Abstract
Abstract. δ18O values of pedogenic minerals forming from soil water are commonly used to reconstruct paleoelevation. To circumvent some of the disadvantages of this method, soil n-alkane δD values were recently proposed as a new tool to reconstruct elevation changes, after showing that soil n-alkane δD values track the altitude effect on precipitation δD variations (r2=0.73 along Mt. Gongga, China). To verify the suitability of soil n-alkane δD values as a paleoelevation proxy we measured the δD of soil n-alkanes along Mt. Kilimanjaro (Tanzania). At midslope, soil n-alkane δD values are possibly influenced by the present precipitation belt, causing D-depletion in precipitation, and hence in the soil n-alkanes. Consequently, soil n-alkane δD values do not linearly relate with altitude (r2=0.03), suggesting that, in this case, they can not serve as an unambiguous proxy to infer past elevation changes. In contrast, it was recently shown that the MBT/CBT temperature proxy, which is based on the distribution of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids, is linearly related with MAT, and thus altitude (r2=0.77), at Mt. Kilimanjaro. This suggests that this proxy may be more suitable for paleoelevation reconstruction for this region. However, application of the MBT/CBT proxy on the altitude gradient along Mt. Gongga showed that, although the MBT/CBT-derived temperature lapse rate (−5.9°C/1000 m) resembles the measured temperature lapse rate (−6.0°C/1000 m), there is a relatively large degree of scatter (r2=0.55). Our results thus show that both proxies can be subject to relatively large uncertainties in their assessment of past elevation changes, but that a combination of the soil n-alkane δD and MBT/CBT proxies can likely result in a more reliable assessment of paleoelevation.
Highlights
Stable isotope values of authigenic and pedogenic minerals are a common tool for the reconstruction of paleoelevation changes in mountain ranges (e.g. Poage and Chamberlain, 2001; Rowley and Garzione, 2007, and references therein). δ18O and δD values of precipitation and meteoric water become more negative with increasing elevation due to rain-out caused by the decrease in temperature and relative humidity, the so-called “altitude effect” (Dansgaard, 1964), a trend that has been recognized in almost all mountain belts of the world (Poage and Chamberlain, 2001)
Precipitation and surface water isotopic composition along an altitude gradient provides an isotopic lapse rate that can be used as a reference line for the reconstruction of historical elevation changes
The membranes of these bacteria are composed of branched glycerol dialkyl glycerol tetraether (GDGT; Fig. 1) lipids, of which the molecular structure can vary in the amount of methyl branches (4 to 6) attached to the alkyl chain, and in the number of cyclopentane moieties (Sinninghe Damsteet al., 2000; Weijers et al, 2006)
Summary
Stable isotope values of authigenic and pedogenic minerals are a common tool for the reconstruction of paleoelevation changes in mountain ranges (e.g. Poage and Chamberlain, 2001; Rowley and Garzione, 2007, and references therein). δ18O and δD values of precipitation and meteoric water become more negative with increasing elevation due to rain-out caused by the decrease in temperature and relative humidity, the so-called “altitude effect” (Dansgaard, 1964), a trend that has been recognized in almost all mountain belts of the world (Poage and Chamberlain, 2001). An alternative method to reconstruct paleoelevation, independent of precipitation, could be the MBT/CBT temperature proxy, which is based on the membrane composition of a yet unknown group of bacteria that occurs ubiquitously in soils worldwide (Weijers et al, 2007c) The membranes of these bacteria are composed of branched glycerol dialkyl glycerol tetraether (GDGT; Fig. 1) lipids, of which the molecular structure can vary in the amount of methyl branches (4 to 6) attached to the alkyl chain, and in the number of cyclopentane moieties (up to 2) (Sinninghe Damsteet al., 2000; Weijers et al, 2006). Both soil n-alkane δD and the MBT/CBT proxy are only indirect recorders of altitude, i.e. through the altitude effect on precipitation δD and temperature lapse rate, respectively, and other factors besides altitude can potentially affect these proxies We, tested these two organicgeochemical proxies on two altitudinal transects to assess their suitability for paleoelevation reconstructions. Gongga by Sinninghe Damsteet al. (2008) and Jia et al (2008), respectively
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