The Cretaceous evolution of calcareous plankton is characterized by a complex history punctuated by accelerated radiation/extinction rates, turnovers, relatively long intervals of stability and major changes in abundance. In particular, calcareous nannoplankton definitively became the micrite-producers in the world ocean and, consequently, introduced feedbacks in the carbonate system and the global carbon cycles at short and long time scales. Fluctuations in nannofloral biocalcification, biodiversity and abundance are correlatable with global environmental changes that are intimately interconnected with and determined by processes operating inside the Earth as represented by large igneous events. Continuous, complete, (hemi)pelagic Cretaceous successions offer the opportunity to date, quantify and model exogenic and endogenic processes that affected and, at least partially, resulted from the evolution of calcareous (phyto)plankton. The Cretaceous history of nannoplankton can be used to trace the response of the marine ecosystem to changes in pCO2, chemistry and thermal structure of the oceans, climate and sea level, availability of nutrients and biolimiting elements. Are evolutionary processes triggered and/or inhibited by environmental changes? Are there causal or causal links between paleobiological pulses and changes in the geosphere, (marine) hydrosphere and atmosphere? In the Jurassic/Cretaceous boundary interval, a major turnover within coccolithophore families is enhanced by appearance of 3 nannolith groups that strongly affected biogenic sedimentation in the oceans. In particular, the big and heavily calcified nannoconids produced massive pelagic carbonates during the Early Cretaceous. Another pulse in nannoplankton evolution is represented by the speciation episode in the latest Barremian – early Aptian, followed by the “nannoconid crisis” during Oceanic Anoxic Event (OAE) 1a. After a brief productive episode (N. truittii Acme), a number of nannofossil extinctions and the final drop of nannoconids mark the late Aptian. A new evolutionary phase occurs through the Albian with appearance of several taxa, especially in intervals related to OAE1b and OAE1d. The Cenomanian/Turonian boundary interval correlates with another turnover, marked by an impressive radiation of Polycyclolithaceae in the Turonian. Maximum nannofossil diversification is reached in the Campanian followed by a Maastrichtian decline truncated by the Cretaceous/Paleocene boundary extinction. It is difficult, if not impossible, to separate single causes triggering and/or inhibiting calcareous nannoplankton evolution, biocalcification and production. Rather a combination of abiotic and biotic paleoecologic factors seems responsible for evolutionary innovations, stability and extinctions. Most consistent correlations suggest that pCO2, chemistry, structure and fertility of the oceans are instrumental, whereas climate and sea level apparently play a secondary role. Increased rates of volcanism during the formation of the Paranà-Etendeka large igneous province (LIP), Ontong Java (and Manihiki) and Caribbean Plateaus are believed to have caused the geological responses associated with the Valanginian OAE, early Aptian OAE1a, and latest Cenomanian OAE2, respectively. Excess CO2 in the atmosphere probably turned the climate into a greenhouse mode, accelerating continental weathering and increasing nutrient content in oceanic surface-waters via river run-off. Higher fertility was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes. Emplacement of LIPs has been associated with mass extinctions. However, after a phase of stress (biocalcification crises), LIP formation positively stimulated the production/evolution of calcareous nannoplankton. Because Mesozoic OAEs are often represented by carbonate-poor sediments, quantitative studies of calcareous nannofossils have been applied to explore dissolution events and biological processes such as photosynthesis and biomineralization affecting adsorption/release of CO2. During OAEs higher abundances of small coccoliths are associated with biocalcification crises (Valanginian “nannoconid decline” and Aptian “nannoconid crisis”) presumably under conditions of excess CO2 and possibly concurrent higher fertility. Most extreme environmental conditions during the latest Cenomanian OAE 2 exerted different influences on calcareous nannoplankton that experienced a turnover. Precise timing of the events before, during and after Cretaceous OAEs indicate that they were intervals of enhanced oceanic productivity and that anoxia/dysoxia postdate biotic changes.
|Titolo:||Evolution of calcareous phytoplankton, anoxia, pCO2, climate change and igneous events during the Cretaceous: causal or casual link?|
ERBA, ELISABETTA (Primo)
|Settore Scientifico Disciplinare:||Settore GEO/01 - Paleontologia e Paleoecologia|
|Data di pubblicazione:||2005|
|Tipologia:||Book Part (author)|
|Appare nelle tipologie:||03 - Contributo in volume|