Toward the understanding of (post)pedogenic processes behind the peaks and lows of commonly used climate proxies

Different quantitative methods are used in loess research to provide environmental proxies, such asenvironmental magnetic measurements, grain-size distribution, and indices from diffuse reflectancespectroscopy. Those proxies are successfully applied in many cases, especially in reconstructing glacialinterglaciallevel climate fluctuations and cross profile correlations (Horváth-Bradák ۲۰۱۴; Past InterglacialsWorking Group of PAGES, ۲۰۱۶). However, our understanding of the processes represented by suchparameters (proxies) is still limited. The application of soil micromorphology and comparison withcommonly used parameters may fill this gap. In this study, we used two Middle Pleistocene paleosols, theso-called Basaharc Double (BD۱-۲) and Basaharc lower (BA) from the key late Middle Pleistocene sectionsof Basaharc (Hungary) (Pécsi ۱۹۶۵, ۱۹۸۷). Based on the chronometric dating and the presence of theapproximately ۳۵۰ka old Bag Tephra (BT) horizon appearing in the underlying loess of BA paleosol, BAand BD۱-۲ soils are dated back to Marine Isotope Stage (MIS) ۹ and ۷, respectively (Frechen et al. ۱۹۹۷;Horváth ۲۰۰۱). The oriented sampling and the description of the profiles, and the sampling forluminescence dating were carried out in ۱۹۹۶, the other proxies (low field volumetric magneticsusceptibility (κlf), grain-size distribution, redness, and brightness index) were measured and calculatedlater from the same profiles, and from other parts of the site, but they were not used in this study. Soilmicromorphology was conducted from every soil horizon, and its results were compared with the otherproxies (Stoops et al. ۲۰۱۸). The paleosol thin sections show a high degree of reworking in every studiedhorizon. Besides the secondary carbonates formed in situ (e.g., coatings and hypocoatings, calcified rootcells, and earthworm biospheroids), many fragments suggest reworking. The lack of in situ intact claycoatings in Bt soil horizons indicates the reworking of such horizons - only smaller reworked fragmentsare visible in the thin sections. There is some concern whether the high low field volumetric magneticsusceptibility (κlf) value suggests a strong interglacial, or is it the sign of a more hydromorphic localmicroenvironment, a former soil formation, or a continuous accumulation of a reworked soil material withhigher magnetic mineral content in a lower topographical position (e.g., in a depression). It may also occurthat the applied proxies provide controversial results. Such concerns can be addressed by usingmicromorphological investigations, as they were in the case of a brown horizon in the loess below the BApaleosol and above the BT. Based on the high κlf and redness index, this horizon was described as apaleosol, but the clay content (a possible indicator of pedogenesis) was the second-lowest in the whole loess-paleosol sequence, and it contains a significant amount of sand. The analyses of the thin sectionconfirmed that this is a paleosol with reworked clay coatings, fragmented snails, and secondary carbonatesin the matrix. The reworking may suggest high biological activity. It could be the result of the relocationof a soil horizon too, but in this case, there should be another soil formation before MIS۱۰, which has nodetectable trace at the site. As proved by our research, a deeper understanding of (post)pedogenicprocesses, by e.g., the study of pedogenic microstructures, is crucial when interpreting commonly appliedclimate proxies. The lack of such knowledge about pedogenesis and post-pedogenic alterations may leadto bias and misinterpretation of those climate indicators. Some part of this research was supported by theHungarian NRDIO K۱۱۹۳۶۶ project.

Keywords:

loess , paleoclimate proxies , paleosol micromorphology , Middle Pleistocene , Hungary