The Early Eocene To Pliocene Palaeoenvironment Evolution And Biota In The Northern Qaidam Basin

The global distribution of both marine and terrestrial sedimentary sequences allows us to compare the different climatic records. Long term stable and continuous ancient sedimentary strata are key carriers of a variety of paleoclimate parameters. Numbers of Tertiary sedimentary basins were formed in the internal and surrounding regions of the Tibetan Plateau, as a result of the India-Asia collision and subsequent uplift of the Tibetan Plateau. With its thick sequences of Eocene to Pliocene terrestrial records, the Qaidam Basin at the NE margin of the Tibetan Plateau provides an important sedimentary archive for understanding the paleoclimate and surface uplift history of the Plateau.In this study, we present detailed data of geochemistry, clay mineralogy, sedimentary color and palynology, which are investigated from the excellently preserved sedimentary record in the Qaidam Basin, with a view to achieving a better understanding of middle Eocene to early Miocene paleoweathering history and paleoclimatic characteristics of the NE Tibetan Plateau. These data correlate well with the oceanic818O record during the middle Eocene to early Miocene, and provide evidence for the early uplift of the Tibetan Plateau.Based on changes of chemical element ratios in the vertical, chemical weathering index and the color records, three major climatic episodes could be identified in the middle Eocene to early Miocene sedimentary sequences of the studied section.①Early-middle Eocene episode (52.0--43.0Ma).The first major climatic episode corresponds to Unit1, which displays a remarkable feature of chemical element ratios, chemical weathering index and sedimentary color records. In this unit, the CIW’is characterized by the highest value and suggests the maximum chemical weathering degree in the whole sedimentary sequences. In addition, the redness of Unit1is also marked by the highest values compared to the other units. All these observations strongly suggest a warm and humid condition in the source regions during the middle Eocene episode (52.0--43.0Ma). This assumption is also supported by the previous palynology study. In the studied Unit1, the chlorite content is characterized by the lowest average content in the whole sedimentary sequences, which suggests a warm and humid condition.㎝idde Eocene to early Oligocene episode (43.0-～30.0Ma).This major climatic episode corresponds to the sediment in Unit2. In this unit, overall CIW,K/Na and Rb/Sr ratios are characterized by a significantly decrease compared to Unit1, which is consistent with a dramatic decrease of redness of the edimentary sequences. From the pollen record, vegetation changes in Unit2are probably the most dramatic in the whole studied sedimentary sequences. This dramatic decrease in chemical weathering degree and vegetion condition suggested a significant climate change occurred at-43.0Ma in the studied area.③Early Oligocene to early Miocene episode (30.0--18.5Ma). The long-term chemical weathering degree is characterized as a significantly decreasing trend since21.5Ma, which suggests a transition to cold and dry conditions in the source regions.The early elevation history of Tibetan Plateau has been studied by many researchers. In this study, we interpret that the～43.0Ma cooling event could be used as evidence for intensification of central Asia aridification, which could be attributed to attainment of high elevations in the southern-central Tibet, which in turn, formed an orographic barrier, preventing moisture from the Neotethyan oceans into the studied area at about43.0Ma. However, previous climate modeling studies have shown that the epicontinental sea that extended across Eurasia was the main source of moisture for northwest China and that its retreat would notably increase aridity in that region.The warmest temperatures of the last90m.y. occurred during the early Eocene Climatic Optimum (EECO;52-50Ma). In this study, we present a detailed whole geochemistry, clay mineral, combined with carbon and oxygen isotopes analyses in the well exposed early-middle Eocene sedimentary record in Qaidam Basin. Our goal is to achieve a better understand of the early Eocene Climate Optimum (EECO) event in the Qaidam Basin and assess variation in regional response during a time of warm global climate and elevated CO2levels.To the Qaidam basin, our sedimentological study on the sedimentary sequence of Lulehe Formation suggests that the red paleosol horizon (118-130m) is characterized by significantly high redness compared to its overlying and underlying strata and can be correlated in the region. Our detailed magnetostratigraphic study has bracketed the depositional age of this red paleosol horizon of50.68～50.48Ma, which is consistent with the peak interval of the Early Eocene Climatic Optimum (EECO). In addition to the significantly high redness, this paleosol horizon is also marked by the highest values for the chemical weathering index (CIW) and (S+Ka)/(Ch+I) ration in the whole sedimentary sequences. As a consequence, we can conclude that all these observations strongly indicate a severe intensification in chemical weathering across the red paleosol horizon (50.68-50.48Ma) occurred in the Qaidam basin.The red paleosol horizon represents a weathering signature of the peak interval of the Early Eocene Climatic Optimum (EECO). Using the average of all of the δ13Cc values-7.93‰(1σ=±0.45), measured from the paleosol carbonate nodules of the red paleosol horizon (50.68-50.48Ma), atmospheric pCO2during the peak interval of the Early Eocene Climatic Optimum (EECO) was604ppmV (±129based on1standard deviation of δ13Cc values),which is much higher than the preindustrial or natural modern value of280ppmv.We conclude that the development of the red paleosol horizon (50.68-50.48Ma) is the result of significantly increased chemical weathering intensity, in response to higher atmospheric pCO2during the peak interval of the Early Eocene Climatic Optimum (EECO).As the largest inland arid region in the world, the central Asian area is sensitive to the regional and global climate change. In this paper, we focus on the continuous lacustrine-deltaic sedimentary successions in Qaidam basin and present the results of ostracods record and stable isotope analyses of ostracods shells, as well as whole rock geochemistry, to investigate the paleoenvironmental changes and better constrain the evolution of drying in central Asia in middle Miocene.The paleontological data for the studied sequences of the north Qaidam Basin indicate marginal freshwater-to-saline subenvironments. Two main stages are recognized in the hydrological evolution of the marginal zone of the middle Miocene lacustrine-delta system of the Qaidam Basin, and they are differentiated on the basis of the ostracods assemblages, isotopic features, and paleoweathering characteristics.Stage I (13.78-13.35Ma):Stage I corresponds to the unit1and is characterized by the relatively low mean values in δ18O and δ13C, which can be related to periods of lake expansion in a humid climate. Furthermore, the O and C stable isotopic compositions in Unit1do not display covariance, which can be interpreted as indicative of low-residence time, open-system lacustrine dynamics. In addition, the ostracod fauna from this unit is dominated by fresh water species. In particular, both the CIW and K/Na rations in this stage are characterized by high value and suggest strong chemical weathering intensity in the studied area, which also indicate a humid conditions. Stage Ⅱ (13.35-12.36Ma):Stage II corresponds to the unit2. Both the δ18O and δ13C values in this unit are characterized by high values, narrow distribution and a stronger correlation, which is typical of a closed lake system with noticeable changes in the E/P balance. Meanwhile, the ostracods fauna becomes dominated by Cyprideis torosa and rapid increase ratio of evaporite minerals (gypsum and halite) in this unit indicate a transition from a freshwater to a saline lake. At the same time, the CIW and K/Na ratios in this stage are marked by significant decrease of chemical weathering degree compare to the Stage I. All these observations indicate an important drying event occurred at the studied regions since13.35Ma and can be interpreted as evidence for intensification of aridification in study area.The aridification of central Asia in the mid-Miocene (-13Ma) has been documented in the several paleoclimatic records from the sedimentary basins in the northern Tibetan Plateau. Several observations point to major changes in the tectonic evolution of the Tibetan Plateau and these are interpreted as responses to the uplift of the plateau. As mentioned above, these geologic evidences suggest that an extensive strong uplift event occurred in or around the Tibetan Plateau during the middle Miocene (-13Ma), which have changed the local atmospheric circulation and rainfall patterns precipitation patterns and contributed to the intensified aridity of the Qaidam Basin in13.35Ma because of topographic effects. Furthermore, as the contribution and timing of final retreat of the Paratethys from the central Asia are still poorly constrained, we can not rule out its influence to this major drying event in the Qaidam Basin.