Abstract
PurposeThe existence of black horizons (BHs) is often highlighted in European soils, and in the Po River plain of northern Italy. Nevertheless, BH chronological frameworks and genetic models are still debated. The present study investigated the genesis of BHs in the eastern Po Plain where they are buried at various depths.Materials and methodsSoil sequences were investigated with a multidisciplinary approach integrating geomorphologic, stratigraphic, pedologic, geochemical, isotopic, palynological, and radiometric analyses.Results and discussionThe formation of the studied BHs was scattered over time from the Last Glacial Maximum to at least the middle Holocene. The new data indicate that BHs developed when the landscape was dominated by coniferous forest during conditions that were totally different from the current pedoclimatic setting. The recurrent presence of black particles indicates that this vegetation cover was systematically affected by fire episodes that induced soil degradation and mineralization processes of the original organic compounds, thus contributing to darkening of the upper soil horizons.ConclusionsBH formation clearly coincided with cold time lapses. Evidence for repeated fire events (natural or human-induced?) provides insights for the controversial debate on early anthropogenic impacts on the environment.
Highlights
Pedogenetic processes and the resulting soil types are strictly dependent on existing climatic conditions (Schaetzl and Anderson 2005; Clark et al 2012; Walker et al 2012; Meier et al 2014; Mauri et al 2015; Tabor and Myers 2015; Binney et al 2017; Pérez-Lambán et al 2017; Zhou et al 2021)
As described by the relevant authors, they are “black soil horizons” that are morphologically similar to the black horizons (BHs) studied in this paper, and their thicknesses rarely exceed the value of 50–60 cm
Our review of BHs in the areas surrounding Bologna provides a tool to evaluate climatic changes that occurred during the Late Glacial-Holocene period
Summary
Pedogenetic processes and the resulting soil types are strictly dependent on existing climatic conditions (Schaetzl and Anderson 2005; Clark et al 2012; Walker et al 2012; Meier et al 2014; Mauri et al 2015; Tabor and Myers 2015; Binney et al 2017; Pérez-Lambán et al 2017; Zhou et al 2021). Case studies performed in wide alluvial plains with high discontinuous sedimentation rates are rare (Ravazzi et al 2006; Marchesini et al 2017; Bruno et al 2020; Marcolla et al 2021) In this framework, during recent decades, excavation for building and engineering activities in the eastern Po Plain (northern Italy) targeted stratigraphic sequences that were well constrained from archeological and chronological points of view (Bianchini et al 2014, 2019; Cacciari et al 2017; Vittori Antisari et al 2011, 2013a). The significant thickness of the sedimentary cover is effective in sealing and preserving the main soil features These sequences often contain black soils, which, already described by previous studies in the Po Plain (e.g., Amorosi et al 2014a, 2017; Bruno et al 2020; Marcolla et al 2021), have not been interpreted in terms of paleoenvironmental conditions. In other European countries (e.g., Spain), black soils have been ascribed to specific climatic conditions, vegetation types, and the occurrence of fires (Tallón-Armada et al 2014; Armas-Herrera et al 2019), and related hypotheses need to be tested in northern Italy
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