PCO2 EFFECTS ON THE PRODUCTION OF PELAGIC BIOGENIC CARBONATE AND OCEAN CHEMISTRY: A CASE HISTORY FROM THE CRETACEOUS.
Tesi di Dottorato
Data di Pubblicazione:
2011
Citazione:
PCO2 EFFECTS ON THE PRODUCTION OF PELAGIC BIOGENIC CARBONATE AND OCEAN CHEMISTRY: A CASE HISTORY FROM THE CRETACEOUS / C. Bottini ; tutor: Elisabetta Erba ; cotutor: Anthony S. Cohen ; coordinatore: Stefano Poli. Universita' degli Studi di Milano, 2011 Feb 03. 23. ciclo, Anno Accademico 2010.
Abstract:
In the early Aptian, ocean experienced a global phenomenon of widespread deposition of organic carbon-rich sediments under oxygen-poor conditions, named the Oceanic Anoxic Event 1a (OAE1a) (~120 Ma) which has its sedimentary expression in the Selli Level. OAE1a represents a profound perturbation of the ocean-atmosphere system caused by natural CO2 emissions related with climate change, ocean fertilization and acidification, probably associated with the Ontong Java Plateau (OJP) emplacement.
The goal of this PhD thesis was to study in high detail the OAE1a through an integrated micropaleontological (calcareous nannofossils) and geochemical (Re-Os isotopes, C and O stable isotopes) approach. Calcareous nannoplankton, responsible for primary productivity and effective producers of calcite, affect the C-cycle on both long- and short-term time scales. They are sensitive to changes in temperature, fertility and chemistry of surface-waters, and, consequently, their abundance and composition are used to reconstruct paleoclimatic and paleoceanographic fluctuations and the functioning of the ocean/atmosphere system.
Osmium isotopes are used as geochemical proxy to reconstruct changes in the composition of ancient seawater related to volcanic activity and/or continental weathering which are two natural phenomena capable of exerting a direct control on the ocean-atmospheric CO2 budget.
The study was performed on pelagic sediments from two distant drillsites in order to discriminate between global and regional changes: the DSDP Site 463 (Mid-Pacific Mountains) which was relatively close to the Ontong Java province at the time of its emplacement, and the Cismon core (northern Italy) representing sediments deposited in the Mesozoic Tethys. Both sections are well-dated through bio-, magneto-, chemo-, and cyclo-stratigraphy, providing high-resolution time control and correlation. A quantitative approach was applied to the investigation of calcareous nannofossil assemblages (relative and absolute abundances, paleofluxes) and morphometric analyses were performed on selected taxa to identify changes in size and ultrastructure. Geochemical data were obtained from samples studied for calcareous nannofossils in order to achieve a coherent picture of the events through the OAE1a interval.
Calcareous nannoplankton responded to variations in surface-water fertility and temperature as well as to CO2-induced acidification. They underwent a biocalcification decline and crisis, started ~1 Ma before OAE1a, deriving from combined progressive surface-water nutrification and acidification, probably induced by early construction phases of the OJP. The initial stage of OAE1a is marked by: (a) a drastic drop in carbonate, (b) demised nannoconids, (c) abundant mesotrophic species B.constans, Z.erectus, and D.rotatorius, represented by dwarf coccoliths, and (d) a large number of W.barnesiae deformed specimens affected by reduced size and marked ellipticity. Coccolith dwarfism and “deformation/malformation” are interpreted as the coccolithophorid species-specific response to surface-water acidification. Nannofossil abundance and paleofluxes recover after most extreme conditions are reached in the early phase of OAE1a, but still under persisting anoxia/dysoxia, volcanogenic CO2 emissions and climate perturbation.
Calcareous nannofossil Nutrient Index (NI) and biogenic calcite paleofluxes reveal intervals of increased nutrient supply before and during OAE1a. Calcareous nannofossil Temperature Index (TI) shows a significant increase at the onset of OAE1a and a few cooling interludes during the rest of the perturbation.
C and O stable isotopes, Os isotope composition and lithology highlighted major variations in pCO2 derived from the interplay between OJP volcan
Tipologia IRIS:
Tesi di dottorato
Elenco autori:
C. Bottini
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