High-resolution stratigraphy of Central and Southern Adriatic Quaternary deposits of sub-Milankovian climate change on Mediterranean circulation

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 (14C 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).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 temperate-intermediate-climate (Se1, Se3, Se9, Se’, Se10) conditions, consistently with literature. Cold-climate 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 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 14C AMS ages, on estimates of the secular variation of the magnetic field, on geochemical estimates of the activity depth of 210Pb 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)).

Tectonics and kinematics of curved mountain belts: examples from the Andes and the Alps

Curved mountain belts have always fascinated geologists and geophysicists because of their peculiar structural setting and geodynamic mechanisms of formation. The need of studying orogenic bends arises from the numerous questions to which geologists and geophysicists have tried to answer to during the last two decades, such as: what are the mechanisms governing orogenic bends formation? Why do they form? Do they develop in particular geological conditions? And if so, what are the most favorable conditions? What are their relationships with the deformational history of the belt? Why is the shape of arcuate orogens in many parts of the Earth so different? What are the factors controlling the shape of orogenic bends? Paleomagnetism demonstrated to be one of the most effective techniques in order to document the deformation of a curved belt through the determination of vertical axis rotations. In fact, the pattern of rotations within a curved belt can reveal the occurrence of a bending, and its timing. Nevertheless, paleomagnetic data alone are not sufficient to constrain the tectonic evolution of a curved belt. Usually, structural analysis integrates paleomagnetic data, in defining the kinematics of a belt through kinematic indicators on brittle fault planes (i.e., slickensides, mineral fibers growth, SC-structures). My research program has been focused on the study of curved mountain belts through paleomagnetism, in order to define their kinematics, timing, and mechanisms of formation. Structural analysis, performed only in some regions, supported and integrated paleomagnetic data. In particular, three arcuate orogenic systems have been investigated: the Western Alpine Arc (NW Italy), the Bolivian Orocline (Central Andes, NW Argentina), and the Patagonian Orocline (Tierra del Fuego, southern Argentina). The bending of the Western Alpine Arc has been investigated so far using different approaches, though few based on reliable paleomagnetic data. Results from our paleomagnetic study carried out in the Tertiary Piedmont Basin, located on top of Alpine nappes, indicate that the Western Alpine Arc is a primary bend that has been subsequently tightened by further ~50° during Aquitanian-Serravallian times (23-12 Ma). This mid-Miocene oroclinal bending, superimposing onto a pre-existing Eocene nonrotational arc, is the result of a composite geodynamic mechanism, where slab rollback, mantle flows, and rotating thrust emplacement are intimately linked. Relying on our paleomagnetic and structural evidence, the Bolivian Orocline can be considered as a progressive bend, whose formation has been driven by the along-strike gradient of crustal shortening. The documented clockwise rotations up to 45° are compatible with a secondary-bending type mechanism occurring after Eocene-Oligocene times (30-40 Ma), and their nature is probably related to the widespread shearing taking place between zones of differential shortening. Since ~15 Ma ago, the activity of N-S left-lateral strike-slip faults in the Eastern Cordillera at the border with the Altiplano-Puna plateau induced up to ~40° counterclockwise rotations along the fault zone, locally annulling the regional clockwise rotation. We proposed that mid-Miocene strike-slip activity developed in response of a compressive stress (related to body forces) at the plateau margins, caused by the progressive lateral (southward) growth of the Altiplano-Puna plateau, laterally spreading from the overthickened crustal region of the salient apex. The growth of plateaux by lateral spreading seems to be a mechanism common to other major plateaux in the Earth (i.e., Tibetan plateau). Results from the Patagonian Orocline represent the first reliable constraint to the timing of bending in the southern tip of South America. They indicate that the Patagonian Orocline did not undergo any significant rotation since early Eocene times (~50 Ma), implying that it may be considered either a primary bend, or an orocline formed during the late Cretaceous-early Eocene deformation phase. This result has important implications on the opening of the Drake Passage at ~32 Ma, since it is definitely not related to the formation of the Patagonian orocline, but the sole consequence of the Scotia plate spreading. Finally, relying on the results and implications from the study of the Western Alpine Arc, the Bolivian Orocline, and the Patagonian Orocline, general conclusions on curved mountain belt formation have been inferred.

Paleosecular variation of the magnetic field recorded in Pleistocene-holocene volcanics from Pantelleria (italy) and Azores archipelago (portugal): implications for local volcanic history

The primary goal of volcanological studies is to reconstruct the eruptive history of active volcanoes, by correlating and dating volcanic deposits, in order to depict a future scenario and determine the volcanic hazard of an area. However, alternative methods are necessary where the lack of outcrops, the deposit variability and discontinuity make the correlation difficult, and suitable materials for an accurate dating lack. In this thesis, paleomagnetism (a branch of Geophysics studying the remanent magnetization preserved in rocks) is used as a correlating and dating tool. The correlation is based on the assumption that coeval rocks record similar paleomagnetic directions; the dating relies upon the comparison between paleomagnetic directions recorded by rocks with the expected values from references Paleo-Secular Variation curves (PSV, the variation of the geomagnetic field along time). I first used paleomagnetism to refine the knowledge of the pre – 50 ka geologic history of the Pantelleria island (Strait of Sicily, Italy), by correlating five ignimbrites and two breccias deposits emplaced during that period. Since the use of the paleomagnetic dating is limited by the availability of PSV curves for the studied area, I firstly recovered both paleomagnetic directions and intensities (using a modified Thellier method) from radiocarbon dated lava flows in São Miguel (Azores Islands, Portugal), reconstructing the first PSV reference curve for the Atlantic Ocean for the last 3 ka. Afterwards, I applied paleomagnetism to unravel the chronology and characteristics of Holocene volcanic activity at Faial (Azores) where geochronological age constraints lack. I correlated scoria cones and lava flows yielded by the same eruption on the Capelo Peninsula and dated eruptive events (by comparing paleomagnetic directions with PSV from France and United Kingdom), finding that the volcanics exposed at the Capelo Peninsula are younger than previously believed, and entirely comprised in the last 4 ka.