8th International Nannoplankton Association Conference
Elisabetta Erba, Fabrizio Tremolada:
Coccolithophores are the major primary producers which convert dissolved CO2 into CaCO3 in the ocean, and calcareous nannofossils (coccoliths and nannoliths) are the principal contributors to pelagic micrite from the Jurassic to Recent. Since calcareous nannofossils constitute the bulk of micritic limestone, many authors in the last decade have carried out investigations about extant coccolith volumes to estimate their weights (CaCO3 density = 2.7g/cm3) and the carbonate fluxes directly produced by coccoliths. The absolute abundances of single taxa are based on counts of coccoliths in unit area of sediment traps. Estimates of coccolith carbonate fluxes are then obtained using single-taxon volume and absolute abundance.
This kind of investigation has not been attempted on Cretaceous micrites, although a number of quantitative studies have provided insights into nannofossil palaeoecology, and a few indices have been recognised. The Cretaceous is a period recording extreme palaeoclimatic and palaeoceanographic conditions, perhaps provoked by major tectonic and volcanic events. In particular, the volcanic and tectonic events of the Early Cretaceous (formation of the Mahiniki and Ontong Java Plateaus and the Nova Canton Trough System) produced excessive CO2 levels in the atmosphere and caused changes in the nutrient content of oceanic surface-waters which influenced productivity and biogenic sedimentation. In the Tethyan area, rhythmically-bedded sediments of Cretaceous age reflect high-frequency variations in the structure of phytoplankton palaeocommunities, whereas low-frequency nannofloral variations are recorded in major lithologic changes. The higher fertility, and concomitant production of organic matter, resulted in sediment anoxia during 'black shale' deposition (Oceanic Anoxic Events), whereas lower fertility resulted in calcareous-rich layers. Assemblages of higher fertility conditions are mostly dominated by A. infracretacea, R. terebrodentarius, B. constans, Z. erectus and D. rotatorius, while the lower fertility assemblages mainly consist of W. barnesiae, Nannoconus spp. and pentaliths.
We calculated the coccolith volumes/masses of the most abundant species of Cretaceous age and estimated biogenic marine fluxes characterising lower and higher fertility conditions recorded in the different lithologies. Such fluxes are used to estimate the response of phytoplankton to nutrification events and to calibrate the real contribution of coccoliths to pelagic carbonates. Additionally, the amount of coccoliths in the carbonate fluxes might be used to model the biosphere reactions to palaeoenvironmental changes.
Copyright © 2000, most recent revision July 24, 2000Tania Hildebrand-Habel (email@example.com)