The Origin And Distribution Of Carbonates In The Loess Deposits From Northern China:Implications For Palaeomonsoon Changes

The variations of monsoon rainfall affect production,lives,survival and economic society of the East Asia.Thus,the precise prediction of monsoon rainfall can help people to avoid disasters under future glocal warming scenarios.It is useful to study East Asian summer monsoon(EASM)rainfall variability during the past warm interglacial periods,e.g.,early Pleistocene(2.6-1.65 Ma)and post-Mid-Brunhes event(MBE,(0.43-0 Ma)interglacial periods,might help to better understand monsoon dynamics and provide a geologically analogous mechanism for future EASM rainfall changes under global warming scenarios.However,it remains uncertain how the EASM intensity has varied with global temperature during above two past interglacial periods.This is due to possible lack of the reliable monsoon rainfall proxies.Loess deposits,located on monsoon areas in the northern China,are the thickest aeolian deposits over the world.These aeolian deposits document East Asian monsoon rainfall changes during Quaternary interglacial periods.Abundant carbonate minerals in these loess materials consist of calcite and detrital dolomite.Their dissolution,migration,and leaching loss phenomenon are sensitive to monsoon rainfall.However,previous studies on soil carbonates have encountered three challenge:quantification,Identification between primary and secondary carbonate as well as migration.Stoichiometric dolomite in Quaternary soil has to be inherited and has potential to indicate dissolution degree of primary carbonate and constrain leaching depth of carbonate.Here,the thesis develops a new quantification method of dolomite and calcite using Fourier transform infrared spectrophotometry,together with Inductively Coupled Plasma-Atomic Emission Spectroscopy and Gas Chromatograph Mass Spectrometer etc.methods.The thesis carries out a systematically survey on distributions,origins and geochemical characteristics of carbonate minerals sampling from North China Deserts and multiple loess sections located on the central CLP and Chifeng,northeastern China.With these information,the main sources areas of carbonate in loess of CLP are delineated and the correlation between dissolution degree of carbonate and monsoon rainfall are built.Based on carbonate dissolution phases,we reconstruct interglacial EASM rainfall records from the CLP and Northeast China since 2.6 Ma to better understand the dynamics of EASM on tectonic timescales,providing references for future prediction in EASM rainfall changes.1.Carbonate and dolomite contents in bulk samples decrease from western Taklimakan desert(16%and 2.4%,respectively)to eastern sandy lands(0%)controlling by surrounding bedrocks types.Some desert samples were divided into three grain sizes ranges(<63,63-154,>154 ?m).The results show that carbonate and dolomite contents in<63 ?m components form Taklimakan desert and adjacent areas(19.4%and 4.6%,respectively)are solely higher than their initial contents in loess on the CLP(14.7%and 3.2%,respectively).Whereas,other deserts/sandy lands have obviously lower carbonate and dolomite contents compared with loess except for the Qaidam desert which has similar their content to loess.This suggests Taklimakan desert and adjacent areas are major sources of carbonate in loess on the CLP.In addition,large ranges survey of carbonate in the loess deposits on the CLP also implies that initial detrital carbonate input during interglacials has probably remained constant since 2.6 Ma in the middle-southern CLP.2.The dolomite and calcite contents from eight loess sections since 130 ka on the CLP decrease from northwest to southeast and from glacial to interglacial on the CLP.The dolomite and calcite disappear in modern surface soils where the mean annual precipitation(MAP)reaches?610 mm and?690 mm,respectively.Based on the distribution degree of dolomite and calcite in loess-paleosol sequences,four dissolution phases of carbonate and each of which corresponds to a different EASM rainfall intensity are identified as follows:Dissolution Phase 1,the coexistence of dolomite and calcite indicates very low EASM rainfall(MAP<610 mm);Dissolution Phase 2,calcite without dolomite suggests low EASM rainfall(610<MAP<690 mm);Dissolution Phase 3,when both dolomite and calcite are absent but no downward leaching is observed indicating high EASM rainfall(690<MAP<725 mm);and Dissolution Phase 4,both dolomite and calcite are absent and downward leaching of the underlying layer is observed signifying very high EASM rainfall(MAP>725 mm).3.The distributions of dolomite and calcite in the typical Luochuan,Lingtai and Baoji loess-paleosol sequences are detailedly investigated.The dolomite and calcite contents from three loess sections generally decrease from loess units(glacial)to paleosol unites(interglacial).Applying new carbonate-based monsoon rainfall proxy,we have reconstructed interglacial EASM rainfall variations since 2.6 Ma.These records show that the interglacial EASM rainfall on the CLP is low during the warm early Pleistocene(1.65-2.6 Ma,Dissolution phase 2)and then gradually increases with global cooling during the middle and late Pleistocene(Dissolution phase 2 or 3 or 4).In contrary to previous suggestions that a warmer climate leads to higher monsoon rainfall on tectonic timescales,we propose that the low interglacial EASM rainfall during the early Pleistocene is caused by reduced sea surface temperature gradients across the equatorial Pacific,providing a testable hypothesis for climate models.Therefore,the variation of atmospheric circulation may need more attention in future prediction of EASM rainfall.4.The C-O isotopic characteristics of secondary carbonate and the acid soluble element concentrations from paleosol layers of Luochuan and Zhaojiachuan sections on the CLP are measured.The results show that Ca,Mg and Sr elements contents and their ratios in the acid soluble phase are controlled by carbonate content The three elements are also higher at early Pleistocene than mid-late Pleistocene.The ?3C of carbonate in the paleosol layers is distinctly related to the intensity of soil respiration,indicating the change of monsoon rainfall.The average value decrease from-5.5‰ to-7‰ from the early Pleistocene to the mid-late Pleistocene,generally decreasing with age.The ?13C of carbonate and elements concentrations in acid soluble phase in the paleosol layers form two sections show that the EASM rainfall is lower during early Pleistocene than late Pleistocene,consistent with carbonate-based reconstruction of EASM rainfall.These support our proposed hypothesis that the equatorial Pacific Walker circulation affects the EASM rainfall of interglacial period on tectonic timescale since 2.6Ma.5.We systematically investigate the distributions of the carbonate and redness in the Niuyangzigou section,Chifeng,Northeast China.The results show two proxies mainly reflect monsoon rainfall changes with heavy monsoon rainfall during the interglacials and low monsoon rainfall during the glacials.Interestingly,after MBE(?430ka),carbonate content increases and the redness decreases abruptly during the interglacial period,implying that the summer monsoon in Northeast China suddenly weakens after the MBE.We further compile the reliable EASM records over the China,finding the interglacial EASM in both the northeast China and the western region of Sichuan province abruptly weakens while other regions remain unchanged.These suggest that the EASM rainfall varied in the vast East Asian monsoon regions in response to MBE,one possible driver is the south-north shift of Intertropical Convergence Zone and/or the changes in the cross equatorial flow.But the detailed mechanism needs further study.Wheather or not,after the MBE,global temperature increased,while monsoon rainfall in above two regions decreased,indicating that the EASM rainfall in two regions may be reduced under global warming.