| Sedimentary rocks record important information on the evolution of Earth’s surface. The depositional and diagenetic processes of marine carbonate and the records of carbonate on the composition of seawater and carbon cycle have long been the frontiers and hotspots in Earth science. Dctrital sediments can also provide important information on chemical weathering as a function of paleoclimatic changes. Carbonate and detritai sedimentary rocks from Carboniferous to Triassic outcrop at Chaohu. Anhui, China, providing an excellent natural laboratory for studying the depositional and diagenetic processes of carbonates and detritai sediments, to reconstruct the composition of ancient seawater, the intensity of chemical weathering and the change of palcoclimate, and to trace terrigenous influx during deposition of marine carbonates in continental shelf. In this PhD thesis, I have focused on various types of carbonate and detrital sediments with different ages from the Chaohu area. An integrated study of carbonate carbon-oxygen isotopes and major-trace elements was carried out for this purpose. The results are used to understand the record of carbonate on the composition of ancient seawater and terrigenous influx, to discern the controlling factors on each elements of detrital sediments in order to establish new indexes for chemical weathering, to find a geochemical method to distinguish between authigenic carbonate sink and primary marine carbonate sink and to seek a geochemical framework in retrieving the linked sedimentary and diagenetic histories of marine carbonates.A combined study of major-trace elements and carbon-oxygen isotopes was carried out for Carboniferous limestones from Chaohu. The results provide insights into the records of marine carbonate on the composition of ancient seawater and terrigenous influx. According to the correlations between various elements with Al and Th, different elements exhibit different degrees of two-component mixing between seawater and terrigenous detritus. They are primarily dictated by such parameters as calcite-seawater partition coefficients (Dx), seawater-upper crust partition coefficients (Kx) and the redox sensitivity of trace elements. Water-soluble elements with high and medium Kx values such as Na, Mg, P, V, Cr, Ni, Cu, Zn, Sr, Mo and U have relatively high concentrations in carbonate lattices, which may record the composition of seawater. In contrast, water-insoluble elements with low Kx values such as Be, Be, Al, Sc, Co, Ga, Cs, REE, Hf and Th cannot be substantially taken up by carbonate and thus can be used to trace terrigenous influx. Carbonate carbon isotope profiles exhibit three positive and three negative shifts from the late Visean to the Gzhelian, which arc well comparable with those of contemporaneous strata in the world. The low δ13C intervals are associated with high terrigenous influx indicated by Y Ho, (Nd/Yb)pAAS. Th and Sc. Detritus from the intervals with high terrigenous inputs have high chemical index of alteration (CIA), representing high degree of chemical weathering. In this regard, high temperature may be the reason for negative δ13 shifts and the enhancement of chemical weathering may be the reason for high terrigenous influx. On the other hand, positive δ13 shifts and the decreases of terrigenous influx may be correlated with cold intervals with enhanced glaciations of Carboniferous, those arc C1 (the latest Viscan to the early Scrpukhovian), C2 (the Bashkirian to the early Moscovian) and C3 (the Gzhelian).After a systemic understanding of the behaviors of various elements in carbonates, it is possible to use these elements to study the depositional and diagenetic processes of marine carbonates. One type of diagenetic carbonates, authigenic carbonate, was recently invoked as a third major global carbon sink in addition to primary marine carbonate and organic carbon. It is fundamental to distinguish between authigenic and primary marine carbonates because this is important in our understanding of global carbon cycle and its role in regulating the evolution of atmospheric oxygen. Using microscale geochemical measurements of carbonates in the Early Triassic Yindushan section at Chaohu, we find that the growth of authigenic carbonate follows a different trajectory from primary marine carbonate in the plot of uranium concentration versus carbon isotope composition, indicating that a combination of the two geochemical variables is able to distinguish between the two carbonate sinks. The occurrence of authigenic carbonates in the Smithian suggests that the increase in the extent of carbonate authigenesis was negative feedback to the elevated atmospheric CO2 concentration. Using the method of identifying authigenic carbonate, we find that the Shuram excursion in the Doushantuo Formation was not caused by the incorporation of authigenic carbonate.A geochemical study of major-trace elements in detrital sediment and carbon-oxygen isotopes in carbonate was carried out for the west Pingdingshan section of early Triassic at Chaohu. This section is composed of argillaceous limestone and calcareous mudstone. The results place constraints on the geochemical behavior of various elements in the detrital sediments in order to establish new indexes for chemical weathering. According to the correlations between element concentrations and their variations in the profile, the elements of detrital sediments are categorized into four groups with different geochemical behaviors. The first group is composed of Al, Th, Sc, Be, In, Ga, K, Rb and Cs that are tightly correlated due to their immobility during chemical weathering. The second group is composed of Ca and Na that show the opposite variation trends with Th and Sc, on account of their mobile behavior in the weathering profile. The third group is composed of high field strength elements such as Ti,Nb, Ta, Zr and Hf that arc closely correlated with each other because they were primarily taken up by heavy minerals and thus arc sensitive to hydraulic sorting as well as aeolian contribution. The fourth group is composed of redox-sensitive elements such as Co, Cu, Fe, Mn and Ni that are correlated with S and thus mainly hosted by sulfides. Th. Sc, Ca, and Na were not amenable to changes in sedimentary provenance or hydraulic sorting, and hence are selected to establish proxies for chemical weathering. Geochemical proxies such as logarithmic parameters log(Th’Ca). log(Sc/Ca), log[Th/(Na/5+Ca)] and log[Sc/(Na/5+Ca)] exhibit synchronous increases at the Permian-Triassic boundary, the middle Gricsbachian and the early Smithian, representing the enhancement of chemical weathering. High proxy values approaching the values for the extremely weathered product of granodiorite occur in the middle to late Gricsbachian and early Smithian, demonstrating the occurrences of extremely chemical weathering and very warm palcoclimate in those periods. These palcoclimatic changes are parallel with results from studies elsewhere in the world.In order to find a method to comprehensively understand the sedimentary environments and the diagenetic processes of carbonate sediments, a combined study of REE+Y and C-O isotopes was carried out for the Carboniferous-Triassic marine carbonates in Chaohu. In modem shallow-water carbonate platforms, there is a successive changes of diagenetic zones linked to sea level drop. This relationship is important in the identification of diagenetic zones and in the interpretation of carbon isotope excursions and positive correlations between carbonate carbon and oxygen isotopes. However, it is not readily recognized in ancient carbonates because it is probably hard to obtain the information on the change of sea level. By plotting inter-REE+Y ratios such as Y/Ho against (Nd/Yb)pAAS for pure carbonates, it is demonstrated that mixing is evident between seawatcr and fresh water. The REE+Y distributions of pure carbonates show regular stratigraphic variations, reflecting regular changes in the amount of fresh water addition that were likely caused by the variations in relative sea level. Variations in diagenetic zones shown by the relationships between carbonate C-O isotopes and other evidences followed the changes in relative sea level suggested by the REE+Y distributions. Thus, a combination of REE+Y and C and O isotopes of pure carbonates can be used to obtain the changes in the linked dcpositional environments and diagenetic zones of marine carbonates. The drop of relative sea level and the final addition of fresh water in the Early to Middle Triassic were caused by basin elevation due to the collision of the South China and the North China blocks. |
PDF Full Text Request