Biotic and geochemical response to anoxic events: the Aptian pelagic succession of the Gargano Promontory (southern Italy)

Microfossil distribution patterns and high-resolution delta C-13 and delta O-18 curves, calibrated against planktonic foraminiferal and calcareous nan nofossil data, are provided for the Aptian pelagic Coppitella section of th e Gargano Promontory (southern Italy). The succession consists of cyclicall y arranged couplets of bioturbated grey marlstones and off-white marry lime stones, referable to the Marne a Fucoidi. In the lower portion of the secti on, two thin black shares were recognized. The high-resolution delta C-13 c urve presented here correlates with those of other Alpine-Tethyan sections, albeit with lower absolute values. The onset of deposition of organic-rich sediments falls at the top of the interval of unchanging carbon-isotope va lues, whereas the upper black shale is documented from the interval of the main Aptian positive delta C-13 excursion. According to our biostratigraphi c data, the deposition of organic matter in the Gargano Promontory persiste d through Early/Late Aptian boundary time. Using a chemostratigraphic defin ition, only the lower black shale is referred to the Selli Level. As far as the biotic response is concerned, the onset of the 'nannoconid crisis' is recorded considerably below the lower black shale, whereas the 'Globigerine lloides eclipse' is recorded below and within the upper black shale. The di stribution of meso-eutrophic indices (Zygodiscus spp., radiolaria) vs, mode rate-fertility indices (Rhagodiscus asper and Lithraphidites carniolensis) testifies to a modest increase of surface-water fertility only throughout t he stratigraphically higher black shale. The occurrence of a benthic forami niferal fauna, albeit impoverished, in both the basal and upper black horiz ons clearly documents dysaerobic rather than completely anoxic conditions o n the sea floor. Relative sea-level rise at the time of the Selli Event in the Gargano Promontory is documented by drowning and foundering of the Apul ia platform margin, situated adjacent to the basin in which the Marne a Fuc oidi accumulated.