Variations of organic carbon isotopic composition and its environmental significance during the last glacial on western Chinese Loess Plateau

Abstract

A high-resolution loess section in the western Chinese Loess Plateau, Yuanbao Section, was sampled for organic carbon isotopic analyses. The soil organic carbon isotope (δ 13 C org ) varied between −22.6‰ and −27.5‰ during the last glacial at the section. During the last interstadial, the δ 13 C org values were more negative than those in both early and late periods of the last glacial by 4‰. The isotopic composition indicates a coupled response of the pure C 3 plants to the temperature, precipitation and the concentration of atmospheric CO 2 . Decrease in temperature and the atmospheric CO 2 concentration from the last interstadial to Last Glaicial Maximum (LGM) caused the organic carbon isotopes to become positive by 1.5‰-2.0‰. The amplitude of 4‰ in the δ 13 C org variation during the last glacial should be mainly caused by the precipitation change.Therefore, the δ 13 C org variations of the Yuanbao Section during the last glacial period documented the large-amplitude fluctuation of the monsoon precipitation, which is estimated to be 250-310 mm more during the last interstadial than that in the LGM, and 100 mm more than that during early last glacial. The rapid changes of the monsoon precipitation on millennial scale during the last glacial have also been recorded in the isotopic variations in Yuanbao loess section. As the isotopic composition varies complicatedly as shown in the loess-paleosol sequence, it cannot be simply attributed to the abundances of C 3 and C 4 plants or be used as an indicator of the summer monsoon variations.Keywords: western Chinese Loess Plateau, last glacial, organic carbon isotopes, C 3 plants.The last glacial period is characterized by a cold and dry climate with low atmospheric CO 2 concentration. The relatively arid climate and low CO 2 concentration are favorable to the growth of C 4 plants, but the low temperature limits the development of this type of plants. Researches in low-latitude Africa and equator Columbia suggest that C 4 plants expanded during the last glacial period in those regions [1][2][3] . The organic carbon isotope records from two lakes in Mesoamerica indicate a reverse trend of the C 4 plants in the last glacial [4] . Researches on organic carbon isotopes (δ 13 C org ) in Chinese loess also suggest low abundances of C 4 plants during the last glacial [5][6][7][8] . The expansion of C 4 plants in low latitudes during the last glacial was interpreted as the response of C 4 plants to the low atmospheric CO 2 concentration or the increasing aridity or to both. In Chinese loess records, the reduction of C 4 plants in last glacial could be attributed to either low temperatures or increased precipitation during the winter as the climate condition was not sufficient to drive the expansion of C 4 plants though the CO 2 concentration was low [5 -7] . The δ 13 C org record in European loess [9] showed that the C 3 plants had been dominant throughout the whole last climatic cycle. The more positive δ 13 C org values during the last glacial were the response of C 3 plan...

Spatial and temporal variations of C3/C4 relative abundance in global terrestrial ecosystem since the Last Glacial and its possible driving mechanisms

“…When environmental temperature is below the "threshold value", especially during the growing seasons, C 4 plants will be absent in local ecosystem, such as the reconstructed paleovegetation in the high-latitudes since the Last Glacial [53,56,59] and that during the Last Glacial on the western edge of the CLP [35]. When environmental temperature is higher than the "threshold value", extremely arid, especially during the growing seasons, could also be adverse to the growth of C 4 plants, such as the modern vegetation in the Mediterranean climate zone (Pasadena in California and Seville in Spain [5] in Figure 1).…”

“…Luochuan, Jiaodao, Jixian, Yanshi, Xunyi, Weinan, Lantian, Lingtai, Yuanbao, Xifeng, Huanxian, Baoji, Figure 1) on the Chinese Loess Plateau (CLP) [18,19,[33][34][35] show an increase of C 4 relative abundance from the Last Glacial to the Holocene. In the eastern and central CLP, the magnitude of C 4 increase in several profiles is ca.…”

“…Even in the Holocene with higher C 4 abundance, C 4 plants only dominated on the eastern edge of CLP (Yanshi [19] in Figure 1). Organic   13 C data from the western edge of CLP indicate that this area was dominated by pure C 3 vegetation during the Last Glacial (Yuanbao [35] in Figure 1). Because   13 C values of modern plants are variable under different environmental conditions [36][37][38][39], and to a certain extent, it will become more positive during the decomposition of plant remains [39,40].…”

“…On the western edge area of the CLP, organic   13 C data indicate that the local biomass was predominated by C 3 plants during the Last Glacial period, consequently, the   13 C data have been attempted to be used in paleoprecipitation reconstruction [35].…”

Organic carbon isotope and molecular fossil records of vegetation evolution in central Loess Plateau since 450 kyr

“…Over the past few decades, researchers have studied past vegetation and climate changes throughout different periods from sediments on the Chinese Loess Plateau with various methods, such as soil microstructure (Guo et al, 1993), sporopollen in loess deposits (Liu and Su, 1994;Sun et al, 1997;Wu et al, 2004;Long et al, 2011), phytoliths (Lv and Liu, 1999), and stable carbon isotopes (Ding and Yang, 2000;Gu et al, 2003;Zhang et al, 2003;Vidic and Montanez, 2004;Liu et al, 2005;Rao et al, 2006;Chen et al, 2006;Zhou et al, 2009Zhou et al, , 2014Yang et al, 2012). This previous research has generally found that grassland vegetation on the CLP was a mixture of C 3 and C 4 herbaceous plants, with a dominance of C 3 plants since late Quaternary, and at no previous time was the CLP dominantly forested (Gu et al, 2003;Zhang et al, 2003;Zhou et al, 2009;Zhang et al, 2013).…”