Settore scientifico-disciplinare: GEO/01 Paleontologia e paleoecologia XIX CICLO Stratigrafia ad alta risoluzione dei depositi quaternari in Adriatico centrale e meridionale: impatto di cambiamenti climatici a scala sub-Milankoviana sulla circolazione in Mediterraneo High-resolution stratigraphy of Central and Southern Adriatic Quaternary deposits: impact of sub-Milankovian climate change on Mediterranean circulation Dottorando Relatore

William Cavazza, Andrea Piva, Battista Gian, Co-Relatori Vai, Alessandra Asioli, Fabio Trincardi, R Ralph, Schneider Bologna
2007 unpublished
Trincardi et al., Marine Geology, in press ii Riassunto Questa tesi racchiude il lavoro di tre anni di Dottorato di ricerca, durante i quali sono stati analizzati sedimenti marini dell'Adriatico centrale e meridionale, ottenuti mediante la perforazione di pozzi e carote, grazie ad una ottima conoscenza sismo-stratigrafica dell'area indagata. Il lavoro è stato svolto nell'ambito dei progetti EC-EURODELTA (coord. Fabio Trincardi, ISMAR-CNR), EC-EUROSTRATAFORM (coord. Phil P. E. Weaver, NOC, UK) e
more » ... EC-PROMESS1 (coord. Serge Bernè, IFREMER, Francia). Le successioni sedimentarie analizzate presentano intervalli stratigrafici espansi, in particolare per gli ultimi 400, 60, e 6 kyr BP. I tre intervalli di tempo indagati sono riflessi in una tripartizione della tesi stessa. Lo studio è consistito nell'analisi quantitativa delle associazioni a foraminiferi planctonici e bentonici, per oltre 560 campioni analizzati, nella preparazione del materiale per l'analisi degli isotopi stabili di ossigeno e carbonio e nella interpretazione e discussione del dato ottenuto. Si è provveduto all'inquadramento cronologico per gli ultimi 400 kyr del pozzo PRAD1-2, perforato in Adriatico centrale alla profondità di 186.5 m (ciò ricadeva nell'unità operativa WP6 del progetto PROMESS1). La cronologia proposta è il frutto di un approccio multidisciplinare, dato dall'integrazione di parametri diversi ed indipendenti, alcuni dei quali forniti dagli altri specialisti coinvolti nel progetto. La cronologia del pozzo è basata su: micropaleontologia (bioeventi a foraminiferi e nannoplancton calcareo), ciclicità climatica (associazioni a foraminiferi), geochimica (isotopi stabili dell'ossigeno su record planctonico e bentonico), paleomagnetismo, datazioni radiometriche ( 14 C AMS), tephrocronologia, riconoscimento di livelli sapropel-equivalenti (Se). Va notato anche come il record isotopico dell'ossigeno del pozzo PRAD1-2 mostri un'ottima correlabilità con record più profondi del Mediterraneo. I parametri indagati hanno consentito di riconoscere nel pozzo PRAD1-2 la presenza di tutti gli stadi isotopici dal MIS1 al MIS10, mentre la base del pozzo è stata ascritta alla parte terminale del MIS11. Gli stadi interglaciali e glaciali incontrati in PRAD1-2 sono stati analizzati in dettaglio anche dal punto di vista del paleoambiente. Ad esempio, gli intervalli glaciali (in particolare MIS6, MIS8 e MIS10) presentano associazioni a foraminiferi peculiari, dominate da specie bentoniche tipiche di regioni polari, non più presenti attualmente nell'Adriatico Centrale. Si evidenzia inoltre un trend di approfondimento nella paleoprofondità durante i glaciali dal MIS10 (ambiente di piattaforma interna) al MIS4 (ambiente di piattaforma media). Abstract This volume is a collection of the work done in a three years-lasting PhD, focused in the analysis of Central and Southern Adriatic marine sediments, deriving from the collection of a borehole and many cores, achieved thanks to the good seismic-stratigraphic knowledge of the study area. The work was made out within European projects EC-EURODELTA (coordinated by Fabio Trincardi, ISMAR-CNR), EC-EUROSTRATAFORM (coordinated by Phil P. E. Weaver, NOC, UK), and PROMESS1 (coordinated by Serge Bernè, IFREMER, France). The analysed sedimentary successions presented highly expanded stratigraphic intervals, particularly for the last 400 kyr, 60 kyr and 6 kyr BP. These three different time-intervals resulted in a tri-partition of the PhD thesis. The study consisted of the analysis of planktic and benthic foraminifers' assemblages (more than 560 samples analysed), as well as in preparing the material for oxygen and carbon stable isotope analyses, and interpreting and discussing the obtained dataset. The chronologic framework of the last 400 kyr was achieved for borehole PRAD1-2 (within the work-package WP6 of PROMESS1 project), collected in 186.5 m water depth. The proposed chronology derives from a multi-disciplinary approach, consisting of the integration of numerous and independent proxies, some of which analysed by other specialists within the project. The final framework based on: micropaleontology (calcareous nannofossils and foraminifers' bioevents), climatic cyclicity (foraminifers' assemblages), geochemistry (oxygen stable isotope, made out on planktic and benthic records), paleomagnetism, radiometric ages ( 14 C AMS), teprhochronology, identification of sapropel-equivalent levels (Se). It's worth to note the good consistency between the oxygen stable isotope curve obtained for borehole PRAD1-2 and other deeper Mediterranean records. The studied proxies allowed the recognition of all the isotopic intervals from MIS10 to MIS1 in PRAD1-2 record, and the base of the borehole has been ascribed to the early MIS11. Glacial and interglacial intervals identified in the Central Adriatic record have been analysed in detail for the paleo-environmental reconstruction, as well. For instance, glacial stages MIS6, MIS8 and MIS10 present peculiar foraminifers' assemblages, composed by benthic species typical of polar regions and no longer living in the Central Adriatic nowadays. Moreover, a deepening trend in the paleo-bathymetry during glacial intervals was observed, from MIS10 (inner-shelf environment) to MIS4 (mid-shelf environment). 4 Ten sapropel-equivalent levels have been recognised in PRAD1-2 Central Adriatic record. They showed different planktic foraminifers' assemblages, which allowed the first distinction of events occurred during warm-climate (Se5, Se7), cold-climate (Se4, Se6 and Se8) and temperateintermediate-climate (Se1, Se3, Se9, Se', Se10) conditions, consistently with literature. Coldclimate sapropel equivalents are characterised by the absence of an oligotrophic phase, whereas warm-temeprate-climate sapropel equivalents present both the oligotrophic and the eutrophic phases (except for Se1). Sea floor conditions vary, according to benthic foraminifers' assemblages, from relatively well oxygenated (Se1, Se3), to dysoxic (Se9, Se', Se10), to highly dysoxic (Se4, Se6, Se8) to events during which benthic foraminifers are absent (Se5, Se7). These two latter levels are also characterised by the lamination of the sediment, feature never observed in literature in such shallow records. The enhanced stratification of the water column during the events Se8, Se7, Se6, Se5, Se4, and the concurring strong dilution of shallow water, pointed out by the isotope record, lead to the hypothesis of a period of intense precipitation in the Central Adriatic region, possibly due to a northward shift of the African Monsoon. Finally, the expression of Central Adriatic PRAD1-2 Se5 equivalent was compared with the same event, as registered in other Eastern Mediterranean areas. The sequence of substantially the same planktic foraminifers' bioevents has been consistently recognised, indicating a similar evolution of the water column all over the Eastern Mediterranean; yet, the synchronism of these events cannot be demonstrated. A high resolution analysis of late Holocene (last 6000 years BP) climate change was carried out for the Adriatic area, through the recognition of planktic and benthic foraminifers' bioevents. In particular, peaks of planktic Globigerinoides sacculifer (four during the last 5500 years BP in the most expanded core) have been interpreted, based on the ecological requirements of this species, as warm-climate, arid intervals, correspondent to periods of relative climatic optimum, such as, for instance, the Medieval Warm Period, the Roman Age, the Late Bronze Age and the Copper Age. Consequently, the minima in the abundance of this biomarker could correspond to relatively cooler and more rainy periods. These conclusions are in good agreement with the isotopic and the pollen data. The Last Occurrence (LO) of G. sacculifer has been dated in this work at an average age of 550 years BP, and it is the best bioevent approximating the base of the Little Ice Age in the Adriatic. Recent literature reports the same bioevent in the Levantine Basin, showing a rather consistent age. Therefore, the LO of G. sacculifer has the potential to be extended to all the Eastern Mediterranean. Within the Little Ice Age, benthic foraminifer V. complanata shows two distinct peaks in the shallower Adriatic cores analysed, collected hundred kilometres apart, inside 5 the mud belt environment. Based on the ecological requirements of this species, these two peaks have been interpreted as the more intense (cold and rainy) oscillations inside the LIA. The chronologic framework of the analysed cores is robust, being based on several range-finding 14 C AMS ages, on estimates of the secular variation of the magnetic field, on geochemical estimates of the activity depth of 210 Pb short-lived radionuclide (for the core-top ages), and is in good agreement with tephrochronologic, pollen and foraminiferal data. The intra-holocenic climate oscillations find out in the Adriatic have been compared with those pointed out in literature from other records of the Northern Hemisphere, and the chronologic constraint seems quite good. Finally, the sedimentary successions analysed allowed the review and the update of the foraminifers' ecobiostratigraphy available from literature for the Adriatic region, thanks to the achievement of 16 ecobiozones for the last 60 kyr BP. Some bioevents are restricted to the Central Adriatic (for instance the LO of benthic Hyalinea balthica, approximating the MIS3/MIS2 boundary), others occur all over the Adriatic basin (for instance the LO of planktic Globorotalia inflata during MIS3, individuating Dansgaard-Oeschger cycle 8 (Denekamp)).
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