| Quantitative analyses of calcareous nannofloras and geochemical investigations were carried out on the Berrisian-Hauterivian Polaveno roadcut (Northern Italy), a very expanded and continuous section with a good bio- magneto- and chemostratigraphic control.
Nannofossils were quantified in smear slides and thin sections. On each smear slide at least 300 specimens were counted, whereas all the specimens present in 1 mm 2 were counted in each thin section. Fluctuations in nannofossil total abundance, diversity and abundance of single taxa reflect paleoenviromnental changes in the surface waters. The most marked fluctuations in both absolute and relative abundance concern the group of calcareous nannoplankton named nannoconids. This group experienced a sharp decline across the carbon isotope positive excursion, globally documented.
The nannofossil assemblages of the Late Valanginian were dominated by W. barnesae, with relative increase of Diazomatolithus spp., probably indicating higher fertility conditions as further supported by the decline in the oligotrophic nannoconids. An ascent in Sr/Ca ratios in these nannofossil carbonates in the Late Valanginian supports this interpretation of enhanced productivity. This increase slightly leads the carbon isotope excursion, and is coeval with the onset of the nannoconid decline. The tight correlation between increase in Sr/Ca ratios in the Polaveno section and in Atlantic sediments further suggests that this productivity event was global.
The nannoconid decline and productivity increase probably were induced by changes in nutrient content caused by the Early Cretaceous volcanic activity connected with the implacement of the Paranà Plateau and the "pulse" in the seafloor production.
These volcanic and tectonic events provoked excessive CO2 levels in the atmosphere favouring warm and humid conditions that induced an accelerated transfer of nutrients from the continents to the oceans increasing the fertility of surface waters. Also, a nutrification event might have been directly caused by hydrothermal processes connected with the igneous activity. |