PROTODOLOMITE IN THE LATE MIOCENE-PLIOCENE RED CLAY FORMATION, CHINESE LOESS PLATEAU: TELECONNECTION TO THE PLIOCENE PERMANENT EL NIÑO-LIKE CLIMATE

The closest analog to contemporary global warming is the Pliocene (5.3 -2.6 Ma). During the Pliocene northern hemisphere glaciation was nearly absent and atmospheric CO₂ concentration, sea level and temperature were higher than preindustrial values. However, the climate during Pliocene warm period (PWP) is rather controversial. To explore the Pliocene warming, we studied carbonate minerals in the Red Clay Formation on the Chinese Loess Plateau by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and stable isotope mass spectrometry. Red Clay sediments on Chinese Loess Plateau shows diagnostic FTIR absorption features and XRD signals for calcite and protodolomite, thereby allowing quantitative determination of the two carbonate minerals. The concentration and occurrence of protodolomite is higher from ~7 - 4.2 Ma, then decreased markedly and finally disappeared at ~2.7 Ma when Northern Hemisphere Glaciation started. SEM observation indicates that Red Clay protodolomite is made of euhedral rhombic crystals coexisting with calcite and palygorskite. The crystals vary from 1 – 20 microns and grow into soil voids. These morphologic features imply that the protodolomite grew in situ, that is, is secondary. The depletion of C and O isotopes further indicates that the protodolomite is freshwater, secondary dolomite. 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 high evaporation rates leads to the formation of calcrete from soil pore waters thereby significantly increasing the Mg/Ca and resulting in the formation of protodolomite. The occurrence of Red Clay protodolomite appears to be a response of Chinese Loess Plateau sediments to the global Pliocene warming and the permanent El Niño-like state in the Pacific, and implies that warm, dry and highly evaporative climate controlled the Chinese Loess Plateau as early as late ~7 Ma ago.