Compositional, micromorphological and stratigraphic characterization of Holocene Tiber floodplain deposits (Rome, Italy)

The sequestration of sediment deposited on alluvial floodplains may release an identifiable fingerprint on sediment composition and texture. In this regard, soil micromorphology, can be used to better discriminate the role of autocyclic and long-term factors in controlling the environmental condition of soil formation when framed into Quaternary stratigraphic record. In this study, we report a high-resolution micromorphological characterization of the Tiber River floodplain soils for a better understanding of the soil processes related to changes in water table, biological activity, weathering, and accommodation space and integrate soil micromorphology with sequence stratigraphy. Petrographic and micromorphological features from a borehole advanced 60 m into the Tiber channel belt and floodplain, document incipient pedogenetic modifications across stratigraphic markers evidenced by faunal and plant activity, accumulation of peat, and typified by precipitation of heavy metals, iron oxides and secondary carbonates. All these features developed in correspondence of surfaces of stratigraphic significance and reveal depositional features and post-depositional modifications associated with different environmental changes. These observations tell us about a specific history case of incipient soils formation in the Tiber Depositional Sequence but may serve as a model to reconstruct the stratigraphic evolution of ancient relict soils in similar alluvial settings. This work demonstrates that a combination between sedimentological and stratigraphic observations and soil micromorphology can be critical to supplement field observations and determine the relative effect of pedogenic and depositional processes on the organization, composition and texture, and geotechnical properties of floodplain in urban areas.