| Abstract: | The Cretaceous was a period of extreme climatic conditions accompanied by major perturbations in biogeochemical cycling, preserved in the geological record as long- and short- term, abrupt and transitional changes. High-resolution integrated stratigraphy has demonstrated that during the Cretaceous a complex series of events affected the ocean-atmosphere system. While we are still taking a census of such events, it has become more and more evident that exogenic and endogenic processes affected and, at least partially, resulted from the evolution of marine and terrestrial biota. Rates of biomineralization played a key-role in the evolution of the oceans, by affecting their chemico-physical-biological conditions as well as the gas exchange between surface seawater and the atmosphere. Organic carbon-rich sediments can be viewed as end-member lithotypes formed under peculiar paleoceanographic conditions hampering biocalcification in both pelagic and neritic environments. The Cretaceous offers a unique opportunity to explore causes and effects of climate changes and their relationship with skeletal mineralogy and biodiversity. Evolutionary rates in both pelagic and neritic environments correlates with carbon and strontium isotope excursions, Oceanic Anoxic Events (OAEs) and climate changes closely coincident with major volcanic-tectonic events.
A first episode of greenhouse climate occurred in the late Valanginian as testified by a globally recorded C-isotope positive excursion, associated with a crisis of pelagic and neritic carbonates and increased evolutionary rates that are interpreted as a nutrification event, perhaps linked to the Paranà volcanism. The late Valanginian warm and humid climate was followed by a cooler Hauterivian-Barremian interval, then replaced by the mid-Cretaceous greenhouse climate representing the most extreme warmth of the past 150 my. OAE1a and OAE2 correlates with the onset and end of such greenhouse conditions, interrupted by some brief cooling events in the Aptian to Cenomanian interval alternating with extremely warm conditions leading to black shale deposition during OAsubEs.
In the Late Cretaceous, climate gradually returned to cooler conditions, although the entire water column was much warmer than today. The Turonian-Maastrichtian interval is characterized by an overall cooling trend interrupted by short periods of warming and cooling as documented by changes in biodiversity of planktonic and benthic communities.
The Cretaceous biosphere was severely affected by the peculiar paleoclimatic and paleoceanographic conditions. Biological processes such as photosynthesis and biomineralization affected the organic and inorganic C-cycle as well as adsorption of atmospheric CO2 in the oceans. These biotic sinks for CO2 imply nutrification events and interactions of the C-cycle with other biogeochemical cycles. Large volcanic-tectonic events and greenhouse conditions must have altered the content and distribution of nutrients in the oceans. Hydrothermal megaplumes related to formation of oceanic plateaus and crust probably introduced high concentrations of dissolved and particulate biolimiting metals into the oceans and CO2 in the atmosphere. Under greenhouse conditions, the hydrological cycle and weathering were altered, perhaps accelerating the introduction of nutrients, iron and other biolimiting elements via run-off and eolian fluxes. Subaerial eruptions must have increased the release of volatiles such as CO2 and S and particulate material in the atmosphere and stratosphere, inducing global climatic effects and perhaps diminishing light intensity available for photosynthesis (volcanic winter). |