Grain-size records at ODP site 1146 from the northern South China Sea: Implications on the East Asian monsoon evolution since 20 Ma

Abstract

samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS)were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10-19 μm/1.3-2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2+EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma. grain size, eolian dust, East Asian monsoon, South China Sea, ODP Leg184East Asian monsoon circulation is driven by differential heating between the WPWP (western Pacific warm pool) and the Asian continent. Seanally reversing summer-and winter-monsoon winds drive seasonal precipitation and runoff regimes which determine, in part, the soil and vegetation characteristics of eastern Asia [1] . Past fluctuations of the East Asian winter and summer monsoons over long time scales (>1 Ma) have been studied using the loess-paleosol sequences in the Chinese Loess Plateau [2][3][4][5][6] . By comparison, similar studies of deep-sea sediments in the South China Sea (SCS), which is the main sediment sink for fluvial material from the East Asian continent and an ideal location for recording the erosional response to both winter and summer monsoons were hampered by the lack of long sediment cores until six sites were drilled in two areas in the SCS during ODP Leg 184 [7] . Until now, most of the paleoclimatic studies which permit the reconstruction of long-term evolution of the East Asian paleo-monsoon from sediments in the SCS are mainly based on studies of radiolarians [8] , foraminifera [9,10] , stable isotopes [11][12][13] , sediment geochemistry [14] and even broad scale climatic and botanical zones of China [15] . However, evidence of longterm evolution of the East Asian monsoon over tectonic

Sensitive grain-size records of Holocene East Asian summer monsoon in sediments of northern South China Sea slope

“…On the basis of the data of LSR and DBD (dry bulk density = mass of oven dry bulk sample/ core sample volume), the total MAR values were calculated, and in turn the MAR of terrigenous materials was determined by multiplying the total MAR by the weight percent value of the terrigenous components. Using these accumulation rates, dilution effects by other components were excluded, and furthermore, sediment compaction with burial was corrected (Wan et al, 2007b).…”

“…Much knowledge about global and regional climate has been gained by extracting different grain-size components of sediments. Grain-size distribution of marine sediments also provides much information about climate and environmental change such as source region's climate, monsoon evolution and ocean currents, which was well applied to studies in the north Atlantic (Gröger et al, 2003), the north Pacific (Rea and Hovan, 1995), Arabian Sea (Prins et al, 2000a,b), offshore southwest coast of Africa (Stuut et al, 2002;Pichevin et al, 2005), the East China Sea (Xiao et al, 2006;Xu et al, 2009a), the South China Sea (Boulay et al, 2003;Wan et al, 2007b) and so on.…”

Evolution of the East Asian monsoon and its response to uplift of the Tibetan Plateau since 1.8 Ma recorded by major elements in sediments of the South China Sea

“…Because biogenic carbonate has been removed from the fine grain-sized sediments, it is generally accepted that residual carbonate-free detritus consists of silicate, a small quantity of ferromanganese oxide, and biogenic silicon [12,17], whereas the fine grain-sized silicate detritus is predominantly composed of terrestrial materials from rivers and volcanic ash [14,[18][19][20]. Therefore, three factors were associated with sources according to the geochemical characteristics of major elements and the exclusivity of element association in the geological process.…”

“…On the other hand, studies on the East Asian monsoon in the South China Sea, using core sediments drilled in ODP Leg 184, have found little association between the long-term evolution of the East Asian monsoon and the uplift of the Tibetan Plateau and have related the evolution of the East Asian monsoon only to glacial-interglacial cycles. In particular, there has been no study on the relationship between the evolution of the East Asian monsoon and the wholly stepwise, quick uplift of the Tibetan Plateau since the Quaternary in the South China Sea [12][13][14]. In our study, therefore, we compare the stepwise changes of the terrestrial factor, which was extracted from major-element data of fine grain-sized carbonate-free sediments (< 4 μm) of the upper 185 mcd (meters composite depth; corresponding to 1.81 Ma [15]) splice at ODP Site 1146 drilled during Leg184 in the South China Sea using R-mode analysis, with the tectonoclimatic cycles revealed by the magnetic susceptibility of Chinese loess.…”

Monsoon‐Enhanced Silicate Weathering as a New Atmospheric CO2 Consumption Mechanism Contributing to Fast Late Miocene Global Cooling

“…The less‐altered material (with a low chemical alteration index but high chlorite and K/Al ratios) that appeared in the post‐∼8 Ma sediments in the South China Sea was previously assumed to be caused by a weakened EASM (Clift et al., 2014; Wan et al., 2007a, 2010), which is inconsistent with our SWI records. In an alternative view, however, the less‐altered material might reflect a strong eolian input of high contents of illite and chlorite caused by the intensified East Asian winter monsoon since ∼8 Ma rather than a weak EASM (Wan et al., 2007b), which can be supported by the enhanced Asian dust input into the Japan Sea (Shen et al., 2017) and North Pacific Ocean (Rea et al, 1998) since ∼8 Ma. In addition, an enhanced EASM may also induce changes in erosion and transport regimes in the source‐to‐sink systems over southern East China, thereby leading to less‐altered material accumulation in the South China Sea.…”