Environmental controls on paleo-pH at mid-latitudes: A case study from Central and Eastern Europe

Here we present the first reconstruction of the Black Sea catchment soil pH evolution during the last glacial period and insights into its environmental controls. The pH reconstruction is derived from the CBT index (cyclization ratio of branched tetraethers—brGDGTs; Weijers et al., 2007a), a proxy based on the relative abundance of branched tetraethers, applied in this study to a northwestern Black Sea sediment core. Since branched tetraethers are produced primarily in soils but also in aquatic environments, we first evaluate the potential of the CBT proxy to reconstruct soil pH regarding the possible origins of brGDGTs. Based on a previous study, we propose a correction of the reconstruction to account for the increase in brGDGTs derived from in situ production in the Black Sea paleo-lake. We also use the CBT-pH record to track the soil signal of the Black Sea catchment as well as to investigate the internal (i.e., soil parent material) and external (i.e., climatic) controls on long-term soil pH change at mid-latitudes. Given that CBT-pH was first proposed as a qualitative proxy of paleoprecipitation at lower latitudes, we investigate, in particular, the influence of precipitation on our setting. We show that, in Central and Eastern Europe, soil pH evolution during the last glacial–interglacial transition cannot be explained by precipitation amounts only but can be explained by (i) variations in the spatial source of brGDGTs associated with that of soil parent material, (ii) soil water balance and (iii) soil vegetation litter. Notably, the CBT-pH is significantly influenced by a spatial change of brGDGT sources during Heinrich Stadial 1. Moreover, the CBT-pH record suggests a strong decrease in the soil moisture budget from the LGM towards the early Holocene, interrupted by an abrupt increase during the Younger Dryas.Our study stresses the importance of considering evapotranspiration in addition to precipitation shifts in order to better understand chemical changes of mid-latitude soil under major climate change. Furthermore, we suggest that mid-latitude soil chemistry during the last glacial-interglacial climate transition was more prone to and/or more sensitive to vegetation cover changes through (i) the role of vegetation cover in evapotranspiration and thus in soil leaching and (ii) the different quality of vegetation litters. Whatever the latitude, the CBT proxy appears useful as one of the only soil chemistry proxies, to provide insights into the past hydrology of soil.