Potential Fossilized Sulfide-Oxidizing Bacteria in the Upper Miocene Sulfur-Bearing Limestones From the Lorca Basin (SE Spain): Paleoenvironmental Implications

The sulfur-bearing limestones interbedded in the upper Miocene diatomaceous sediments (Tripoli Formation) of the Lorca Basin (SE Spain) are typified, as other Mediterranean coeval carbonate and gypsum deposits, by filamentous, circular and rod-shaped microstructures of controversial origin. These features have been interpreted both as fecal pellets of brine shrimps and/or of copepods, remains of algae or cyanobacteria and fossilized sulfide-oxidizing bacteria. To shed light on their origin, a multidisciplinary study including optical, UV and scanning electron microscopy, Raman microspectroscopy, and geochemical (carbon and oxygen stable isotopes) analyses has been carried out on three carbonate beds exposed along the La Serrata ridge. The different composition of the filamentous and circular objects with respect to the rod-shaped microstructures suggest that the former represent remains of bacteria, while the latter fecal pellets of deposit- or suspension-feeder organisms. Size and shape of the filamentous and circular microfossils are consistent with their assignment to colorless sulfide-oxidizing bacteria like Beggiatoa (or Thioploca) and Thiomargarita, which is further supported by the presence, only within the microfossil body, of tiny pyrite grains. These grains possibly result from early diagenetic transformation of original sulfur globules stored by the bacteria, which are a diagnostic feature of this group of prokaryotes. The development of microbial communities dominated by putative sulfide-oxidizing bacteria at Lorca was favored by hydrogen sulfide flows generated through degradation of organic matter by sulfate-reducing bacteria thriving in underlying organic-rich sediments

CHRONOLOGY OF THE MESSINIAN EVENTS IN THE NORTHERNMOST PART OF THE MEDITERRANEAN: THE GOVONE SECTION (PIEDMONT BASIN, NW ITALY)

In marginal Mediterranean sub-basins the first phase of the Messinian salinity crisis (MSC) is recorded by primary sulfate evaporites (Primary Lower Gypsum unit); in deeper settings, the gypsum makes lateral transition into shales and marls usually barren of calcareous fossils that can hamper the identification of the MSC onset. The Govone section (Piedmont Basin, NW Italy) represents an opportunity to examine in detail the pre-MSC interval and the transition to the first stage of the MSC in a relatively deep marginal basin in the northernmost sector of the Mediterranean. We provide herein an age model for the Govone section, based on an integrated stratigraphic study, including cycloctratigraphy, magnetostratigraphy and micropaleontology (foraminifera and calcareous nannofossils) of the pre-MSC interval and the transition to the MSC. Chron C3An.1n has not been recognized in the study succession, most likely due to early diagenetic processes. Thus, the last occurrence of Turborotalita multiloba occurring two lithological cycles above the second influx of G. scitula, is the event that best approximate the MSC onset and is consistently recorded across the Piedmont Basin, with higher abundance respect to coeval Mediterranean successions. The calibration of the lithological cyclicity by means of these two bioevents allowed to recognize that, unlike other Mediterranean sections, the disappearance of calcareous microfossils occurs before the MSC onset, probably, in response of diagenetic processes favouring the dissolution of calcareous shells

CHRONOLOGY OF THE MESSINIAN EVENTS IN THE NORTHERNMOST PART OF THE MEDITERRANEAN: THE GOVONE SECTION (PIEDMONT BASIN, NW ITALY)

In marginal Mediterranean sub-basins the first phase of the Messinian salinity crisis (MSC) is recorded by primary sulfate evaporites (Primary Lower Gypsum unit); in deeper settings, the gypsum makes lateral transition into shales and marls usually barren of calcareous fossils that can hamper the identification of the MSC onset. The Govone section (Piedmont Basin, NW Italy) represents an opportunity to examine in detail the pre-MSC interval and the transition to the first stage of the MSC in a relatively deep marginal basin in the northernmost sector of the Mediterranean. We provide herein an age model for the Govone section, based on an integrated stratigraphic study, including cycloctratigraphy, magnetostratigraphy and micropaleontology (foraminifera and calcareous nannofossils) of the pre-MSC interval and the transition to the MSC. Chron C3An.1n has not been recognized in the study succession, most likely due to early diagenetic processes. Thus, the last occurrence of Turborotalita multiloba occurring two lithological cycles above the second influx of G. scitula, is the event that best approximate the MSC onset and is consistently recorded across the Piedmont Basin, with higher abundance respect to coeval Mediterranean successions. The calibration of the lithological cyclicity by means of these two bioevents allowed to recognize that, unlike other Mediterranean sections, the disappearance of calcareous microfossils occurs before the MSC onset, probably, in response of diagenetic processes favouring the dissolution of calcareous shells