Buried palaeosols in Tuscan inland basins provide clues to changing climate conditions across the Pliocene

Although Pliocene palaeosols can supply palaeoclimate information and help to disentangle stratigraphic reconstructions, soils are sparsely represented in global datasets for Pliocene climate. Pliocene was characterised by global mean annual temperatures 2–4 °C warmer than today, representing an analogue for future climate prevision. We aimed, thus, to investigate palaeosols as suitable archives for reconstructing geomorphic processes, ecosystems and climate patterns in the Pliocene.

We compared the palaeosol record stored in Zanclean and Piacenzian successions locate in central and southern Tuscany, respectively. Palaeosol morphological features observed in the field were used for the sequence-stratigraphic interpretation. To qualitatively evaluate soil development, we considered the time needed to attain various soil properties. Pedogenetic processes and the intensity of weathering were also quantified by applying a range of proxies based on geochemical analyses. Climofunctions based on major element ratios were used to estimate mean annual palaeoprecipitation (MAP). Carbon and oxygen isotopes analysis were performed on carbonate nodules, to verify their pedogenic origin and as a proxy for palaeovegetation and palaeotemperatures.

Though all soils are unconformities in the record, the rank and type of unconformity were defined in detail, to precisely place the soil formation time within the sedimentary sequence and to correlate soil-forming intervals with general environmental changes. Then, soil characteristics were interpreted, considering the geomorphological setting reconstructed by the stratigraphic and sedimentological analysis.

The Zanclean-age soils represent a presumably long-time span, likely a few thousand years, and exhibit strong redoximorphic features such as low-grade plinthite, suggesting that they developed in intervals of humid climate. The mid Piacenzian-age soils represent rhythmic and short intervals of pedogenesis, which correlate to sea level highstands. The main characteristics of palaeosols are due to clay shrink-swell properties (vertic) and carbonate translocation (calcic). Calcic features were interpreted as recording the duration of pedogenesis and thus of the sedimentary hiatuses, suggesting a range of development from 1,000 to 10,000 years. These soil features and the isotopic proxies converge towards indicating a highly seasonal rainfall pattern. This inference matches the sedimentological facies, which point to sediment transport and deposition from highly laden flood flows, recalling the sedimentary dynamics of seasonal fluvial systems. The hypothesis of a very high degree of rainfall seasonality would match well with existing palaeoclimatic records for the Mediterranean Middle Pliocene Warm Period (MPWP), and could shed light on certain unusual features in such records.

Comparing soils developed in the Early and in the Late Pliocene, the MAP estimates confirm the differences in climate condition which likely led to the divergent pedogenesis pathways. Furthermore, the palaeotemperatures estimated from the oxygen isotopic composition of pedogenic carbonates within Piacenzian calcic horizons, point to values matching the modelled temperatures for the MPWP.