Università degli Studi di Milano, 2011-02-03

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 volcanism, weathering, carbonate dissolution/precipitation and organic matter burial. Moreover, O stable isotopes concurred in the reconstruction of paleotemperatures. Ocean changes prior to, during and at the end of OAE1a comprise: deep-water acidification, calcite-lysocline and CCD shallowing and deepening, submarine volcanism, weathering, paleotemperatures fluctuations. The direct calibration of the nannofossil and geochemical data gathered during this PhD project was crucial to build a coherent paleoceanographic/paleoclimatic reconstruction in high-resolution. These results were compared and integrated with previous data and reconstructions to provide a more complete picture of the complex ecosystem perturbation of the OAE1a. The effects of pCO2 on surface-water and bottom-water acidification were identified and traced, indicating that bottom-water acidity is delayed by 25-30 kyr with the early effects on surface-waters recorded by calcareous nannoplankton. The maximum phase of ocean acidification occurs ~76 kyr after the onset of OAE1a. Nannoplankton and carbonate recovery develops over ~160 kyr, under persistent global dysoxia-anoxia. Oxygen-derived paleotemperatures and calcareous nannofossil TI indicate that paleotemperatures were not constant through OAE1a as the onset of OAE1a is marked by the highest temperatures followed by two distinct cooling episodes, interrupting generally warm conditions. Supposedly, volcanogenic CO2 emissions during the major constructional phases of the OJP had a strong impact on climate, imposing (super)greenhouse conditions. The Os isotopic composition of seawater reconstructed for the Tethys and Pacific Oceans provides independent evidence of (at least) two major volcanic phases, presumably associated with the OJP: the first preceding the OAE1a onset of ~200 kyr, the second starting in the core of negative carbon isotope shift and lasting for ~880 kyr. The Os isotopic record further traces a short-lived increase in weathering rates soon preceding the long-lasting submarine volcanic phase. Such weathering peak seems linked to a methane release from clathrate dissociation, following rapid global warming. This study has implemented the geological data-set characterizing the OAE1a. Under extreme climatic-oceanographic conditions, marine biota responded in a very dynamic way, with significant adaptations and reactions. In particular, calcareous nannoplankton were able to survive severe climatic, trophic and chemical perturbations. In fact, the nannofossil calcification failure and nannoconid crisis is a case of false extinctions (Lazarus effect) and no disappearances are detected. Survival ability is further evidenced by a major origination episode through the OAE1a interval. The occurrence of dwarf and malformed specimens in the phase of most extreme paleoenvironmental conditions (maximum acidification) suggests that increasing pCO2 triggered coccolith malformation and solicited production of r-strategist taxa, which secreted dwarf coccoliths as a strategy to overcome acidification. My results emphasize the complex nature of the OAE1a paleoenvironmental perturbation and indicate that ocean anoxia is not synchronous with global warming or ocean acidification or fertilization. Rather, it seems that organic carbon-rich black shales sedimented after threshold conditions were reached. Moreover, under long-lasting (~1.3 million years) anoxia-dysoxia, biotic, chemical, physical conditions fluctuated and interacted at various time scales. While the onset of OAE1a may be justified by progressive and concurrent paleoenvironmental changes, its termination remains to be fully understood.

diritti: info:eu-repo/semantics/closedAccess
tutor: Elisabetta Erba ; cotutor: Anthony S. Cohen ; coordinatore: Stefano Poli
Settore GEO/01 - - Paleontologia e Paleoecologia

Tesi di dottorato. | Lingua: Inglese. | Paese: | BID: TD16000274