The Yangtze River Delta: Holocene Sedimentary Environmental Evolution And Implications Of East Asian Summer-monsoon Inferred From Sediment Records

Monsoon is an important planetary climate system at a global scale. There are about half of the global population living in the monsoon climate zone. Monsoon has attracted a great scientific attention of numerous scholars and government and intergovernmental organizations because of its large seasonal and inter-annual changes, which often leads to climate flood and drought disasters. In a longer time scale, monsoon climate change is an important part of climatic evolution. Therefore, it is extremely important for us to study deeply its active role, influence, formation of mechanism and future prediction, in the view of its theoretic and practical implications to the survival and development of our human being.The Holocene is the newest geological Epoch of the world, the last glacial ablation period in the Quaternary. During the very short geological history of the Holocene, the fluctuations of cold/warm and dry/wet climate have always had a profound impact on human societal advancement and development. Considered as an historical analogue for the study of regional/global change in future, Holocene climatic fluctuation continues to be an important aspect for the Global Change studies.The Yangtze River Delta is located at the typically subtropical monsoon climate region, where it is profoundly influenced by the East Asian monsoon, and where the Holocene climate and sea level change and variety of associated natural disasters have brought about significant impact on human societal development. Probably known by most of us, the Yangtze River estuary, located at the mid-latitude coast of the N-hemisphere, where there is an intensive interaction between the land and sea, is a unique sedimentary environment sensitive to the climate-sea level fluctuations. The present study is also important for the ancient environment evolution because of rich monsoon evolution information can be derived from terrestrial and marine sediments, with reasonable sedimentation rates occurring throughout the Holocene time period. However, due to constraint of research funding, and suitable materials for dating, the process of research on subaqueous delta is far less active than that on the subaerial part of the delta system, making it difficult when compared with each other. At the same time, owing to various uncertainties, including multiple interpretations of the sediment proxies in relation to sedimentary facies evolution, etc., there have been still considerable gaps to fill out in order to establish effective proxies for the East Asia monsoon evolution from the Holocene sediment records in the Yangtze River estuary.The core HYZK5 was taken from the subaqueous Yangtze River delta with relatively higher deposition rates and relatively continuous sedimentary strata. This sediment core can be in general comparable to those obtained from adjacent subaerial delta area, providing a suitable sedimentary sample for tracing the information of sedimentary changes and climatic fluctuations of the Holocene. Based on sediment provenance, environment nature, lithology, particle size, magnetic susceptibility, foraminiferal abundance, geochemistry and organic carbon isotope obtained from in the core HYZK5, the purpose of this paper attempts for suitable environmental proxies that can be used to analyze the delta environment change and the East Asian summer monsoon nature. This would highlight the regional response to the Global climate change in Yangtze River estuary during Holocene, in order to help predict the future climatic change with its potential impacts on human development.Following results and conclusions can be acquired:1. Average clay content of the three sub-layers in core HYZK5 decreases from bottom upward, and clay content ofⅠsub-layer in the bottom is higher than that of the upper part of other two sub-layers; and so are TOC and TN contents accordingly. Clay content ofⅡsub-layer in the middle is higher than that ofⅢsub-layer on the top, while TOC and TN contents are lower on the contrary, showing that organic carbon or nitrogen content in the sediments may also be related to its sources, decomposition rate and preservation conditions, in addition to clay content.2. The core HYZK5 of nearly about 60m long was taken from the subaqueous Yangtze River delta. Homogeneous lithology occurs throughout in the core HYZK5. It consists of fluvial, coastal marsh, estuarine-shallow marine (prodelta) and distributary facies deposited since the latest glacial maximum. There are thicker early Holocene sediment strata (8.4m), with relatively continuous sedimentation. While comparing to the adjacent subaerial region in delta, the core HYZK5 of the present study serves as a good material for studying paleo-climate and paleo-environment evolution of Holocene. The estuarine-shallow marine (prodelta) and distributary facies were formed in the core HYZK5 from the early to mid-and later Holocene. According to lithology, particle size, magnetic susceptibility and foraminiferal abundance, relatively stable and continuous estuarine-shallow marine facies of the early Holocene can be further subdivided into estuarine-coastal facies deposited by marine transgression of the early Holocene, relatively stable shallow marine facies of the mid-Holoccne, and energetic shallow-marine sedimentary facies of the late Holocene.3. A-CN-K diagram reflects the source rocks of the HYZK5 sediments that have the same chemical compositions at the beginning of chemical weathering. Mean values of Ti and Ti/Zr of the H YZK5 sediments are very close to those of the Yangtze River sediments, so the sediments of the HYZK5 may come primarily from the Yangtze River. R-factor analysis of elements in the core HYZk5 sediments shows that the source and composition of its elements is chiefly controlled by factors such as the source rocks, particle size and marine biology. Based on the distance by R-type cluster analysis, its elements can be divided into three categories, which is exactly consistent with the classification results by correlation coefficient among the elements.4. Carbon isotope ratio (δ:3C) is not only affected by climate fluctuations, but also by different provenance duo to sea level change in complex estuary sedimentary environment, where the links betweenδ13C and climate may not be matchable. The sedimentary environment of C layer in the core HYZK.5 is relatively stable, and there the provenance of the clayey silt is mainly from the sediments of the Yangtze River according to the analysis of geochemical elements. Inorganic geochemical element indicators such as K/Na, Al/Na and CIA from sediments of C layer in Core HYZK5, are more sensitive than that ofδ13C in reflecting climate fluctuations in the estuary area.5. Based on the sedimentary records of the core HYZK5, four main periods of the East Asian summer-monsoon evolution have been revealed since Holocene:1) the early Holocene period, showing a rapid warming climate to reflect the initiation of enhancing East Asian summer monsoon; 2) the early and middle mid-Holocene period, demonstrating a relatively stable and warm-wet climate and implying a stronger East Asian summer monsoon; 3) the late mid-Holocene period, denoting a cold and dry climate to show a decline of the East Asian summer monsoon; 4) the late Holocene period, suggesting a relatively warm and humid climate with a re-enhanced East Asian summer monsoon. Of note, climate was very unstable with higher frequent and significant fluctuations during the early and late Holocene; while warm, wet and relatively stable during the Mid-Holocene. Furthermore, in the process of Holocene climatic evolution, at least 6 obvious cold and dry events occurred at the core depth of 51.6m,45.4m,39～41m,22～38m,17.3m and 13.5m, respectively.6. The similarity of paleoclimate evolution processes can be seen among the cores of HYZK5 and others in the eastern China regions since Holocene. This reflects that the climate fluctuations in the Yangtze estuary region was not only isolated events but throughout the entire east coast of China. In the core HYZK5. the rapid warming trend during the early Holocene may be directly forced by the changes in solar radiation in the mid-low latitude; the Holocene climatic optimum lagged behind the time when the summer insolation of the mid-low latitude in the Northern Hemisphere reached its peak, which may be explained by two aspects. One is that the primary factor is probably ice-volume forcing during the early Holocene, but since the mid-Holocene when the most glaciers melting prevailed in the Northern Hemisphere, changes in solar radiation could have played a dominant role in determining the strength of the East Asian monsoon; the other is the buffering action of the ocean. Therefore, the main reason is the changes of the coupled ocean-atmosphere-land climate system affected by variation of solar radiation, leading to significant changes of the Holocene summer monsoon as possibly recorded by the core HYZK.5 sediments. Global ice volume changes are influenced by solar radiation finally, and also contribute to the strength of the East Asian monsoon in Holocene, but with less important insolation forcing.