Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 5165–5174 www.elsevier.com/locate/gca Soil n-alkane dD vs. altitude gradients along Mount Gongga, China Guodong Jia *, Kai Wei, Fajin Chen, Ping’an Peng State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Received 10 December 2007; accepted in revised form 6 August 2008; available online 20 August 2008 Abstract The altitude effect on the isotopic composition of precipitation and its application to paleoelevation reconstruction using authigenic or pedogenic minerals have been intensively studied. However, there are still no studies on variations in biomarker dD along altitude transects to investigate its potential as a paleoelevation indicator, although it has been observed that dDof higher plant lipid may record changes in precipitation dD(dDp). Here, we present dD values of higher plant-derived C27,C29, and C31 n-alkanes from surface soil along the eastern slope of Mount Gongga, China with great changes in physical variables and vegetation over a range from 1000 to 4000 m above sea level. The weighted-mean dD values of these n-alkanes (dDwax) 2 show significant linear correlations with predicted dDp values (R = 0.76) with an apparent isotopic enrichment (ewax–p)of À137 ± 9&, indicating that soil dDwax values track overall dDp variation along the entire altitudinal transect. Leaf dDwax is also highly correlated with mountain altitude by a significant quadratic relationship (R2 = 0.80). Evapotranspiration is found declining with altitude, potentially lowering dDwax values at higher elevations. However, this evapotranspiration effect is believed to be largely compensated by the opposing effect of vegetation changes, resulting in less varied ewax–p values over the slope transect. This study therefore confirms the potential of using leaf dDwax to infer paleoelevations, and more generally, to infer the dD of precipitation. Crown copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. 1. INTRODUCTION bonate, kaolinite, and muscovite that precipitate in equilib- rium with surface waters. Hence paleoelevation of Numerous studies have characterized the relationship mountainous regions, a crucial variable in tectonics and between elevation and the stable isotope composition of climate research, can be reasonably constrained (e.g., Poage precipitation or meteorically derived waters (Ambach and Chamberlain, 2001; Rowley and Garzione, 2007). et al., 1968; Siegenthaler and Oeschger, 1980; Ramesh However, in addition to source water isotopic composi- and Sarin, 1992; Bartarya et al., 1995; Garzione et al., tions, mineral isotopic compositions are influenced by 2000a,b; Poage and Chamberlain, 2001; Gonfiantini et al., many other confounding factors; for example, the tempera- 2001; Dalai et al., 2002). The isotopic values of precipita- ture at which minerals precipitate, the timing of mineral 18 tion (d Op and dDp) decrease as a result of distillation of formation, and diagenesis alteration after mineral forma- 18O and D in precipitation as the air masses move from tion (Poage and Chamberlain, 2001; Morrill and Koch, 18 the oceans to the continents. This decrease of d Op and 2002; Garzione et al., 2004, 2006). These factors introduce dDp values is enhanced as air masses pass over mountains, large uncertainties to paleoelevation reconstructions, sug- resulting in the so-called ‘‘altitude effect”. It has been shown gesting the needs to refine the reconstructions by multipr- that the altitude effect could be well documented in d18O oxy studies. Consequently, investigation of new proxies and/or dD of authigenic or pedogenic minerals, such as car- for paleoelevation is of great value. Compound-specific hydrogen isotope ratios of plant- derived lipids are emerging as a new paleoclimatic and * Corresponding author. Fax: +86 (20)85290706. paleohydrological proxy. Although the implicit assumption E-mail address: [email protected] (G. Jia). of nearly constant biological fractionation in many works 0016-7037/$ - see front matter Crown copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.08.004 5166 G. Jia et al. / Geochimica et Cosmochimica Acta 72 (2008) 5165–5174 has been suggested not strictly true (Sessions, 2006; Hou ical zonations (Thomas, 1999; Zhong et al., 1999)(Fig. 1). et al., 2007), making it difficult to establish quantitative The eastern slope is covered by dense clouds and fogs dur- environmental proxies based on lipid dD, surveys of mod- ing late spring and summer resulting in attenuation of solar ern environments spanning a range of environmental condi- radiation. Mean annual precipitation generally increases tions for establishing the relationship between lipid dD and with altitude, while evaporation is reverse, resulting in environmental water (Sauer et al., 2001; Huang et al., 2002, RH increase with elevation and positive water balance (pre- 2004; Yang and Huang, 2003; Sachse et al., 2004, 2006; cipitation–evaporation, P–E) above 2700 m. Mean annual Smith and Freeman, 2006; Hou et al., 2008) were still of sig- temperature declines upward from 11.8 °C at 1600 m asl to nificance, as indicated by successful applications of lipid dD 3.4 °C at 3000 m asl. Over a vertical range of 3800 m from to many paleoenvironmental studies that often use multipr- the subtropical arid Dadu River valley to the snowline, an oxies to corroborate each other (e.g., Huang et al., 2002; intact, continuous vertical vegetation spectrum from the Hou et al., 2006; Pagani et al., 2006; Jacob et al., 2007). subtropical zone to the frigid zone can be observed The correlations between organic dD and source water (Fig. 1). Mount Gongga thereby is an area with very high dD were found to vary significantly between different stud- biodiversity: about 2500 plant species belonging to 869 gen- ies, but the linear fits between the two variables still ac- era and 185 families have been identified (Thomas, 1999). counted for most of the observed variances suggested by Surface soil samples (0–5 cm) in A horizon for this study the coefficient of determination (R2) larger than 0.6 (Huang were collected after removing the litter layer along the slope et al., 2002; Sessions and Hayes, 2005; Hou et al., 2008). A transect of Mount Gongga over 1-week period in late May, high correlation between leaf lipid dD and precipitation dD 2004. According to Wang et al. (2005), all samples from A along a large natural transect with marked changes in cli- horizon of soil along the eastern slope of Mount Gongga mate and vegetation has recently been attributed to the off- have D14C values greater than zero, suggesting A horizons set of the opposing isotopic effects of relative humidity contain a substantial amount of bomb carbon and have (RH) and vegetation (Hou et al., 2008). Therefore, leaf lipid ages of about only several decades. Therefore, vegetation dD has the potential to record changes in the isotopic com- and moisture source can be assumed constant for sampled position of the hydrogen source at different climate settings, soil lipid signals. The altitude of each locality was deter- and further, to be used as paleoelevation proxy. However, mined using a handheld GPS unit with an error of in addition to common environmental parameters such as ±10 m. Samples at each locality were the mixture of three precipitation, evaporation, RH, and vegetation along a nat- subsamples randomly taken within a radius of 10 m, using ural horizontal transect, the mountain environment along a small metal scoop. All samples were wet, and each of an altitudinal transect has its own particularities, e.g., them was tightly sealed on site in a polyethylene zipper decreasing pressures of air, moisture, and CO2 with eleva- bag. The first bag was then sealed in a second, ‘‘outer”, zip- tions, that are crucial to plant transpiration and hence to per bag to insure against possible damage and leakage. isotopic fractionation. Therefore, the feasibility of using Bagged samples were frozen immediately after being carried leaf lipid dD in paleosols or sediments to infer paleoeleva- in the laboratory. The duration between sampling in the tions remains unclear and is pending to be answered. Here, field and freezing in the laboratory was about 2 weeks. dD data of higher plant-derived n-alkanes extracted from soil samples at various altitudes on Mount Gongga, China 2.2. Analysis of soil water isotopic values are presented in order to assess the potential of lipid dDasa paleoelevation proxy. In order to obtain both soil waters and lipids, we referred to an existing method which uses a dense, water- 2. STUDY AREA, SAMPLING, AND EXPERIMENTS immiscible liquid to displace soil solution during high speed centrifugation (Litaor, 1988; and references therein). 2.1. The altitudinal transect and samples We chose dichloromethane (DCM) as both the water- immiscible liquid and lipid extractant. Briefly, a thawed soil Mount Gongga, located approximately 30°N, 102°E sample was divided into several subsamples, and each sub- (Fig. 1) on the eastern side of the Tibetan Plateau in Sich- sample was immersed in DCM in a 50-mL Teflon tube for uan Province, southwest China, is the highest part (peak ultrasonic extraction for 10 min. Then phase separation was at 7556 m above sea level, asl) of the north-south trending performed by centrifugation at 6000 rpm, resulting in three mountain ranges collectively known as Daxueshan. The phases from the top to the bottom: water, DCM, and soil. eastern slope of Mount Gongga reaches down into the deep Subsequently, water and DCM were taken out carefully in Dadu River valley at 1100 m asl with a horizontal distance turn by pipettes. Usually after three times of extraction and of less than 30 km, and the western slope blends into the centrifugation, water content was too low to separate from eastern Tibetan Plateau at 3000 to 3500 m (Thomas, DCM.