Sedimentation Patterns Of Lake Kuhai,NE Tibetan Plateau During The Late Holocene Based On X-ray Fluorescence Analysis

Lake basins on the north-eastern Tibetan Plateau are considered important archives for the reconstruction of environmental and climate changes during the Late Quaternary.Lake deposits serve as ideal indicators for various transportation and depositional processes under changing hydro-climatic settings.Understanding dynamic processes is of crucial interest to decipher the process-response behavior of lakes by their sedimentary characteristics through time.The close and saline Kuhai Lake basin is ideal to unravel sediment dynamics and related lake level changes during the last 4.3 ka BP,revealed by two sediment cores KH16 and KH17 which were retrieved from the littoral and central part of the lake.The thesis explored the methods of calibration and statistical analysis based on X-ray fluorescence scanning(XRF)and Loss on ignition(LOI)data with the test of portable X-ray fluorescence(p-XRF)quantitative analysis.The calibrated XRF data together with LOI and grain size data of two cores are applied to investigate the sedimentary patterns in Lake Kuhai during the late Holocene.The cross-check between XRF scanning and p-XRF data shows that the accuracy of 80%selected elements could be improved with Normalization by Total Counts(NTC)method.Hence,NTC is suitable to remove the signal artifacts related to pronounced lithological changes especially for the elements prepared for multivariate analysis.To discuss the specific processing of the samples with similar lithological or compositional characteristics and visualize them directly,two grouping methods were applied.Multiple comparison test(MCT)is a good choice for the samples with clear lithological units and high resolution to distinguish the similarity of certain components,while K-mean clustering is more focused on the composition,which can provide clusters with specific geochemical characteristics.The compositional data like LOI and XRF scanning results should be treated with full caution due to problems of spurious correlation before operating multivariate analysis.Using centered log ratio transform(clr)and additive log ratio transform(air)for raw datasets would be a trial.Additionally,principle component analysis(PCA)and hierarchical clustering can reveal the relationship among the components and better understand the processes behind them.Results reveal four main groups of components:carbonate(Ca,Sr and CO3%),detrital flux(Fe,Rb,K and Ti),background material(Al and Si)and organic matter(OM%).However,the divisions vary subtly in different periods and sample groups.Based on these aspects,the process differences between two cores could be revealed.Core KH16 from the littoral zone reacted more sensitively to the inflow change compared with KH17 from the lake center.KH17 in the lake's center can better record changes in water level.The results of multi-proxy analysis including Ti,CO3%,Rb/Ti,Sr/Ca,OM%,Fe/Mn and clay%show,that the lake experienced six phases of hydrological changes during the last 4.3 ka.The lake was shallow during 4.3-2.9 ka BP while KH 16 site experienced wetland conditions.Generally,it was a relatively dry period with abundant OM productivity due to shallower water conditions.Due to these conditions river input became more pronounced during 4.1-3.7 ka BP at the site.The lake level increased to a high stage during 2.9-1.5 ka BP,however,river inflow to site of KH16 was limited probably caused by the changes in the river course and/or reduced discharge from the sub-catchment during 2.6-1.6 ka BP.The lake level decreased during 1.5-1.0 ka BP and increased thereafter during the Medieval Warm Period.The lake level decreased under relatively dry and cold conditions possibly related to deteriorated climate during the Little Ice Age.Afterwards,the ameliorating climate induced abundant primary productivity,and decrease of water salinity similar to the present time.The presented results and interpretation are based on a set of XRF and LOI data,which cannot fully explain all recorded differences between the two analyzed sediment cores.Comparison with additional proxies may improve the quality of inferred hydrological changes during the last 4.3 ka BP.Further analysis and comparison with other sediment cores from the lake will lead to a better understanding of water and material sources,local sedimentary processes and the interactions with climate.