n-Alkane lipid biomarkers in loess: post-sedimentary or syn-sedimentary?

Michael Zech1, Tomasz Goslar2, Sebastian Kreutzer3, Markus Fuchs4, Tobias Krause5, Sascha Meszner5, Dominik Faust5
1 LS Geomorphologie & Abt. Bodenphysik, Universität Bayreuth
2 Poznan Radiocarbon Laboratory, Poland
3 LS Geomorphologie, Universität Bayreuth
4 Physische Geographie, Justus-Liebig-Universität Gießen
5 Physische Geographie, TU Dresden

P 2.15 in Loess and terrestrial archives

There is an ongoing discussion on whether n-alkane biomarkers – and loess organic matter (OM) in general – reflect a syn-sedimentary paleoenvironmental and paleoclimate signal or are significantly post-sedimentary contaminated by root-derived OM. We present first radiocarbon data for the n-alkane fraction of lipid extracts for the Middle to Late Weichselian loess-paleosol sequence Gleina in Saxony, Germany. Comparison of these biomarker ages with sedimentation ages as assessed by optically stimulated luminescence (OSL) dating shows that one n-alkane sample features a syn-sedimentary age (29.2±1.4 cal ka BP versus ~27.3±3.0 ka). By contrast, the three other n-alkane samples are clearly younger (20.3±0.7 cal ka BP, 22.1±0.7 cal ka BP and 29.8±1.4 cal ka BP) than the corresponding (partly interpolated) OSL ages (26.5±3.1 ka, 32.0±3.5 ka and 45.6±5.3 ka, respectively). This finding suggests that a post-sedimentary n-alkane contamination presumably by roots has occurred.

In order to estimate the post-sedimentary n-alkane contamination more quantitatively, we applied a 14C mass balance calculation based on the measured pMC values, the calculated syn-sedimentary pMC values and pMC values suspected to reflect possible post-sedimentary contamination (current, modern, 3 ka, 6 ka and 9 ka). Accordingly, current, modern and 3 ka old root-contamination would account for up to 10% of the total sedimentary n-alkane pool, whereas an Early and Middle Holocene root-contamination would account for up to 19% of the total sedimentary n-alkane pool.

We acknowledge and encourage that these first radiocarbon results need further confirmation both from other localities and for different biomarkers, e.g. carboxylic acids or alcohols as further lipid biomarkers, which can be easily extracted in high enough amounts even from OM-poor loess.

last modified 2012-07-26