Paleovegetation reconstruction using δ13C of Soil Organic Matter

Abstract

The relative contributions of C3 and C4 plants to vegetation at a given locality may be estimated by means of δ 13C of soil organic matter. This approach holds a great potential for paleoecological reconstruction using paleosols. However, two main uncertainties exist, which limits the accuracy of this application. One is 13C-enrichment as the plant carbon becomes incorporated into soil organic matter. The other is due to environmental influences on δ13C of plants. Two types of data were collected and analyzed with an objective of narrowing the error of paleovegetation reconstruction. First, we investigated δ13C variations of 557 C3 and 136C4 plants along a precipitation gradient in North China. A strong negative correlation is found between the δ13C value of C 3 plants averaged for each site and the annual precipitation with a coefficient of -0.40%/100mm, while no significant coefficients were found for C4 plants. Second, we measured13C of soil organic matters for 14 soil profiles at three sites. The isotopic difference between vegetation and soil organic matter are evaluated to be 1.8% for the surface soil and 2.8% for the soil at the bottom of soil profiles. We conducted a sample reconstruction of paleovegetation at the central Chinese Loess Plateau during the Holocene and the Last Glacial (LG), and conclude that, without corrections for 13C-enrichment by decomposition, the C4 abundance would be overestimated. The importance and uncertainties of other corrections are also discussed. in the vegetation at a given locality in the past. Many researchers have used δ13CSOM of paleosols and/or loess to reconstruct paleovegetation and paleoclimate (e.g. Stanley et al., 1991; Boutton, 1996; Boutton et al., 1998; Guillaume et al., 2001; Wang and Zheng, 1989; Gu, 1991; Frakes and Sun, 1994; Han et al., 1996; Wang and Follmer, 1998; Lin et al., 1991; Ding and Yang, 2000; Vidic and Montañez, 2004; Liu et al., 2005; An et al., 2005). For reconstructions of paleovegetation, the following isotope mass-balance equations are used: C3(%)= (δ13CSOM- δ13CC4)/( δ13CC3-δ13CC4 ) × 100; (1) C4(%)=100-C3(%) (2) where δ13CC3 , δ13CC4 are the mean δ13C values of C3 and C4 plants at a given locality at the time the soil under study was developed; C3 (%) and C4 (%) are percentages of C 3 and C4 biomass in the local vegetation. The accuracy of the reconstruction depends upon the accuracy of the end member values of δ13CC3 and δ13CC4 . The use of these equations also assumes that δ13CSOM represents the δ13C value of bulk local vegetation. To acquire the δ13C values for the pure C3 and C4 plants at the time paleosols formed, isotopic values for modern plants are first obtained. However, modern plants may be isotopically different from ancient plants due to environmental changes. An array of environmental factors influence the δ13C values of plants, and many scientists have investigated the effects (see the summary in Arens et al., 2000). Attempts have been made by several authors to correct the effects of these enviromental factors before Eqs. 1 and 2 were applied (Liu et al., 2005; Chen et al., 2005). Another source of uncertainty comes from the assumption that δ13CSOM equals the δ13C of vegetation. It is known that 13C-enrichment occurs during decomposition such that the soil organic matter tends to have higher δ13C values than the vegetation.