Unusual Miocene hydrocarbon-seep faunas from the Brisighella area in northern Italy: embedded in clastics and first records of the lucinid bivalves Megaxinus and Miltha

hydrocarbon-seep sites

Methane seeps along continental margins harbor lush faunal communities composed mainly of animals living in symbiosis with chemotrophic bacteria, from which they derive their nutrition (Paull et al. 1985, Dubilier et al. 2008).Due to this in situ food source, these faunas are believed to have a unique evolutionary history, closely linked to that of similar faunal communities living at hydrothermal vents (McArthur and Tunnicliffe 1998;Vrijenhoek 2013).The anaerobic oxidation of methane at these sites facilitates carbonate precipitation (Peckmann and Thiel 2004;Cochran et al. 2022) and hence fossil examples of these faunas are a common feature of uplifted deep-water strata, especially from the late Mesozoic onward (Kiel 2010;Hryniewicz 2022).This fossil record has provided unique insights into the evolution of these ecosystems, showing a major role for brachiopods in their earlier history (Campbell and Bottjer 1995;Kiel and Peckmann 2019), recurrent morphological adaptations among different clades at different times (Hryniewicz et al. 2017), resistance towards extinction events (Kiel and Little 2006), and marine sulfate concentrations as a potential major driver of ecologic and evolutionary change (Kiel 2015).
The Mediterranean Sea today has a rather depauperate seep fauna compared to that of the major oceanic continental margins (Olu et al. 2004;Taviani et al. 2013;Taviani 2014).The fossil record indicates that this is a geologically young feature, as typical "oceanic" faunal elements such as large bathymodiolin mussels and vesicomyid clams were common and widespread at Miocene methane-seep deposits throughout Italy (Moroni 1966;Taviani 1994;Kiel and Taviani 2017).These seep deposits, locally known as "Calcari a Lucina" (Lucina limestone), occur from the Turin hills along the Apennine Mountain chain all the way to Calabria in the south (Manzoni 1876;Taviani 1994;Conti et al. 2021;Hryniewicz 2022).Despite their biogeographic and evolutionary importance, the documentation and identification of the taxa composing these faunas is still incomplete.The first thorough monograph of the "Calcari a Lucina" mollusk fauna was published by Moroni (1966), though this was before the discovery of vent and seep communities in the modern oceans (Corliss et al. 1979;Paull et al. 1984) and the rich taxonomic work on their fauna (Desbruyères et al. 2006).The present contribution is part of our ongoing efforts to document the taxonomic and ecologic diversity of seep faunas in the Neogene of Italy (Taviani et al. 2011;Vinn et al. 2012;Sami and Taviani 2015;Kiel andTaviani 2017, 2018;Kiel et al. 2018), to better understand the biogeographic and evolutionary history of the Mediterranean seep fauna.

Results
Here we comment on two deposits with mollusks characteristic of the "Calcari a Lucina" fauna.They were found only a few meters apart, on the northern corner of a vineyard along Via Monte Mauro, at 44°13'59.6"N11°42'21.2"E,SWW of Faenza and just to the west of Brisighella (Fig. 1).Stratigraphically, the sediments enclosing these deposits belong to the upper Tortonian-?lower Messinian (Upper Miocene) pre-evaporitic "Euxinic Shales" (Sami and Taviani 2019).The vineyard bordering the outcrops belong to the Tenuta Cassano vinery, and the sites were hence reported as Ca' Cassano 1 and 2 by Sami and Taviani (2019), who briefly discussed their stratigraphy, lithology and associated macrofauna.
Ca' Cassano 1 is a lens, about 60 cm thick and at least 250 cm wide, of unconsolidated silty sandstone, gray with ochre weathering surfaces.Molluscan fossils are preserved mainly as internal molds with little or no shell material (Table 1, Fig. 2), though small shell fragments are dispersed throughout the lens.The most abundant taxa are the bathymodiolin mussel Bathymodiolus moroniae Kiel and Taviani, 2017, reaching 10 cm in length, and the vesicomyid clam Archivesica aharoni Kiel and Taviani, 2017, up to 10 cm in length.Less common are specimens of the lucinid Lucinoma perusina (Sacco, 1901) and of the mytilid Samiolus iohannesbaptistae Kiel and Taviani, 2017.Gastropods are rare but relatively diverse, including the neritid Thalassonerita megastoma Moroni, 1966, a large buccinid previously identified as "Chrysodomus hoernesi Bellardi, 1873" by Sami and Taviani (2019) up to 8 cm tall, the colloniid Homalopoma domeniconii Moroni, 1966, a large gastropod (possibly cf."Galeodea sp."), and specimens com-   The Ca' Cassano 2 deposit, stratigraphically about 7 meters above Ca' Cassano 1, is an isolated limestone deposit, consisting of gray, carbonate-cemented, detrital siltstone with occasional micritic patches, calcite-filled voids, finely dispersed, small shell fragments, and a diverse mollusk fauna (Table 1).The most common bivalve is the lucinid Megaxinus bellardianus (Mayer, 1864), reaching 43 mm in length, 40 mm in height, and 23 mm in thickness (Fig. 3D-F).It shows some variation in shell shape, especially regarding the position of the umbo, but the specimens match those reported as Megaxinus bellardianus by Sacco (1901: 75-77, pl. 17: 29-37).This taxon is recorded by Sami and Taviani (2019) as "Lucinoma sp.".An extremely flat lucinid bivalve reaching 40 × 40 mm at a width of 13 mm is here identified as Miltha sp. (Fig. 3G-J).It was reported previously as "Myrtea sp." by Sami and Taviani (2019), the ten available specimens show a style of preservation similar to that of Megaxinus bellardianus, indicating that the extremely flat shell is original and not the result of a taphonomic process.In contrast to Ca' Cassano 1, the Ca' Cassano 2 site also included a few large specimens of the lucinid bivalve Meganodontia hoernea (Des Moulins, 1868), reaching 15 cm in length.The gastropod fauna is similar to that of Ca' Cassano 1.

Discussion
Carbonate formation is a characteristic feature of hydrocarbon seeps because the main geochemical process, the anaerobic oxidation of methane, results in a local increase in alkalinity, triggering carbonate precipitation (Peckmann and Thiel 2004;Cochran et al. 2022).Hence, the mass occurrence in the unconsolidated siltstone lens of Ca' Cassano 1 of taxa that are typically considered obligate to cold seeps, such as Bathymodiolus moroniae and Archivesica aharoni, is unusual; elsewhere in Miocene strata of the Apennines, these species are typically found in massive carbonate mounds (Taviani 1994(Taviani , 2011;;Conti et al. 2021).One exception are the vesicomyid-rich horizons (reported as ?Calyptogena sp., but most likely belonging to Archivesica) from carbonate-free sandy silty mudstone associated with submarine slides in the turbiditic Miocene Marnoso-Arenacea Formation in the Romagna Apennines (Lucente and Taviani 2005).Occurrences of seep-inhabiting bivalves, especially vesicomyids, in bedded siltstone or mudstone, are common in the Oligocene of Alaska (Kiel and Amano 2010) and the Neogene of Japan (Majima et al. 2005) and are considered as allochthonous.Re-deposition of the shells by a turbidity current is certainly a possible explanation for Ca' Cassano 1.
Alternatively, a few accumulations of "seep taxa" without associated carbonate precipitation, but on organic-rich sediments, are known from the modern ocean.Turbidity current deposits resulting from the 1929 Grand Banks earthquake on the Laurentian Fan in the deep NW Atlantic Ocean were colonized by vesicomyid and thyasirid clams, and siboglinid tube worms (Mayer et al. 1988).Likewise, vesicomyid bivalves are common in organic-and sulfide-rich sediments of the Congo deep-sea fan in the tropical eastern Atlantic Ocean, and are believed to sustain themselves on hydrogen sulfide generated by the degradation of the locally abundant organic matter (Olu et al. 2017).Yet another example are solemyid and nucinellid bivalves inhabiting sediments in fjords in New Zealand, where sulfide is generated by decomposing forest litter and marine algae (McLeod et al. 2010).Such settings with organic-rich sediments could be an alternative explanation for the mass occurrence of seep taxa in the unconsolidated siltstone lens of Ca' Cassano 1.However, the faunal communities in the Laurentian and Congo fans and the New Zealand fjords host mainly infaunal and semi-infaunal bivalves but no bathymodiolins, unlike the Ca' Cassano 1 deposit.Thus, re-deposition might be the more likely scenario for the formation of the Ca' Cassano 1 deposit.
The lucinid Megaxinus is common in Neogene strata of the Mediterranean region (Sacco 1901), including at Pliocene seep deposits (Cau et al. 2015;Kiel and Taviani 2018) and whale falls (Danise et al. 2010).The genus occurs mostly in Neogene deposits of the Mediterranean region, and today in the Mediterranean Sea and western Indian Ocean (Glover and Taylor 1997).Megaxinus bellardianus found at Ca' Cassano 2 is the first record of this genus and species at a Miocene Mediterranean seep deposit.A species of Megaxinus was recently repor ted from Upper Miocene seep deposits in the Philippines (Kiel et al. 2022), thus Megaxinus bellardianus from the Ca' Cassano 2 deposit emphasizes faunal relationships between the Mediterranean region and the tropical Indo-West Pacific Ocean during the Miocene.Together with the flat Miltha sp., it also emphasizes the heterogeneity among faunal assemblages in the "Calcari a Lucina" seep deposits, as recently indicated by an Upper Miocene faunule including the lucinid bivalve Anodontia mioinflata, the bathymodiolin Idas aff.tau roparva, and abundant serpulid worms (Kiel et al. 2018).

Fig. 1 .
Fig. 1.The location of the Ca' Cassano sites near Brisighella in northern Italy

Table 1 .
List of species found at the Ca' Cassano sites in the Late Miocene-aged pre-evaporitic "Euxinic Shales" in the Emilia-Romagna province in northern Italy.