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Climatically driven glacial-interglacial variations in C₃ and C₄ plant proportions on the Chinese Loess Plateau

¹ Department of Agronomy, University of Ljubljana, 1000 Ljubljana, Slovenia
² Department of Geology, University of California, Davis, California 95616, USA

High-resolution ¹³C records of bulk organic matter (OM) from three loess-paleosol successions from the central Chinese Loess Plateau document systematic fluctuations in C₃ and C₄ plant abundance for certain intervals during the past 620 k.y. Measured ¹³C values of OM in paleosols are less negative by 0.5–4, and total organic carbon (TOC) contents are higher by up to 0.5 wt% relative to corresponding values in loess deposits. The ¹³C_OM and TOC decrease from peak values in paleosols progressively into overlying and underlying loess deposits. Integrated ¹³C_OM, TOC, and magnetic susceptibility rec ords indicate an increase in C₄ vegetation to 50% during peak paleosol formation and a decrease in C₄ vegetation to 30% during loess deposition for three of the past glacial-interglacial cycles. These results indicate that C₄ plant abundance declined during glacials in this mid-latitude region driven primarily by a shift to enhanced winter precipitation and lower temperatures, and further contribute to the rapidly growing body of evidence that low pCO₂ during Pleistocene glacials was insufficient to trigger C₄ plant expansion without coincident favorable climatic conditions.

Key Words: paleosols • Chinese Loess Plateau • carbon isotopes of organic matter • C₄ and C₃ plants • glacial-interglacial cycles

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