Geochemistry Of Carbonates In The Late Miocene To Pliocene Red Clay Deposits On Chinese Loess Plateau

The Pliocene Warming Period (PWP) is often considered the closest analog of modern global warming. Comparing with the pre-industrial conditions, mean annual temperature was2.5℃higher, with the atmospheric concentration of CO2between300-400ppm. During the PWP, higher latitudes were warmer, the ice cap was largely absent from Northern Hemisphere, and the sea level was higher. The Pacific climate was also quite different. The surface sea temperature (SST) of Pacific increased and the SST gradient between the equatorial Pacific was very small. The Northern Hemisphere atmospheric circulations show a robust response to meridional SST gradient changes. The reduced SST gradient widens the Intertropical Convergence Zone (ITCZ) and decreases precipitation amount at the western coast of Pacific. These Pliocene climate conditions were often called "permanent El Nino". Under modern El Nino-like climate, the landfalls will decrease in North China. However, there is still considerable debate over the characteristics of El Nino/southern oscillation (ENSO) during PWP. The opposing evidence suggests that a La Nina-like state was dominant during PWP. During a modern La Nina event, monsoonal rainfall will be enhanced in North China.The Red Clay Formation, which underlies the well-known Quaternary loess sequence, extends the eolian deposits from2.6Ma through the late Miocene making it a good archive to reconstruct Pliocene climate in North China. Paleoecological analysis of terrestrial snail communities from the Red Clay and pollen assemblages indicates the existence of monsoonal precipitation. However, the temporal sampling resolution in these studies is low and detailed information of EAM variability remains poorly understood. The Pliocene climate pattern was rather controversial and focused on the state of East Asian monsoon (EAM).The goal of this study is to explore the evolution of EAM during the Pliocene. To accomplish this goal, we studied the carbonate minerals formed in the warmer Pliocene epoch, as carbonate minerals have been suggested as potential paleoclimate proxies in Pleistocene loess-paleosol sequence. However, little work has been done on the carbonates from the underlying Red Clay. Few attempts have been made to determine Red Clay carbonate genesis or distinguish differences between carbonate species. As a result, the lack of information about the relative abundance of different carbonate mineral species in the Red Clay is a potential hindrance to paleoclimate reconstructions. We studied four continuous sections (Duanjiapo, Lingtai, Bajiazui and Jiaxian) of the Red Clay Formation on the Chinese Loess Plateau (CLP) by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Stable Isotope Mass Spectrometry. The fundermantal objective of these studies is to clarify Pliocene paleoclimate state by relative abundances, micro-morphology, stable isotope compositions of primary pedogenic protodolomite and calcite from four typical Red Clay sequences.The summary of innovations in this study was listed as follows including four aspects.1) Red Clay on CLP shows diagnostic FTIR absorption features of calcite and protodolomite, which allowed the two carbonate minerals to be identified and quantified using the FTIR method. Linear regression of the713cm-1and728cm-1absorption band area versus calcite and dolomite concentration, respectively, produces regression equations with R2’s of0.92and0.96. The protodolomite and calcite minerals contents were acquired for the four studies Red Clay sections. In all the four sections, protodolomite concentrations and the frequency of occurrence increase downward.2) SEM observations indicate that Red Clay protodolomite is composed of euhedral rhombic crystals coexisting with calcite and palygorskite. The crystals vary from1-20μm and grow into the soil voids, implying that the protodolomite is authigenic. The depletion of δ18O further indicates that this protodolomite formed in a fresh water environment. The above results demonstrate that the protodolomite originated from pedogenesis. Dolomitization in the Red Clay sequence appears to be the result of overcoming kinetic barriers. A high pore water Mg/Ca ratio is the critical kinetic factor controlling Red Clay dolomitization. Warm and seasonal dry conditions lead to the formation of protodolomite. Protodolomite is indicative for the warm and dry climate during Pliocene.3) Red Clay Formation contains the leached and calcareous horizons. Their carbonate content varies with high frequency and large amplitude. High MS values, Rb/Sr ratios and lower Zr/Rb ratios are found in the leached horizons, whereas low MS values, Rb/Sr ratios and higher Zr/Rb ratios are found in the calcareous horizons. These variations are comparable with those observed in the overlying paleosol-loess sequence. Seven highly leached horizons, whose carbonate content is near zero, reflect climate intervals dominated by large climate events. They were in phase with the deep sea δ18O isotope fluctuations, indicating a close connection with global climate change. At least two strong leaching/carbonate accumulation cycles occurred during the PWP (5-3Ma). The dramatic differences in the precipitation suggested by these cycles do not appear to support the hypothesis that past and future warmer climate would lead to a permanent El Nino-like climate.4) The carbon isotopes of carbonate nodules in Red Clay show a northward positive gradient on CLP, revealing that the abundance of C4plants was in proportion with aridity. C4plants greatly expanded in two intervals from6.7-6.4Ma and at～3.6Ma on CLP. The second C4expansion interval is in phase with the increasing dust deposition rate in North Pacific, suggesting that the drying of Asian interior may have triggered the C4plants expansion. Three important climate evolution stages were revealed from the oxygen isotopes of carbonate nodules in Red Clay. Between6.1-3.6Ma, oxygen isotopes of carbonate nodules gradually increased and after～3.6Ma, the increasing trend was even more evident. The enhancement of oxygen isotopes from carbonate nodules correlated with deep sea oxygen isotopes, indicating a close dependence on global temperatures.