Paleoenvironment reconstruction of Parnaíba Basin (north, Brazil) using indicator species analysis (IndVal) of Devonian microphytoplankton

The microphytoplankton record of Parnaíba Basin is typically of Devonian age and commonly associated with changes in marine environments. The method of Indicator Species Analysis (IndVal) was applied as a tool for reconstructing marine environment changes during the Devonian (Pragian-Famennian). The IndVal expresses which taxa are strongly associated with particular groups of samples. The Devonian microphytoplankton distribution reflects the paleoenvironment, and the IndVal indices illustrated how the taxa interacted through time and reveals patterns in the changing composition of the microphytoplankton. The material studied derives from three well sections, ARN (12 samples), NA (22 samples) and LT (13 samples), drilled through the Itaim, Pimenteira and Cabeças formations in the Parnaíba Basin, respectively. Samples were prepared using the standard palynological method. The indicator species coincidently are age-diagnostic. For the Devonian succession, the IndVal indicates fourteen taxa, viz. Schizocystia saharica, Triangulina alargada, Bimerga paulae (Pragian-Emsian Interval), Onondagella asymetrica, Tunisphaeridium tentaculiferum, Estiastra spp. (Early Givetian Interval), Mediocorpore conspicuus, unattributed species of Unellium (Frasnian 1 Interval), Petrovina connata, unattributed species of Acriora, (Frasnian 2 Interval), Winwaloeusia distracta, unattributed species of Stellinium (Frasnian 3 Interval), Arkonites bilixus and unattributed species of Duvernaysphaera. (Frasnian-Famennian Interval). Microphytoplankton typical of more nearshore settings are recorded in the basal interval (Pragian-Emsian Interval) accompanied by high values of phytoclasts and continental:marine ratio (C/M). A change in microphytoplankton is recorded, with an increase in acritarchs and prasinophycean algae typical of open marine conditions, in particular during the Frasnian 3 Interval. The change reflects sea level rise.