HIGH-RESOLUTION DELTA13C VARIATIONS IN C3 AND C4 PLANTS ON THE CHINESE LOESS PLATEAU - A COMPARISON OF COMPOUND SPECIFIC AND BULK ORGANIC CARBON ISOTOPIC ANALYSES

High-resolution d¹³C records of bulk organic matter and compound specific analysis from three loess-paleosol successions from the central Chinese Loess Plateau document systematic fluctuations in C₃ and C₄ plant abundance for discrete intervals that support a dominant climatic control on C₃ and C₄ plant distributions over the past 620 k.y.  Total lipid extracts range from 125 to 368 µg/g, and are higher in paleosols than in overlying and underlying loess deposits.  The n-alkane distributions of all samples show strong odd-carbon preference.  Ratios of lower (C₁₅, C₁₇, C₁₉) to higher (C₂₇, C₂₉, C₃₁) homologues range from 0.22 to 0.38 for the bounding loess and 0.28 to 0.51 for the paleosols indicating the dominance of leaf wax components in the paleosols. The similarity in the d¹³C values of n-C₂₉ and n-C₃₁, which range from -29.8 to -33.1 per mil for the bounding loess and from -26.9 to -29.6 per mil for the paleosol, indicates their common sources. Paleo-floral reconstruction based on bulk d¹³C of organic matter documents a decrease in C₄ vegetation driven by enhanced winter precipitation and lower temperatures associated with low pCO₂ during glacial intervals.  Calculations of the C₄ component based on the d¹³C values of n-C₂₉ and n-C₃₁ support the findings of Vidic and Montanez (2004) based on d¹³C of bulk organic matter.  A d¹³C value of -38 per mil for the C₃ end member, which is within the recognized range (-32 to -39 per mil; Collister et al. 1994), yields a direct correspondence between the abundance for the C₄ component derived from bulk and compound-specific methods. Other components within the sediment extracts include abietic acid and carboxylic acid methyl esters that hint at the possibility of employing a broader diversity of compounds to further assess the origins of the organic matter in loess-paleosol sequences.