Rhizoliths in loess – evidence for post-sedimentary incorporation of root-derived organic matter in terrestrial sediments as assessed from molecular proxies

Abstract

Loess–palaeosol sequences are important terrestrial archives for studying Quaternary climate change. A major assumption for palaeoenvironmental reconstruction based on loess organic matter (OM) is that it represents the signal of syn-sedimentary incorporated plant biomass, i.e. predominantly grass vegetation. However, recent studies on rhizoliths (roots encrusted by secondary carbonate) formed in loess reveal the possibility of post-sedimentary penetration of loess–palaeosol sequences by deeply rooting plants. This likely led to incorporation of younger root-derived OM into surrounding loess, potentially causing an overprinting of the initial plant-derived signal of loess OM. To obtain information on the source vegetation of rhizoliths and surrounding loess OM we examined rhizoliths and loess from Nussloch, SW Germany, using alkane and fatty acid (FA) molecular proxies. Moreover, the lipid composition was compared in transects from rhizoliths via rhizosphere loess towards root-free loess for a preliminary assessment towards quantifying the post-sedimentary input of root-derived OM in loess. Loess OM gave a combined signal from shoot and root biomass from grass vegetation, confirming the assumption of deposition during glacial periods with scarce grass vegetation cover. In contrast, the lipid composition of rhizolith OM reflected roots of woody vegetation, indicating the post-sedimentary character of rhizoliths. Stronger degradation of OM, together with a large content of microorganism-derived FAs in the former rhizosphere indicated rhizodeposition associated with high microbial activity in loess adjacent to rhizoliths, at least up to a distance of 5 cm. Rhizosphere loess and reference loess at a distance of 50–70 cm showed a significantly different OM composition, thereby revealing the incorporation of considerable portions of root-derived OM into loess in the vicinity of roots. Further studies are necessary for an exact quantification of this potential overprint of the syn-sedimentary loess OM, which might cause uncertainty in palaeoenvironmental studies.