Did an extensive forest ever develop on the Chinese Loess Plateau during the past 130 ka?: a test using soil carbon isotopic signatures

摘要

Pleistocene vegetation history on the Chinese Loess Plateau has been traditionally investigated using palynological methods, and questions remain regarding whether an extensive broadleaf deciduous forest ever developed on the loess table under favorable climatic conditions. The authors have employed a C isotope approach to address this question by comparing delta(13)C values in soil organic matter from different loess ecological domains with known source vegetation to the C isotope values obtained from a paleosol section that can be dated back to 130 ka. The C isotopic compositions of modern soils from the loess table and the loess-desert transition gave delta(13)C values of -24.5parts per thousand to -18.2parts per thousand and -25.7parts per thousand to -20.7parts per thousand, respectively. These C isotopic ratios are consistent with the standing modern vegetation that is dominated by a mixture of C-3 and C-4 plants and can be distinguished from that in the patchy forest areas where exclusive C-3 trees yield a narrow delta(13)C value range from -26.9parts per thousand to -25parts per thousand (average -26.1parts per thousand). Obtained delta(13)C compositions from paleosols and loess sediments in the Lantian and the Luochuan profiles vary from -24parts per thousand to -16.9parts per thousand, indicating a grass-dominated steppe with shifting C-3 and C-4 contributions controlled mainly by paleoclimatic changes during the late Pleistocene. The present results suggest no extensive forest coverage on the loess table during the past 130 ka even under the most suitable conditions for forest development. This conclusion supports the explanation of natural causes for the development of only patchy forests on the modern loess table and provides critical historical information toward the vegetation restoration project that is currently underway on the Chinese Loess Plateau. (C) 2004 Elsevier Ltd. All rights reserved.