Oxygen isotope ratios (¹⁸O/¹⁶O) of hemicellulose-derived sugar biomarkers in plants, soils and sediments as paleoclimate proxy II: Insight from a climate transect study

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• 作者：Mario Tuthorn ; Michael Zech ; Marc Ruppenthal ; Yvonne Oelmann ; Ansgar Kahmen ; H茅ctor Francisco del Valle ; Wolfgang Wilcke ; Bruno Glaser
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：1 February, 2014
• 年：2014
• 卷：126
• 期：Complete
• 页码：624-634
• 全文大小：889 K

文摘

The oxygen isotopic composition of precipitation (未¹⁸O_prec) is well known to be a valuable (paleo-)climate proxy. Paleosols and sediments and hemicelluloses therein have the potential to serve as archives recording the isotopic composition of paleoprecipitation. In a companion paper (Zech et al., 2014) we investigated 未¹⁸O_{hemicellulose} values of plants grown under different climatic conditions in a climate chamber experiment. Here we present results of compound-specific 未¹⁸O analyses of arabinose, fucose and xylose extracted from modern topsoils (n = 56) along a large humid-arid climate transect in Argentina in order to answer the question whether hemicellulose biomarkers in soils reflect 未¹⁸O_prec.

The results from the field replications indicate that the homogeneity of topsoils with regard to 未¹⁸O_{hemicellulose} is very high for most of the 20 sampling sites. Standard deviations for the field replications are 1.5鈥? 2.2鈥?and 1.7鈥? for arabinose, fucose and xylose, respectively. Furthermore, all three hemicellulose biomarkers reveal systematic and similar trends along the climate gradient. However, the 未¹⁸O_{hemicellulose} values (mean of the three sugars) do not correlate positively with 未¹⁸O_prec (r = 鈭?.54, p < 0.014, n = 20). By using a P茅clet-modified Craig-Gordon (PMCG) model it can be shown that the 未¹⁸O_{hemicellulose} values correlate highly significantly with modeled 未¹⁸O_{leaf water} values (r = 0.81, p < 0.001, n = 20). This finding suggests that hemicellulose biomarkers in (paleo-)soils do not simply reflect 未¹⁸O_prec but rather 未¹⁸O_prec altered by evaporative ¹⁸O enrichment of leaf water due to evapotranspiration. According to the modeling results, evaporative ¹⁸O enrichment of leaf water is relatively low (鈭?0鈥? in the humid northern part of the Argentinian transect and much higher (up to 19鈥? in the arid middle and southern part of the transect. Model sensitivity tests corroborate that changes in relative air humidity exert a dominant control on evaporative ¹⁸O enrichment of leaf water and thus 未¹⁸O_{hemicellulose}, whereas the effect of temperature changes is of minor importance. While oxygen exchange and degradation effects seem to be negligible, further factors needing consideration when interpreting 未¹⁸O_{hemicellulose} values obtained from (paleo-)soils are evaporative ¹⁸O enrichment of soil water, seasonality effects, wind effects and in case of abundant stem/root-derived organic matter input a partial loss of the evaporative ¹⁸O enrichment of leaf water.

Overall, our results prove that compound-specific 未¹⁸O analyses of hemicellulose biomarkers in soils and sediments are a promising tool for paleoclimate research. However, disentangling the two major factors influencing 未¹⁸O_{hemicellulose}, namely 未¹⁸O_prec and relative air humidity controlled evaporative ¹⁸O enrichment of leaf water, is challenging based on 未¹⁸O analyses alone.