Flocculent layers and bacterial mats in the mudstone interbeds of the Primary Lower Gypsum unit (Tertiary Piedmont basin, NW Italy): Archives of palaeoenvironmental changes during the Messinian salinity crisis

The recognition of peculiar laminated layers atypically rich in a biogenic intrabasinal component in the mudstone intervals from the Messinian (late Miocene) Primary Lower Gypsum unit (5.97–5.60 Ma) of the Piedmont basin (NW Italy) provides information on the palaeoenvironmental evolution at precessional insolation maxima. These cyclic layers consist of irregular alternation of cm-thick grey terrigenous laminae and whitish composite packets; the latter is in turn composed of sub mm-thick wrinkled dolomite-rich laminae that alternate with terrigenous ones. Two types of layers can be distinguished: i) peloidal layers, composed of faecal pellets and irregular diatom-rich aggregates, interpreted as marine snow floccules; and ii) filament bearing layers, composed of interwoven filaments up to 150 μm across, corresponding to remains of Beggiatoa-like giant sulphide-oxidising bacteria. By comparison with present-day settings, the peloidal layers are interpreted as flocculent layers, deposited on anoxic sea bottoms, following episodes of phytoplankton bloom in the upper water column. The filament-bearing layers are considered as chemotrophic microbial mats, growing on dysaerobic sea bottoms. The uncommon preservation of both kinds of layers is the result of extensive dolomite precipitation in the shallow subsurface, induced by bacterial sulphate reduction. The laminated layers are the first reported example of a high frequency climate-driven cyclicity in the non-evaporitic portion of the Primary Lower Gypsum unit. Their stacking pattern within the mudstone intervals reflects the gradual increase of the oxygen content of bottom waters during the humid precessional hemicycle, culminating with gypsum deposition at the onset of the arid phase.