A first insight into Mid-Piacenzian Warm Period seasonality: Clumped isotope analysis of bivalve shells

Clumped isotope thermometry (Δ47) is a technique that is being increasingly applied to biogenic carbonates, as it provides a powerful tool for reconstructing seawater palaeotemperatures without requiring past δ18Osw value assumptions. Bivalve shells are recognised as one of the most valuable archives for its application, as they often live in settings affected by changes in δ18Osw values. Here, we perform for the first time clumped isotope analyses to bivalve shells from an Italian Pliocene succession, using a sclerochronological approach to reconstruct seasonal variations. We analyse four pristine shells belonging to taxa with different growth rates: two slow-growing Glycymeris bimaculata (Poli, 1795) and two fast-growing pectinids, Aequipecten opercularis (Linnaeus, 1758) and Perapecten scabrellus (Lamarck, 1819). The combination of species with contrasting growth rates allows reconstruction of seasonality of different record lengths: 2-3 years (Pectinidae) to more than 10-15 years (Glycymerididae). These taxa come from the Rio Vaccaruzza section (Villalvernia, Piedmont, Italy), deposited in the Tertiary Piedmont Basin and attributed to the Mid-Piacenzian Warm Period (3.3-3.0 Ma). Specimens were collected from Bed 3 of the Rio Vaccaruzza section, characterised by muddy sand deposits of the Sabbie di Asti Formation. The depositional environment is interpreted as an infralittoral setting influenced by hyperpycnal flows, leading to bottom-water instability and salinity fluctuations. Under this depositional setting, clumped isotope analysis is crucial as it enables independent reconstruction of seasonal seawater temperatures and δ18Osw variability recorded in bivalve shells. Preliminary Δ47 and δ18O data indicate higher mean annual water temperatures compared to the modern Northern Adriatic Sea, along with seasonal δ18Osw value fluctuations consistent with a shallow-marine environment affected by variable freshwater input.