Abstract
Abstract. Paleoenvironmental and paleoclimate reconstructions based on molecular proxies, such as those derived from leaf-wax biomarkers, in loess-paleosol sequences represent a promising line of investigation in Quaternary research. The main premise of such reconstructions is the synsedimentary deposition of biomarkers and dust, which has become a debated subject in recent years. This study uses two independent approaches to test the stratigraphic integrity of leaf-wax biomarkers: (i) long-chain n-alkanes and fatty acids are quantified in two sediment-depth profiles in glacial till on the Swiss Plateau, consisting of a Holocene topsoil and the underlying B and C horizons. Since glacial sediments are initially very poor in organic matter, significant amounts of leaf-wax biomarkers in the B and C horizons of those profiles would reflect postsedimentary root-derived or microbial contributions. (ii) Compound-specific radiocarbon measurements are conducted on n-alkanes and n-alkanoic (fatty) acids from several depth intervals in the loess section "Crvenka", Serbia, and the results are compared to independent estimates of sediment age. We find extremely low concentrations of plant-wax n-alkanes and fatty acids in the B and C horizons below the topsoils in the sediment profiles. Moreover, compound-specific radiocarbon analysis yields plant-wax 14C ages that agree well with published luminescence ages and stratigraphy of the Serbian loess deposit. Both approaches confirm that postsedimentary, root-derived or microbial contributions are negligible in the two investigated systems. The good agreement between the ages of odd and even homologues also indicates that reworking and incorporation of fossil leaf waxes is not particularly relevant either.
Highlights
Biomarkers or chemical fossils are relatively poorly decomposable components of plants, microorganisms and animals, which, in some cases, record past environmental conditions (Eglinton and Eglinton, 2008)
Organic carbon (Corg)-normalized concentrations of longchain n-alkanes and fatty acids ( C24−34) show a sharp decrease with depth in both sediment profiles in till (Fig. 2). This decrease is pronounced in the Steinhof profile, where the uppermost sample was taken at a depth of only 10 cm, and leaf-wax concentrations are highest (∼ 640 μg/gC for n-alkanes; ∼ 5600 μg/gC for fatty acids)
Concentrations of characteristic plant-wax biomarkers were negligible below the topsoil in the two sediment profiles examined in glacial till
Summary
Biomarkers or chemical fossils are relatively poorly decomposable components of plants, microorganisms and animals, which, in some cases, record past environmental conditions (Eglinton and Eglinton, 2008). The distributions of plant waxes, long-chain n-alkanes and fatty acids, are used to reconstruct past vegetation (Xie et al, 2002; Zhang et al, 2006; Zech et al, 2009b, 2010), while stable carbon isotopic (δ13C) and D/H (deuterium/hydrogen) analyses of these compounds provide valuable information about changes in paleohydrology and paleoclimate (Liu and Huang, 2005; Zech et al, 2011b, 2013). For long-chain n-alkanes and fatty acids, which are thought to be mainly produced above ground as epicuticular leaf waxes, this assumption seems reasonable given their low solubility and limited mobility. Gocke et al (2013, 2014) suggested that significant amounts of long-chain n-alkanes and fatty acids are produced within rhizoliths in the vicinity of root systems.
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