ARE THE MESSINIAN SPAGHETTI-LIKE STRUCTURES FOSSIL COLORLESS SULFUR-OXIDIZING BACTERIA? IMPLICATIONS FOR GYPSUM DEPOSITION

The thick gypsum deposits formed in the Mediterranean basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils (spaghetti-like structures), which were interpreted as fossils of benthic algae or cyanobacteria, pointing to deposition in a marine subtidal or intertidal environment. This interpretation has recently been questioned and it was suggested that the fossils represent sulfur bacteria. This attribution was based on morphological observations only, thus not overcoming the problem that morphology and size alone are not diagnostic for a particular group of bacteria. We present here a petrographic, minerochemical and Raman spectroscopy study of the Messinian fossiliferous gypsum from the Piedmont Basin (NW Italy). The results indicate that the enigmatic filamentous structures represent indeed remains of colorless, vacuolated sulfide-oxidizing bacteria like Beggiatoa or Thioploca. This assignment is supported by the presence of small crystal aggregates of pyrite and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. Besides filamentous fossils, the studied gypsum crystals contain remains of eury- and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage suggests that the Messinian gypsum formed at greater water depth than previously assumed. In addition, it reflects conditions of increased productivity in the water column, which was triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and fresh water discharge. The apparent algal blooms enhanced organic matter degradation by bacterial sulfate reduction in an oxygen-depleted environment, which provided the high hydrogen sulfide flux that was required for the growth of sulfide-oxidizing bacteria. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments on Earth and possibly Mars.