Evolution Of Central Asian Aridity And C4 Plants Since The Middle Miocene Recorded By Marine Sediments In The Japan Sea

Grain size and SEM-EDS of marine samples obtained from Integrated Ocean Drilling Program?IODP?Site U1430 in Ulleung Basin of the southwestern Japan Sea,were used to trace the main sources of the sediment in the Japan Sea since the middle Miocene.Clay minerals,as well as Sr-Nd-Pb of clay-sized silicate,have been established to constrain history of the eolian dust input to the Japan Sea and further to discuss the climate and related driving forces.Based on the results of black carbon content,mass accumulation rate?MAR?and carbon isotope(?¹³C_BC),the C3-C4 plant transition history and related tectonic and climatic control mechanisms were then involved.Acid insoluble sediments of our study site are comprised of three end members based on grain size,the finest end member EM1 among of which is the mixture of eolian dust from central Asia transported by westerly and suspended sediment from the Japan Arcs by rivers and ocean currents;the EM2 with middle mean grain size was mainly derived from eolian dust of central Asia by the near surface East Asian winter monsoon;the coarsest end member EM3 can be considered as mixture of biogenic silica?i.e.,diatom?and volcanic clastics,but compared with biogenic silica,the volcanic clastic is a minor component of EM3.The results of clay mineral assemblages and Sr-Nd-Pb isotope further confirm that the finest clay sized fractions of Site U1430 are mixture of eolian dust from Central Asia and river suspend sediments from Japanese Arc.More than 77%of the clay-size sediment in the Japan Sea is derived from the eolian dust of Central Asia on the basis of?Nd?0?,and that suspend sediment of the Japanese rivers is just a minor component.Besides,Central Asian eolian dust content became more dominant with younger ages.The Central Asian end member supplied illite-rich and high ⁸⁷Sr/⁸⁶Sr and low?Nd?0?eolian dust to the study site by wind,while the Japanese end member,characterized by young volcanic rocks,contributed smectite-rich,low⁸⁷Sr/⁸⁶Sr and high?Nd?0?weathering products via rivers.The ratio of illite/smectite and?Nd?0?values of clay-sized silicate sediments at Site U1430 were used as proxies for tracing the changing strength of central Asian eolian input to the Sea of Japan and thus imply the aridification history of its source region.Our study first time presents continuous high resolution record that highlights the four steps drying of Central Asia occurred at around 11.8 Ma,8 Ma,3.5 Ma and 1.2 Ma.Considered the nature and timing of major climatic and tectonic events in Asia,we conclude that the strengthened aridification of Central Asia starting at¹1.8 Ma was possibly driven by the combined effect of Tibetan surface uplift and global cooling,whereas the rapid drying at⁸ Ma was caused primarily by the uplift the northern Tibetan Plateau.In contrast,the global cooling,overwhelming the influence of Tibetan Plateau uplift,has become the primary control on Central Asia aridification since about 3.5 Ma.The stable carbon isotope(?¹³C_BC)of marine sediment in our study site was used to reconstruct evolution of C4 plants in Central Asia since the Miocene.Carbon isotope of the black carbon records the C3-C4 transition history.The results of SED-EDS and the carbon isotope signatures confirm that black carbon of our study site mainly originate from broad areas in Central Asia such as the Loess Plateau by wind rather than from the Japanese by rivers.The stable carbon isotope value of black carbon(?¹³C_BC)has shown a major shift since the Miocene-Pliocene boundary,suggesting the significant expansion of C4 plants in broad areas of Central Asia,including the inland basins of northwestern China and the Loess Plateau.However,a decline in the content and MAR of black carbon reveals the absence of link between fire and the C4 plant expansion in Central Asia,due to the dramatic decrease in biomass under a drying regime.On a global scale,asynchronous expansion of C4 plants suggests that regional hydroclimatic change rather than decline in CO₂ concentration was the most important factor to influence C4 expansion.We propose that the increased seasonality and the enhanced long-term aridity driven by the concurrent decline in winter westerly vapor and increase in East Asian summer monsoon precipitation were the main driving forces of the C4 plant expansion in broad areas of Central Asia.The water vapor carried by the westerlies during the winter season may have substantially declined since the late Miocene??8 Ma?,because of decreases in winter moisture supply from the windward areas in eastern Eurasia.Moreover,the uplift of the Pamir and Central Anatolian Plateau,as well as the collision of the Pamir Plateau and the Tian Shan ranges,further blocked the eastward transport of winter westerly moisture since the late Miocene to early Pliocene.Such factors together resulted in aridity intensification and a prevailing desert environment in Central Asia and created an environment favoring C4plants over C3 plants,and finally caused C4 plant extensive expanded in Central Asia including the inland basins in northwestern China and the Loess Plateau..This study presents for the first time data showing long-term and well-dated high resolution record of Asian eolian input history from the IODP Site U1430 in the Sea of Japan since about 15 Millions years ago,and tests the three major hypothesized mechanisms that possibly influenced the stepwise drying in central Asia climate during the Cenozoic:retreat of the Paratethys Sea,global cooling,and uplift of the Tibetan Plateau.Our study first time presents continuous high resolution marine record on the path of the westerly and the East Asian monsoon jet that highlights the synthetic C4 plant expansion in broad areas of Central Asia occurred around the Miocene-Pliocene boundary.This study highlights that variations in winter westerly moisture have played a significant role in changes of regional climate and vegetation in Central Asia since the late Miocene,and emphasizes the importance of both the westerly vapor and summer monsoon precipitation variations for Central Asian climate and plant evolution history.Competitive supply of moisture from the winter westerly and the monsoon summer rainfall determines the C3-C4 plant transition in the Asian ecosystem.