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  • Earth-Science Reviews 130 (2014) 86–102

    Contents lists available at ScienceDirect

    Earth-Science Reviews

    j ourna l homepage: www.e lsev ie r .com/ locate /earsc i rev

    Reconstructing chemical weathering, physical erosion and monsoon intensity since 25 Ma in the northern South China Sea: A review of competing proxies

    Peter D. Clift a,b,⁎, Shiming Wan c, Jerzy Blusztajn d

    a Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA b South China Sea Institute of Oceanology, Chinese Academy of Sciences, 164 Xingang Road, Guangzhou, China c Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China d Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA

    ⁎ Corresponding author at: Department of Geology and E-mail address: [email protected] (P.D. Clift).

    0012-8252/$ – see front matter © 2014 Elsevier B.V. All ri http://dx.doi.org/10.1016/j.earscirev.2014.01.002

    a b s t r a c t

    a r t i c l e i n f o

    Article history: Received 9 October 2013 Accepted 7 January 2014 Available online 15 January 2014

    Keywords: Monsoon Erosion Weathering Clay mineralogy Sediment geochemistry South China Sea

    Reconstructing the changing strength of the East Asian summermonsoon has been controversial because different proxies, many being indirect measures of rainfall, tell contrasting stories about how this has varied over long periods of geologic time. Here we present new Sr isotope, grain-size and clastic flux data and synthesize existing proxies to reconstruct changing chemical erosion in the northern South China Sea since the Oligocene, using the links betweenweathering rates andmonsoon strength established in younger sediments as a way to infer inten- sity. Chemical proxies such as K/Rb, K/Al and the Chemical Index of Alteration (CIA), together with clay proxies like kaolinite/(illite+ chlorite) show a steady decline in alteration after a sharp fall following amaximum at the Mid Miocene Climatic Optimum (MMCO; 15.5–17.2 Ma), probably as a result of cooling global temperatures. In contrast, physical erosion proxies, including bulk Ti/Ca and clasticmass accumulation rates (MAR), showpeaks at 21–23 Ma, ~19 Ma and 15.5–17.2 Ma, implying faster run-off in the absence of drainage capture. Rates increase again, likely driven by slightly increased run-off after 13 Ma, but decrease after 8 Ma, which is identified as a period of summer monsoon weakening. Sr isotope composition correlates with hematite/goethite and the spectral proxy CRAT to show stronger weathering linked to more monsoonal seasonality. These proxies argue for a strengthening of the East Asian Monsoon after 22–23 Ma, followed by an extended period of monsoon maximum between 18 and 10 Ma, then weakening. There is some suggestion that the summer monsoon may have strengthened since 3–4 Ma after reaching a minimum in the Pliocene.

    © 2014 Elsevier B.V. All rights reserved.

    Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2. Monsoon Proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 3. Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 4. Links between monsoon and erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5. Earlier monsoon reconstructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 6. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 7. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

    8.1. Physical erosion proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 8.2. Geochemical proxies for alteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 8.3. Clay minerals as alteration proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 8.4. Exploring the CRAT proxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 8.5. Sr isotope evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 8.6. Sediment provenance at ODP Sites 1146 and 1148 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 8.7. Environments in the Plio-Pleistocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 8.8. Weathering in the Late Miocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

    Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA. Tel.: +1 225 578 2153; fax: +1 225 578 2302.

    ghts reserved.

    http://crossmark.crossref.org/dialog/?doi=10.1016/j.earscirev.2014.01.002&domain=f http://dx.doi.org/10.1016/j.earscirev.2014.01.002 mailto:[email protected] http://dx.doi.org/10.1016/j.earscirev.2014.01.002 http://www.sciencedirect.com/science/journal/00128252

  • 87P.D. Clift et al. / Earth-Science Reviews 130 (2014) 86–102

    8.9. Erosion at the MMCO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 8.10. Erosion at 23.2–21.3Ma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 8.11. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

    9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

    1. Introduction

    The Asianmonsoon is often portrayed as the classic example of how evolution of the solid Earth is coupled to atmospheric processes, yet its temporal evolution is not well defined over periods N10 Myr. This is a major problem because the monsoon and the potentially associated tectonic evolution of Asia may be one of the most significant processes affecting global climatic conditions during the Cenozoic (Raymo et al., 1988; Berner and Berner, 1997; Wang et al., 2003b). However, demon- strating any linkage between climate and tectonics is impossible with- out a better understanding of when and how the monsoon developed, which can then be correlated, or not, with changing topography in central Asia or the tectonics of the Himalaya (Clemens et al., 1991; Prell and Kutzbach, 1992; Molnar et al., 1993; Clift and Plumb, 2008). Reconstructingmonsoon intensity is hampered both by lack of continu- ous sedimentary sequences and by agreement onwhat the best proxies to measure are, because there are few robust rainfall proxies, beyond salinity in seawater, which is only well developed in the Bay of Bengal (Kudrass et al., 2001).

    The key problem in reconstructing monsoon strength is to pick the right proxies to measure because the monsoon is strongly variable across Asia, being wetter in some places, windier in others, while some places are dominated by winter rather than summer monsoons (Wang et al., 2003a). Proxies linked to wind strength are potentially the most reliable, but are often hard to extract at high resolution and over long time periods because they tend to accumulate only in con- densed deep sea sediments (Rea et al., 1998). Moreover, if continental rainfall is the key variable that we wish to examine wind speed is not necessarily the ideal process to measure. Rainfall is the process that is most crucial to those trying to understand how the summer monsoon may have affected the development of orogens in Asia through enhanced erosion (Burbank et al., 2003; Wobus et al., 2003; Clift et al., 2008). Care needs to be exercised in equating total rainfall to monsoon intensity despite the fact that monsoon rain dominates that in much of South and East Asia. This is because climate models predict coastal rain- fall inmost scenarios, with rainfall deep in the continental interior being a feature of monsoons, alongwith a seasonal variation between dry and wet conditions (Webster et al., 1998).Moreover, strong bands of rainfall follow the seasonal migration of the Intertropical Convergence Zone (ITCZ) so that motion of this belt can result in climate changes that mimic monsoon strength changes (Armstrong and Allen, 2011). More often the ITCZ and monsoon merge in Asia to act in concert with one another so that simple separation of these influences is not always possible.

    Alternatively, many paleoceanographic studies have focused on monsoon-related marine upwelling forced by the winds as a measure of monsoon strength (Kroon et al., 1991; Prell et al., 1992; Clemens, 1998; Chen et al., 2003). Unfortunately these proxies also do not always closely track rainfall in Asia. While studies of salinity ne

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