Fossil redox-conditions influence organic matter composition in loess paleosols

The soil memory recorded in paleosols of loess-paleosol sequences is an important contributor to our understanding of past climatic conditions. Molecular proxies based on the organic matter preserved in paleosols form an essential part of this record, but the long-term preservation of SOM is poorly understood, especially for sediment traps and slope profiles. This paper addresses the composition of organic material from the Early Weichselian A-horizons of the Rocourt paleosol, a major paleostratigraphic marker for the Eemian and Early Weichselian in Western Europe. NaOH-extractable organic matter from an exceptionally thick Rocourt profile in the Kesselt Quarry (Belgian Loess Belt) was analyzed by pyrolysis-Gas Chromatography/Mass Spectrometry (pyrolysis-GC/MS) and the results evaluated against paleopedological data. The molecular composition of the organic matter at Kesselt was compared with reference samples from two nearby quarries (including the type locality at Veldwezelt-Hezerwater), and to a sample from the contemporary Nussloch sequence in Germany. The SOM composition found at the four sites indicated a large content of microbial matter and was dominated by carbohydrates and N compounds, many of which were not reported before from SOM pyrolysates. Differences in the molecular composition between samples, both within profiles and between sites, coincided with differences in landscape position (slope-shoulder-plateau) and fossil redox conditions (surface gleys). Samples form drier and more upland situations contained more burnt material, while samples from slope profiles and surface gleys contained even more microbial material, in particular chitin. Results therefore suggest that the admixture of microbial SOM is considerable in loess-paleosols and that differences in edaphic conditions (in particular slope position and soil moisture) and occurrence of wildfires are important for the long-term preservation of SOM. These should therefore be considered when interpreting biogeochemical proxies.