Changes Of Regional Hydrology And Religious Activities Recorded By Sediment From Lake Dalzong,Xiahe,Gansu Province During The Last Millennium

The Tibetan Plateau,affectionately referred to as the"roof of the world"and the"third pole of the Earth",is a geographically significant region.Due to its location at the intersection of three atmospheric circulation systems-namely,the Indian Summer monsoon,the East Asian Summer monsoon,and the westerly monsoon-this area is particularly sensitive to both global changes and human intervention.Considered the"water tower of Asia",the Tibetan Plateau plays a crucial role in the survival and development of billions of people in surrounding countries.However,this region also faces significant environmental challenges due to global warming and is particularly uncertain regarding future climate change.Historical hydrological events can shed light on past changes and assist in assessing current hydrological conditions while predicting future trends.In light of rapid human development,the environmental issues of the Tibetan Plateau-including its proximity to emission sources in heavily populated areas such as the Middle East and South Asia-have become increasingly prominent.The plateau has become one of the regions significantly impacted by the cross-border transport of pollutants,including black carbon.Meanwhile,the Tibetan Plateau has a long history of life and religious sacrifices,including aromatic plant-burning,which were recorded in the geologic record,thus black carbon can be used as a potential indicator to study the change in religious activities.This study focuses on Lake Dalzong,which is an alpine laminae lake located in Xiahe County,Gannan Tibetan Autonomous Prefecture,northeastern Tibetan Plateau.The research involves collecting modern surface sediments and water samples from June 2020 to April 2023 at monthly and seasonal intervals.Additionally,sediment cores with a total length of 457.5 cm were obtained.By analyzing the hydrogen and oxygen isotopes in water,geochemical indices of surface sediments,and typical laminae samples,annual laminae were successfully identified.Meanwhile,a reliable chronological framework for the past thousand years was established by using ¹³⁷Cs,AMS¹⁴C and the varve.Lastly,by measuring grain size,XRF-based elements,and black carbon content changes over the past millennium with high resolution,the study reconstructed the hydrological and religious history of the area.Key findings include:（1）A laminar sedimentary tectonic unit found in Lake Dalzong is composed of three main layers.The first layer,a light layer of bright laminar,is followed by a coarser dark layer of dark laminar,and the topmost layer is a fine layer of dark laminar.The coarse-grained dark layer forms from summer to early autumn due to increased precipitation which brings exogenous detrital materials into the lake.Settling to the bottom of the lake,more organic matter attaches to the surface of detrital minerals forming this layer.On the other hand,the fine-grained dark layer develops during the freezing period of the lake surface（from mid-November to mid-April of the following year）when water movement is minimal,and tiny dead organisms settle slowly.Lastly,the fine-grained brightening layer forms from late spring to summer when the ice melts in spring carrying a large amount of exogenous debris into the lake.With low organic matter content,it creates a relatively fine brightening layer.These findings suggest that a complete laminar sedimentary tectonic unit follows an interannual sedimentary cycle,thus confirming its annual laminar nature.（2）The coarse grain dark layer with a thickness of 0.3～1.2 cm was extracted and defined as the varve event layer.Based on the varve event layer of Lake Dalzong,38heavy precipitation events during the Little Ice Age were identified with annual resolution.According to the classification of heavy precipitation events by the thickness of the event layer,extremely strong precipitation（event layer thickness>1 cm）7 times,moderately heavy precipitation（event layer thickness between 0.5 and 1 cm）21 times,weakly heavy precipitation（event layer thickness<0.5cm）10 times.The occurrence of heavy precipitation may be controlled by El Ni（?）o and volcanic eruption events.On the one hand,when the El Ni（?）o event occurs,the Western Pacific subtropical high becomes stronger and extends westward to the eastern part of the Tibetan Plateau.This phenomenon will lead to the northward transport of water vapor over the South China Sea and the Bay of Bengal in summer,and then form an unusually heavy rain belt within the latitude of 2～10°north of the ridge.On the other hand,volcanic eruptions can strengthen the El Ni（?）o-enhanced Asian summer monsoon,leading to heavier rainfall.（3）The black carbon of Dalzong Lake sediments is mainly accumulated by the proximity wet deposition process,and its content changes reflect the strong and weak variability of surrounding religious activities.The results of the study indicate that the area where Lake Dalzong is located has shown three periods of enhanced religious activities in the last millennium:1490～1565CE（mid-Ming Dynasty）,1810～1890CE（late Qing Dynasty）,and 1920CE to the present（since the Republic of China）,and this enhanced religious activity on a 100-year time scale is a response to the social development in the same period.The results of this research will help promote the study of the historical context of the Anthropocene on the Tibetan Plateau.