Distribution and origin of protodolomite from the late Miocene‐Pliocene Red Clay Formation, Chinese Loess Plateau

Abstract

[1] The Pliocene epoch is considered the most recent analog of modern warming because CO 2 levels were similar to the present. To explore the carbonate minerals formed in the warmer Pliocene epoch, we studied two continuous sections 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 Red Clay Formation on the CLP exhibits diagnostic FTIR absorption features of calcite and protodolomite. This allowed quantification of the two carbonate minerals by the FTIR method. Using the FTIR method we estimate the average concentration of protodolomite in Bajiazui is 3.6% whereas the Duanjiapo section is 6.0%. Protodolomite occurrence is more consistent and the concentration is higher from $6.5-4.2 Ma B.P. and decreases markedly from 4.2-2.6 Ma B.P. Red Clay protodolomite is depleted in both d 13 C PDB and d 18 O PDB , ranging from À4.1‰ to À10.4 and from À6.7‰ to À11.6, respectively, and has a slightly higher d18 O value than the calcites. SEM observations show that Red Clay protodolomite is composed of euhedral rhombic crystals that range from 1-20 mm in diameter, grow into the soil voids and coexist with authigenic calcite and palygorskite. These observations imply that the protodolomite grew in situ and is authigenic from pedogenesis. Dolomitization in the Red Clay sequence appears to be the result of overcoming kinetic barriers. We propose that in the Red Clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to Mg 2+ and significantly increasing the Mg/Ca ratio bringing about the formation of protodolomite.Components: 9100 words, 7 figures, 8 tables.

Miocene climate change on the Chinese Loess Plateau: Possible links to the growth of the northern Tibetan Plateau and global cooling

“…The amplitude of the calcite variability was persistently large during the Miocene, indicating the existence of significant seasonal wet‐dry fluctuations. By contrast, the protodolomite was formed in situ during pedogenesis under a relatively warm and dry climate, as revealed by its micromorphological characteristics and C–O isotopic composition [ He et al ., ]. Since the protodolomite was formed under higher temperatures and much drier conditions compared to the calcite, high contents of the protodolomite correspond to relatively dry climatic conditions.…”

“…Identification of major and clay minerals was made mainly according to the position of basal reflections of these minerals in the XRD diagrams [Liu, 1985;Moore and Reynolds, 1997;He et al, 2012]. XRD diffraction spectra reveal that the minerals in the Red Clay consist of illite (8.868 and 17.788), chlorite (12.58), mica (13.898 and 19.828), quartz (20.868 and 26.668), feldspar (27.5-28.58), calcite (29.488), and protodolomite (30.788) ( Figure 3a).…”

“…Illite is less susceptible to chemical weathering than chlorite [ Zhao et al ., ]; however, the high correlation (r 2 = 0.78, n = 206) between the chlorite/quartz and illite/quartz ratios indicates that chemical weathering during the Miocene was strong enough to cause the simultaneous transformation of these two minerals. Protodolomite, another climatically sensitive mineral, has also been used to infer changes in monsoon intensity during the late Miocene to Pliocene [ He et al ., ]. The protodolomite/quartz ratio exhibits a stepwise pattern of variation, with large‐amplitude fluctuations during 18.5–9.5 Ma and insignificant variability prior to 18.5 Ma and after 9.5 Ma.…”

Dolomite abundance in Chinese loess deposits: A new proxy of monsoon precipitation intensity

“…Dolomite in these loess‐paleosol sections is stoichiometric, well formed, and detrital [ Jeong et al ., ; Li et al ., ], not pedogenic protodolomite as He et al . [] found in the underlying Neogene Red Clay Formation (Text S1 in the supporting information). In this paper we suggest that Chinese loess‐paleosol sections may assist in clarifying the nature of the mineral dolomite as a precipitation indicator.…”

“…In this paper we suggest that Chinese loess‐paleosol sections may assist in clarifying the nature of the mineral dolomite as a precipitation indicator. As the dolomite content is low and difficult to quantify using traditional methods, such as X‐ray diffraction, we employed Fourier transform infrared spectrophotometry (FTIR) [ He et al ., ; Ji et al ., ] to quantify dolomite and calcite in our loess‐paleosol sequences. Quantification of these carbonate minerals permits us to determine the influencing factors which controlled their changes and estimate relative changes in precipitation intensity for the last glacial and interglacial cycle.…”