Tracing freshwater provenance in palaeo-lagoons by boron isotopes and relationship with benthic foraminiferal assemblages. A comparison from late Quaternary subsurface successions in Northern and Central Italy

In order to evaluate for the first time the application of boron isotope (δ11B) composition as a proxy for freshwater provenance in transitional environments, measurements taken from mollusk shells from palaeo-lagoonal deposits are presented herein. The mollusk specimens were sampled from late Quaternary lagoonal successions observed in the cores 187-S1 and Vignarca, respectively from drill-sites south of the Po River delta (Northern Italy) and the Cornia River plain (Central Italy). The latter locality occurs nearby the Larderello boron-rich geothermal area. A freshwater influence in the palaeo-lagoons has been highlighted by benthic foraminiferal analyzes, revealing two main assemblages: the first, dominated by Ammonia tepida (Cushman, 1926) and Ammonia parkinsoniana (d'Orbigny, 1839a), is consistent with high freshwater discharge into the palaeo-lagoon, whereas the second assemblage includes large numbers of Miliolacea, considered as reflecting low freshwater discharge. The inferred salinity variations are possibly constrained between 22‰ and 41‰, based on comparison with modern assemblages observed in various lagoons of the north Adriatic Sea and Tyrrhenian Sea coastal areas (e.g., Zampi & D'Onofrio, 1986; Coccioni, 2000). Additional Sr isotope data, comparable with coeval marine values, are consistent with the salinity range inferred by the fossil assemblages. Boron isotopes clearly record a continental-dominated provenance source of boron in the lagoonal mollusks collected from the Vignarca core. The low δ11B and boron-rich component currently observed in the continental water deriving from the geothermal area, was possibly also present in the freshwater feeding the palaeo-lagoon and gives a unique isotopic imprint to the analyzed samples. Conversely, in the lagoonal mollusks collected from core 187-S1, the δ11B component reveals a dominant provenance source of boron from seawater. In the analyzed successions, boron isotopes have proved useful for recognizing the provenance of the freshwater feeding the palaeo-lagoon, especially in areas characterized by continental water enriched in boron and showing remarkably different δ11B values with respect to marine values.