Extreme weathering/erosion during the Miocene Climatic Optimum: Evidence from sediment record in the South China Sea Shiming Wan, 1,2 Wolfram M. Ku ¨rschner, 3 Peter D. Clift, 4 Anchun Li, 1 and Tiegang Li 1 Received 28 July 2009; revised 24 August 2009; accepted 3 September 2009; published 6 October 2009. [ 1] Investigating the interplay between continental weathering and erosion, climate, and atmospheric CO 2 concentrations is significant in understanding the mechanisms that force the Cenozoic global cooling and predicting the future climatic and environmental response to increasing temperature and CO 2 levels. The Miocene represents an ideal test case as it encompasses two distinct extreme climate periods, the Miocene Climatic Optimum (MCO) with the warmest time since 35 Ma in Earth’s history and the transition to the Late Cenozoic icehouse mode with the establishment of the east Antarctic ice sheet. However the precise role of continental weathering during this period of major climate change is poorly understood. Here we show changes in the rates of Miocene continental chemical weathering and physical erosion, which we tracked using the chemical index of alteration (CIA) and mass accumulation rate (MAR) respectively from Ocean Drilling Program (ODP) Site 1146 and 1148 in the South China Sea. We found significantly increased CIA values and terrigenous MARs during the MCO (ca. 17–15 Ma) compared to earlier and later periods suggests extreme continental weathering and erosion at that time. Similar high rates were revealed in the early-middle Miocene of Asia, the European Alps, and offshore Angola. This suggests that rapid sedimentation during the MCO was a global erosion event triggered by climate rather than regional tectonic activity. The close coherence of our records with high temperature, strong precipitation, increased burial of organic carbon and elevated atmospheric CO 2 concentration during the MCO argues for long-term, close coupling between continental silicate weathering, erosion, climate and atmospheric CO 2 during the Miocene. Citation: Wan, S., W. M. Ku ¨rschner, P. D. Clift, A. Li, and T. Li (2009), Extreme weathering/erosion during the Miocene Climatic Optimum: Evidence from sediment record in the South China Sea, Geophys. Res. Lett., 36, L19706, doi:10.1029/ 2009GL040279. 1. Introduction [2] Continental erosion and weathering can affect both ocean chemistry through fluvial run-off and the atmosphere through draw-down of CO 2 during chemical weathering. These processes have the ability to affect global climate over a range of time scales [e.g., Berner et al., 1983; Walker et al., 1981]. The Miocene is of particular interest for palaeoclimate studies as it is distinguished by an extreme climatic optima alternating with two major glaciations during the Early and the late Middle Miocene [Zachos et al., 2001]. A recent climate model study [You et al., 2009] suggests that the MCO is most likely linked to elevated CO 2 (500 ppmv), which is in agreement to the recent CO 2 estimates [Ku ¨rschner et al., 2008], but contrast with previ- ous low CO 2 levels reconstruction [Pearson and Palmer, 2000; Pagani et al., 2005]. A marked increase in atmo- spheric CO 2 would increase global temperatures through the greenhouse effect, thereby affecting atmospheric and oce- anic circulation, precipitation patterns and intensities, and thus accelerate silicate weathering. An enhanced silicate weathering would in turn limit the increased CO 2 content in the atmosphere through a negative feedback [e.g., Berner et al., 1983; Walker et al., 1981]. A rapid increase in 87 Sr/ 86 Sr of seawater in the middle Miocene has been related to a major episode of erosion and weathering due to the defor- mation in the Himalayan orogen at that time [Raymo, 1994]. But the use of the sea water 87 Sr/ 86 Sr record as an ideal monitor of silicate chemical weathering rates has been refuted because this ratio is controlled by non-unique factors [e.g., Oliver et al., 2003; Quade et al., 1997]. Under these circumstances, an independent method to estimate the Miocene continental weathering and erosion will signifi- cantly help us to understand the response and feedback of weathering and erosion to changing climate from green- house condition to icehouse climate, and thus shed some light on this debate of Miocene CO 2 through consideration of the weathering – climate feedback process in long-term carbon cycle. 2. Materials and Methods [3] Sedimentary basins in the South China Sea preserve some of the best stratigraphic records available for the continental interior of eastern Asia. Cores from ODP Site 1146 and 1148 in the northern South China Sea represent a nearly complete sequence of essentially unaltered Neogene sediments eroded from the Pearl River system in South China [Li et al., 2003; Wei et al., 2006; Wan et al., 2007]. ODP Site 1146 is located at a water depth of 2092 m, within a small rift basin on the mid-continental slope of the northern South China Sea. Three holes were cored to a sub-seafloor depth of 643 meters composite depth (mcd) [Wang et al., 2000]. For this study, a total of 90 samples were sampled at 1.5 m intervals from 435.62–498.94 mcd and at 3 m intervals from 499.84–642.44 mcd. The lithol- ogy of the recovered section is quite homogenous, being GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L19706, doi:10.1029/2009GL040279, 2009 Click Here for Full Article 1 Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China. 2 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 3 Institute of Environmental Biology, Department of Palaeoecology, Utrecht University, Utrecht, Netherlands. 4 School of Geosciences, University of Aberdeen, Aberdeen, UK. Copyright 2009 by the American Geophysical Union. 0094-8276/09/2009GL040279$05.00 L19706 1 of 5