Mediterranean oceanography and North African climate over the Pliocene

Abstract

Mediterranean basin sediments are excellent archives of 1) high-latitude climate change and 2) North African environmental evolution. First, the semi-enclosed nature of the basin amplifies high-latitude climatic signals recorded in the sediments. Second, the episodic formation of organic-rich layers (sapropels) during past North African humid periods allows the reconstruction of past Green Sahara Periods alternating with North African dry periods. In this thesis, I employ a multi-proxy reconstruction (stable oxygen isotope, environmental magnetic and X-ray fluorescence scanning data) for eastern Mediterranean Ocean Drilling Program (ODP) Site 967, to investigate Mediterranean oceanography and North African climate during the past 5.3 Myr (million years), especially focusing on the Pliocene. Prior to the Miocene-Pliocene (M/P) transition (5.3 Myr ago), the Mediterranean was isolated from the global ocean due to tectonic restriction of the Atlantic-Mediterranean gateway, in an event called the 'Messinian salinity crisis'. The 600,000-year long salinity crisis led to a kilometre-scale drawdown of the Mediterranean, resulting in massive evaporite deposition and brine formation. It had been suggested that a catastrophic megaflood due to Gibraltar sill collapse re-established normal marine conditions in the Mediterranean immediately following the M/P boundary. The presence of an unusually lengthy sapropel (the 'mystery sapropel') is recognized in the eastern Mediterranean, at the M/P boundary, while an equivalent is absent in the western Mediterranean. Using physical oceanographic modelling, it is elaborated that an energetic flood transferred most of the western basin Messinian brines across the Sicily sill, yielding salinity stratification in the eastern basin, while normal marine conditions established in the western basin. With additional data from ODP Site 969, it is further explained how the eastern basin initially became stratified and oxygenated, due to a ~1,500-m-high, turbulent and aerated cascade that entered eastern basin. Stratification delayed the establishment of normal marine conditions by ~33,000 years, during which the mystery sapropel was deposited. While the accumulated salts were diffused into outflow that transported them back to the Atlantic, the eastern basin water column shifted from an oxidizing to a reducing redox state. Once the salt export allowed the resumption of convective deep-water formation, eastern Mediterranean deep waters became oxygenated again. A second marked transformation occurred following the Early-Mid Pliocene transition (3.4-3.2 Myr), when eastern Mediterranean organic carbon burial intensified, due to a shift to greater nutrient inputs. It is hypothesized that these inputs resulted from an increase in arid:humid contrast between alternating North African dry-wet episodes. A co-occurrence of various environmental shifts is recognized over the mid-latitudes of both hemispheres. Using paleoclimate modelling, it is shown that the Earth's shift from unipolar to bipolar glaciation resulted in strengthening and shifting of key components in the atmospheric circulation, leading to the observed global-scale phenomena. Furthermore, using climate modelling results and terrestrial weathering indices, this work recognizes the existence of a Pan-North African humid period prior to the Early-Mid Pliocene transition (3.8-3.4 Myr). The lengthy humid period may have resulted in the oldest known early hominin migration to the Chad basin, ~2,000 km west of their origin in east Africa. Finally, a Mediterranean box model is used to show that the North African monsoon was modulated by orbital eccentricity with remarkably stable amplitudes over the past 5 Myr. In summary, this thesis combines proxy data with numerical and climate modelling to explore major Mediterranean oceanographic and North African climatic transitions, and their relation to Pliocene global environmental evolution. Show more