Holocene climate trends in NW Africa: Inferences from grain-size distributions of terrigenous sediments and isotopic provenance studies

Abstract

On a global scale fluvial sediments, transported by rivers to the world s oceans, provide the major component in deep sea sediments, amounting to about ten times the total mass that is transported from all deserts combined. In contrast, offshore the NW African coast the most important constituent of the terrigenous fraction is aeolian dust, delivered by different wind systems to the Atlantic Ocean. In the region downwind the Saharan Desert, the world s biggest source of desert dust, aeolian input exceeds the fluvial supply by a factor of ten. Records of aeolian dust and fluvial matter in marine sediments provide one of the most detailed and best preserved sources of paleoclimatic information, in particular to define past hydrological conditions, changes in transport mechanisms, and provenance of terrigenous material as well as the extent of human-induced dust mobilization by land degradation. The objective of this PhD thesis is the documentation of the temporal and spatial variability of fluvial vs. aeolian input offshore the Saharan Desert and the examination of the provenance of the terrigenous fraction during the Holocene. Compared to the vast glacial-interglacial variations of the Pleistocene, the Holocene climate has been rather stable but is characterized by a number of high-amplitude short-term variations like the Younger Dryas, at the transition between the Pleistocene and the Holocene epoch, and the Holocene Climate Optimum, which is called the African Humid Period within the NW African realm. Therefore, the Holocene is well suited to examine potential controls of climate change in a region which is likely to react extremely sensitive to future climate change. The presented investigations were carried out on a set of marine sediment cores retrieved from the continental slope offshore Morocco, the Western Sahara, Mauritania, and Senegal. Using grain-size measurements and end-member modeling the study shows that the terrigenous material in deep-sea sediments between 30°N and 15°N are composed of variable amounts of aeolian and fluvial sediments, which is linked to prevailing climate conditions on land (Chapters 6 and 7). The results indicate that regions south of 30°N are mainly influenced by the seasonal shifts of the NW African monsoonal rainbelt. During the Younger Dryas coarse aeolian dust is the most important sediment component, demonstrating arid and windy conditions on land. In contrast, the mid-Holocene is characterized by enhanced deposition of fluvial material, coinciding with the African Humid Period. Within late Holocene times, fine aeolian dust is the dominant sediment component, reflecting modern dry conditions over NW Africa. North of 30°N terrigenous sediments are mainly governed by variations of the North Atlantic climate system. Furthermore, the data exhibit that the impact of local features has a stronger influence on the terrigenous sediment supply, than commonly anticipated (Chapter 6). Based on strontium and neodymium isotope measurements, provenance studies and a reconstruction of temporal variations in the NW African weathering regime were carried out. Measured neodymium values indicate that the geological sources of terrigenous material were nearly constant over the last ~12 ka. In contrast, variations in Sr isotope ratios reflect major changes in the evaporation/precipitation balance and hence the rate of chemical weathering. The data document latitudinal shifts of the transition zone between the African monsoonal climate system south of the Saharan Desert, and the Mediterranean climate regime in the North, causing abrupt and persistent changes in the humidity and rate of chemical weathering over NW Africa since the Younger Dryas (Chapters 5 and 6). Furthermore, the results presented in this thesis indicate that human activity influences dust emission through agriculture and land-use in a region extremely sensitive to changes in precipitation (Chapter 8). Data suggest that human dust production through land use exceeds natural dust production by far.