Estuarine Lago Mare fauna from the Tertiary Piedmont Basin indicates episodic Atlantic/Mediterranean exchange during the final stage of the Mediterranean Salinity Crisis

Abstract

The geochemical signature of fossils from Lago Mare deposits at Moncucco Torinese (NW Italy) indicates a temporary marine ingression into the Tertiary Piedmont Basin just before the Miocene/Pliocene boundary. Isotope (δ 18 O, δ 13 C, 87 Sr/ 86 Sr) and trace element (Sr/Ca, Ba/Ca) data from mollusk shells of multiple species and croaker otoliths indicate an upper estuarine environment with considerable seasonal variability in evaporation and freshwater input. Reconstructed salinities b 10 psu and low 87 Sr/ 86 Sr values indicate the close proximity of a watershed draining off of Mesozoic ophiolites and carbonates of the Alps and/or Apennines. Communication with an open marine environment is supported by otoliths of fully marine fishes such as lanternfishes, codlets, and slimeheads found in the same assemblage that carry a euhaline signature close to open ocean values. A thorough taphonomic evaluation renders reworking of the otoliths from pre-evaporitic deposits unlikely; instead we explain their presence in the Lago Mare assemblage as related to large predatory marine fish that periodically migrated into marginal marine and estuarine environments. The presence of a paralic paleobiotope with thalassogenic water in the Tertiary Piedmont Basin contradicts ideas of the Mediterranean as a brackish lake fed by the Paratethys during the latest stage of the Mediterranean Salinity Crisis. Instead, our data support previous notions of temporary Atlantic/Mediterranean exchange during sea-level highstands preceding the permanent re-connection at the Miocene/Pliocene transition. The establishment of short-lived and disjunct estuarine and lagoonal environments such as in the Tertiary Piedmont Basin during marine ingressions also puts highly variable 87 Sr/ 86 Sr values and occurrences of shallow water benthic foraminifera in Lago Mare deposits of other Mediterranean basins into a new perspective.

Late Messinian mollusks and vertebrates from Moncucco Torinese, north-western Italy. Paleoecological and paleoclimatological implications

“…Additional evidence from sea fish remains shows that normal marine conditions were already established in the Mediterranean during the post-evaporitic Lago-Mare phase of the MSC (see also Carnevale et al, 2006aCarnevale et al, , 2006bCarnevale et al, , 2008Grunert et al, 2016 …”

“…The fossiliferous layers of MCC are located at the top of a gypsum quarry. The investigated succession consists of nine layers (MCC1-MCC9, Figure 2), some of which contain a very abundant and extremely diverse fossil assemblage, with ostracods, mollusks and small and large vertebrates (Angelone et al, 2011;Alba et al, 2014;Colombero et al, 2014aColombero et al, , 2014bHarzhauser et al, 2015;Grunert et al 2016). Layers 4, 5 and 7 are constituted by several conglomerate lenses.…”

Birth, evolution and demise of the Mediterranean Lago-Mare

“…Abundant land plant leaves and a diverse terrestrial vertebrate fauna are found in the yellowish siltstones, which have been interpreted as overbank deposits (Harzhauser et al, 2015;Colombero et al, 2017 and references therein). In this continental interval, a lowdiversity fish fauna consisting of otoliths of marine and Paratethyan species is found (Grunert et al, 2016;Carnevale et al, 2018;Schwarzhans et al, 2020). These otoliths were Sr-dated to the early-middle Miocene (Grunert et al, 2016).…”

“…In this continental interval, a lowdiversity fish fauna consisting of otoliths of marine and Paratethyan species is found (Grunert et al, 2016;Carnevale et al, 2018;Schwarzhans et al, 2020). These otoliths were Sr-dated to the early-middle Miocene (Grunert et al, 2016). Nevertheless, they were concluded not to be physically reworked, but rather to have been transported by large marine predators, therefore implying a Piedmont Basin-(marine) Mediterranean connection was in force (Grunert et al, 2016; see section 1.5.6).…”

“…In the relatively full scenario, the revival of marine conditions is ascribed to either connectivity changes (Marzocchi et al, 2016) or to a moderate sea-level rise (Andreetto et al, 2021b). In contrast, Carnevale et al (2006aCarnevale et al ( , b, 2008Carnevale et al ( , 2018 and Grunert et al (2016), based on the recovery of fish remains ascribed to marine species, proposed that fully marine conditions were in force in the Mediterranean already at the end of substage 3.1.…”