Era is reviewed. We focus primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records. We also consider how this improved perspective has led to the recognition of previously unforeseen mechanisms for altering climate.
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            The geochemical evolution of the continental crust

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              Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

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                Author and article information

                Journal
                applab
                Geological Magazine
                Geol. Mag.
                Cambridge University Press (CUP)
                0016-7568
                1469-5081
                June 13 2019
                : 1-17
                Article
                10.1017/S0016756819000608
                1400f325-6277-44f1-806e-726ecdd1d0c6
                © 2019

                https://www.cambridge.org/core/terms

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