CARBON ISOTOPE RECORD OF AUTHIGENIC CALCITE FROM THE NORTHERN CHINESE LOESS PLATEAU: IMPLICATIONS FOR C_4 VEGETATION EVOLUTION DURING LATE MIOCENE-PLIOCENE
The timing for expansion of global C_4-plants is a key issue in understanding relationship between past climate changes and Earth's boundary conditions. However, the possible causes that have motivated the expansion remained disputed. Previous studies on carbon isotopic composition of mammal teeth fossil, soil organic matter and pedogenic carbonate have suggested that the decrease in atmospheric C0_2 concentration may partly contribute to this expansion. Recent studies suggested that the C_4-plants expansion is not globally synchronous, thus regional climate exerted a strong control on the relative abundance of C_4-plants. Jiaxian Red Clay section ( 38° 16'N, 110°05'E) locates in the northern Chinese Loess Plateau near the northern limit of East Asian summer monsoon,an ideal site to study relation between regional vegetation cover and climatic changes. The Red Clay section is 64m thick, dated from 8.2Ma to 2.6Ma. Episodic changes in species and contents of carbonate minerals in the section allow us to study C_4-plants evolution using the stable isotope techniques. Here we develop a simple and fast approach to obtain delta~(13)C changes of secondary calcite minerals totally reflecting paleo-ecological changes. This new approach overcomes the problem of carbonate minerals of detrital origin that complicate an interpretation of delta~(13)C changes in bulk sample of carbonate minerals related with paleo-ecological changes. We identified species of carbonate minerals for 608 bulk soil samples using Fourier Transform Infrared Spectrometry (FTIR) and established dataset with 190 samples that are composed of abundant secondary calcite (>5%) without any of dolomite minerals. We avoided dolomite minerals in measurement of delta~(13)C for following reasons. Dolomite initially eroded from bedrock in dust provenance, having no paleo-ecological significance. The calcite minerals were easily weathered after the Red Clay deposition, thus mostly of secondary origin. Due to two order of magnitude larger rate than calcite formation,the source of carbon in the calcite of secondary origin mostly inherited from soil respiration. Hence, the carbon isotope values for our selected calcites are strongly related to paleo-ecological evolution in site. We analyzed the delta~(13)C values in calcite for 190 samples with equal space interval, so as to obtain a long-term C_3-plants record. The delta~(13)C values varied from -1.67%o to -7.85%o with a mean value of -4.86%o. The delta~(13)C values of calcite formed in 100% C_3-plants conditions ranged from -12%o to -10%o.