Altitudinal effect of soil n-alkane delta D values on the eastern Tibetan Plateau and their increasing isotopic fractionation with altitude

Author:	Bai, Y., C. H. Chen, X. M. Fang, X. M. Liu and H. C. Guo
Abstract:	Stable isotope paleoaltimetry has provided unprecedented insights into the topographic histories of many of the world's highest mountain ranges. However, on the Tibetan Plateau (TP), stable isotopes from paleosols generally yield much higher paleoaltitudes than those based on fossils. It is therefore essential when attempting to interpret accurately this region's paleoaltitudes that the empirical calibrations of local stable isotopes and the relations between them are established. Additionally, it is vital that careful estimations be made when estimate how different isotopes sourced from different areas may have been influenced by different controls. We present here 29 hydrogen isotopic values for leaf wax-derived n-alkanes (i.e., D-delta(wax) values, and abundance-weighted average delta D values of C-29 and C-31) in surface soils, as well as the delta D values of soil water (delta D-sw) samples (totaling 22) from Mount Longmen (LM), on the eastern TP (altitude similar to 0.8-4.0 km above sea level (asl), a region climatically affected by the East Asian Monsoon (EAM). We compared our results with published data from Mount Gongga (GG). In addition, 47 river water samples, 55 spring water samples, and the daily and monthly summer precipitation records (from May to October, 2015) from two precipitation observation stations were collected along the GG transect for delta D analysis. LM soil D-delta(wax) values showed regional differences and responded strongly to altitude, varying fromaEuro'160aEuro degrees toaEuro'219aEuro degrees, with an altitudinal lapse rate (ALR) ofaEuro'18aEuro degrees km(aEuro'1) (R (2)=0.83; p < 0.0001; n=29). These D-delta(wax) values appeared more enriched than those from the GG transect by similar to 40aEuro degrees. We found that both the climate and moisture sources led to the differences observed in soil D-delta(wax) values between the LM and GG transects. We found that, as a general rule, epsilon (wax/rw), epsilon (wax/p) and epsilon (wax/sw) values (i.e., the isotopic fractionation of D-delta(wax) corresponding to delta D-rw, delta D-p and delta D-sw) increased with increasing altitude along both the LM and GG transects (up to 34aEuro degrees and 50aEuro degrees, respectively). Basing its research on a comparative study of D-delta(wax), delta D-p, delta D-rw(delta D-springw) and delta D-sw, this paper discusses the effects of moisture recycling, glacier-fed meltwater, relative humidity (RH), evapotranspiration (ET), vegetation cover, latitude, topography and/or other factors on epsilon (wax/p) values. Clearly, if epsilon (wax-p) values at higher altitudes are calculated using smaller epsilon (wax-p) values from lower altitudes, the calculated paleowater delta D-p values are going to be more depleted than the actual delta D values, and any paleoaltitude would therefore be overestimated.
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Page number:	1664-1673
Issue:	9
Subject:
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PubYear:	2017
Volume:	60
Publication name:	Science China-Earth Sciences
Abstract:	Stable isotope paleoaltimetry has provided unprecedented insights into the topographic histories of many of the world's highest mountain ranges. However, on the Tibetan Plateau (TP), stable isotopes from paleosols generally yield much higher paleoaltitudes than those based on fossils. It is therefore essential when attempting to interpret accurately this region's paleoaltitudes that the empirical calibrations of local stable isotopes and the relations between them are established. Additionally, it is vital that careful estimations be made when estimate how different isotopes sourced from different areas may have been influenced by different controls. We present here 29 hydrogen isotopic values for leaf wax-derived n-alkanes (i.e., D-delta(wax) values, and abundance-weighted average delta D values of C-29 and C-31) in surface soils, as well as the delta D values of soil water (delta D-sw) samples (totaling 22) from Mount Longmen (LM), on the eastern TP (altitude similar to 0.8-4.0 km above sea level (asl), a region climatically affected by the East Asian Monsoon (EAM). We compared our results with published data from Mount Gongga (GG). In addition, 47 river water samples, 55 spring water samples, and the daily and monthly summer precipitation records (from May to October, 2015) from two precipitation observation stations were collected along the GG transect for delta D analysis. LM soil D-delta(wax) values showed regional differences and responded strongly to altitude, varying fromaEuro'160aEuro degrees toaEuro'219aEuro degrees, with an altitudinal lapse rate (ALR) ofaEuro'18aEuro degrees km(aEuro'1) (R (2)=0.83; p < 0.0001; n=29). These D-delta(wax) values appeared more enriched than those from the GG transect by similar to 40aEuro degrees. We found that both the climate and moisture sources led to the differences observed in soil D-delta(wax) values between the LM and GG transects. We found that, as a general rule, epsilon (wax/rw), epsilon (wax/p) and epsilon (wax/sw) values (i.e., the isotopic fractionation of D-delta(wax) corresponding to delta D-rw, delta D-p and delta D-sw) increased with increasing altitude along both the LM and GG transects (up to 34aEuro degrees and 50aEuro degrees, respectively). Basing its research on a comparative study of D-delta(wax), delta D-p, delta D-rw(delta D-springw) and delta D-sw, this paper discusses the effects of moisture recycling, glacier-fed meltwater, relative humidity (RH), evapotranspiration (ET), vegetation cover, latitude, topography and/or other factors on epsilon (wax/p) values. Clearly, if epsilon (wax-p) values at higher altitudes are calculated using smaller epsilon (wax-p) values from lower altitudes, the calculated paleowater delta D-p values are going to be more depleted than the actual delta D values, and any paleoaltitude would therefore be overestimated.
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