The Palaeo-Adriatic Basin during the Mid-Piacenzian Warm Period: Clumped isotope-based seasonality reconstruction from Bivalve shells

Bivalve shells represent one of the most important biotic archives of geochemical proxies for high-resolution palaeoclimatic and palaeoenvironmental reconstructions. In particular, sclerochronological and sclerochemical analyses of calcium carbonate shells allow seasonal investigations of environmental variability (e.g. seawater temperature, δ 18Osw, pH and nutrient supply) and comparisons across modern, historical or deep-time settings. The Mid-Piacenzian Warm Period (3.3-3.0 Ma) is a Pliocene climate warming event, recognised as the most recent example of prolongated global warming in the geological past and regarded as a possible analogue for the future climate conditions [1]. The Rio Vaccaruzza section, Villalvernia (Piedmont, Italy) deposited in the Palaeo-Adriatic Basin, continuously covers this time interval and has rich mollusc assemblages [2]. It represents a key section in which to study the seasonal palaeotemperature variation recorded by its excellently preserved bivalve shells using clumped isotope thermometry (Δ47) through a sclerochronological approach. Here, we analyse four pristine bivalve specimens belonging to two families characterised by different growth rates: two Glycymeris bimaculata (Poli, 1795) having slow growth rates, and two pectinids: one Aequipecten opercularis (Linnaeus, 1758) and one Perapecten scabrellus (Lamarck, 1819) having fast growth rates. The use of species with different growth rates enables the reconstruction of the seasonality at different temporal resolutions, from 2-3 years (Pectinidae) to more than 10-15 years ranges (Glycymerididae). Specimens were collected from Bed 3 of the Rio Vaccaruzza section, which is characterised by a muddy sand lithology belonging to the Sabbie di Asti Formation. The depositional environment is interpreted as an infralittoral setting influenced by hyperpycnal flows that produce bottom water instability and salinity fluctuations. Due to this depositional setting, the application of clumped isotope analyses is crucial as it allows an independent reconstruction of seasonal seawater temperatures and δ18Osw variability recorded by bivalve shells. The preliminary Δ47 and δ18O values of bivalve shells suggest a higher annual mean water temperature than in the modern Northern Adriatic Sea and seasonal fluctuations in δ18Osw consistent with a shallow-marine environment characterised by seasonally varying freshwater input.