| Title: | The Early Cretaceous planktonic foraminiferal radiation: paleoceangraphic implications |
| Author: | Isabella Premoli Silva |
| Date Submitted: | 05/01/2002 |
| Address: | Via Mangigalli 34
Milano
Italy
20133
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| Phone: | +39-02503 15528 |
| Email: | Isabella.Premoli@unimi.it |
| Co-Authors: | |
| Affiliation: | University of Milan, Department of Earth Sciences |
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| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725582 |
| Keywords: | Early Cretaceous planktonic foraminiferal radiation |
| Abstract: | The first radiation of planktonic foraminifera was thought to begin close to the base of the Aptian. However, their recent study from some Lower Cretaceous successions revealed that this group started to diversify much earlier. Direct correlation with ammonites indicated that the first diversification occurred in the early Valanginian with the appearance of the first hedbergellids (H. sigali and H. aptica) followed in late Valanginian by the appearance of H. delrioensis-type and the first planispiral form. This small fauna in both species number and size seems to persist without apparent changes through most of the Hauterivian, the end of which is characterized by an abrupt short flourishing of the gorbachikellids. The lowermost Barremian coincided with a remarkable increase both in abundance and number of planktonic species and genera that continued through the Barremian accompanied by an overall increase in size. However, their stratigraphic distribution and abundance fluctuated rhythmically with increasing fluctuation intensity in late Barremian. This trend continued through the lower and lower upper Aptian with a further increase in number of species and overall abundance and size of specimens after the early Aptian Oceanic Anoxic Event 1a (OAE1a). This increase was accompanied by the progressive appearance of more ornamented morphotypes. By middle late Aptian, several species of the older hedbergellid and globigerinelloidid stocks progressively became extinct. The late to latest Aptian assemblages were mainly composed of large ornamented morphotypes that in turn become extinct around the Aptian/Albian boundary. Contrary to what written in recent years, no turnover in planktonic foraminifera coincided with OAE1a. Detailed study of the organic-rich Livello Selli (Italy) and equivalents revealed that the OAE1a planktonic assemblages are typically dominated by either clavate morphotypes or leupoldinids, cyclically alternating with faunas consisting of normal-sized, round-chambered morphotypes (the same species prior and after OAE1a). The early Valanginian planktonic diversification coincides with several events in other pelagic groups: hedbergellids first occurred as calpionellids drastically decreased in abundance shortly before their extinction; nannoconids decreased in abundance whereas coccolithophorids diversified, boreal dinoflagellates migrated to low latitudes, and diatoms became somewhat better preserved. These events protend changes in the water column derived from increased efficiency of the oceanic current system, which in turn derived from more contrasting seasonality (dry/humid) that increased the nutrient supply in surface waters creating more eutrophic conditions for hedbergellid diversification. This discrete seasonality continued through the Barremian-Aptian as supported by the rhythmic fluctuations in abundance and composition of planktonic assemblages induced by orbitally produced Milankovitch cycles. The dominance of more ornamented, large-sized morphotypes in late Aptian indicates the onset or strengthening of a weak thermocline by that time. The presence of a proto-thermocline was only temporary and was disrupted at the close of the Aptian (= extinction of all large taxa). The OAE1a represented only a temporary paleoenvironmental perturbation characterized in the upper water column by alternating low oxygen levels (proliferation of leupoldinids), to slightly richer oxygen levels (clavate morphotypes) or to better oxygenated waters (round-chambered taxa). The effects of the perturbation related to the OAE1a appear to have terminated only about one million years after the event as clavate taxa and leupoldinids persisted up to the G. ferreolensis Zone. |
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