Marine Geology of Core V28-179 and Plio-Pleistocene climate implications = Geochimica e nannofossili calcarei del Core V28-179: implicazioni climatiche durante il Plio-Pleistocene

The Central Pacific Ocean is an area of great interest for the oceanographers because the current high amount of nutrients and productivity provide an excellent study area for past environmental changes. A study was performed on a Plio-Pleistocene Core VEMA 28-179 (V28-179) positioned at 4° 37’ N, 139° 36' W in the Pacific Ocean, 2081 cm in length and 4502 meters depth below the sea level in order to improve the paleoceanographic knowledges. The core is located along the equatorial zone which extends a range of high nutritional values. Several studies are been performed on the Core V28-179 (Shackleton and Opdyke, 1977; Backman and Shackleton, 1983; Dunn, 1982) and integrated with sedimentation rates, carbonate contents, isotopic values and fossil contents in this thesis. It was possible to emphasize 2.7 to 3 million years ago a change of orbital and tectonic parameters, which would lead progressively to the North Hemisphere Glaciation. Chapter 1 analyzes the visual description of the color, the carbonate content, particle size, the fossil content, sedimentary structures, the presence or absence of bioturbation and contacts that delimit net changes in the characteristics of the sediment. The high degree of compositional and structural changes indicate considerable variations in the carbonate content.Chapter 2 analyzes the magnetic polarity reversals occurred in Core V28-179, based on Laurens et al.(2004). The sedimentation rate has not remained constant over the time. Different causes may have been variations of orbital parameters and variations of physical (particle transport and sedimentation) and chemical (diagenesis) parameters. In addition, the carbonate component from surface productivity involves almost all of the sediment composition in V28-179. Chapter 3 analyzes the fossil content. In particular, we considered three species of calcareous nannofossils Discoaster brouweri, Discoaster tamalis and Discoaster asymmetricus. In particular we carried out the analysis of the abundance of species in the sediment from 1000 to 2065 cm, showing an effective predominance of the species D. brouweri than the other two species, up to about 700 individuals per mm-2 against approximately 110 and 75 respectively for D. asymmetricus and D. tamalis. Chapter 4 is dedicated to the discussion about the processes involved in the carbon cycle (production, transport, deposition and post-depositional processes). V28-179 is positioned in an area of high surface productivity, driven by a high nutrients input, due to upwelling of cold deep waters in the east Pacific Ocean.This results in a high carbonate content. The CaCO3 content analysis also showed a progressive increase from 4,085 Ma to the present time, probably caused by a nutrients increase, and an initial prominent cyclicity of ca. 400 000 years, that becomes indistinguishable in younger sediments. The migration northward of V28-179 position (plate tectonics movements), the migration of the high nutrient content area or its expansion can be the causes of variation in the calcium carbonate content.Chapter 5 analyzes the isotopic values of oxygen (18O / 16O) and carbon (13C / 12C), both reconnected to the carbon cycle. From the carbon isotope at 2.1 Ma is evident a change in the mode of deposition. The comparison with the CaCO3 content shows a stronger anti-phasing trend from 2.1 to 0 Ma. Oxygen isotope values shows an increase in 18O values, representing the transition from a warm climate to the formation of ice sheets in the northern hemisphere. Data were then compared with the isotopic values described by Shackleton and Opdyke (1977) and was summarized the isotopic antithetical diversity with the Atlantic Ocean. In Chapter 6 was performed a research on temperature and orbital parameters variations, occurred during the Plio-Pleistocene. The closure of the Isthmus of Panama, the tibetan uplift and the closure of Indonesian Seaway are among the possible causes that has led to the glaciation of the northern hemisphere, concomitant to changes in orbital parameters from cycles of 41 000 years to 100 000 years. The change in ocean circulation has led to strong variation in the heat distribution in oceanic waters and to the formation of upwelling zones, especially in the equatorial Pacific and along east coasts, where even today there is a seasonal atmospheric disturbance known as El Niño, the proof of the great changes that occurred in the past.