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• W2534451812 abstract "Marine sediment cores provide the opportunity to study past climatic changes, which is essential to understand the ongoing and future climate change. Within this context, the North Atlantic Ocean is one of the best-studied areas. However, most of these studies focus on the continental margins and shelfs, where thick piles of sediment allow a detailed reconstruction of past environmental changes. In contrast, little is known about the low- to midlatitude open ocean areas, which would be crucial regarding the large area covered. Furthermore, the low latitudes of the North Atlantic are considered to play a key role in modulating the global climate on at least orbital and millennial timescales. This key role is attributed to their ability to retain/release heat to the higher latitudes and therefore to influence the Atlantic Meridional Overturning Circulation (AMOC).Especially changes in marine primary productivity, which due to their impact on the ocean carbon cycle are an integral part of the climate system, are of interest to the (paleo-) climate community. Therefore, in this PhD thesis, changes in primary productivity and hy-drography were reconstructed with high temporal resolution in the temperate/subtropical open ocean North Atlantic, from three sediment cores (GEOFAR KF16, MD08-3179Cq and MD08-3180Cq) taken slightly south of the Azores Islands. These reconstructions are primarily based on quantitative analysis of coccolithophore assemblages, which are supplemented by alkenone analysis, X-ray fluorescence (XRF) core scanning and diatom counts. The results indicate that profound changes in productivity and in hydrographic conditions occurred during the selected time periods of the last 130 kyrs in the Azores region, which can convincingly be deduced from changes in coccolithophore assemblages.For example, an increased productivity prevailed in the Azores region during the last deglacial and early Holocene, especially during cold Heinrich event 1 (H1) and the cold Younger Dryas that are characterized by a strongly reduced AMOC. This is evidenced by increased coccolith accumulation rates, increased alkenone concentrations/accumulation rates, increased Ba/Ti ratios and increased diatom abundances. The increased productivity was a response to intensified westerly winds and the advection of northern-sourced, nutrient-rich surface waters. Furthermore, a more southern position of the Azores Front (AzF), as indicated by abundance changes of key coccolithophore species (G. muellerae, grouped subtropical species), contributed to the observed increased productivity. In conjunction with other studies from the midlatitude North Atlantic, these results indicate that there was a band of high productivity at the northern rim of the contracted North Atlantic subtropical gyre during times of a reduced Atlantic Meridional Overturning Circulation (AMOC).A similar pattern of changes can be observed during weak AMOC phases between 130 and 48 kyrs BP. During the penultimate deglacial and cold Marine Isotope (sub-) Stages (MIS) 5.4, 5.2 and 4, an increased productivity and a more southern position of the Azores Front is observed. On the other hand, during warm periods (MIS 5.3, MIS 5.1, MIS 3) a decreased productivity and a more northern position of the Azores Front are found. An exception to this general pattern is the last interglacial period (MIS 5.5), which is characterized by strongly increased coccolithophore productivity. This increased productivity was probably caused by the opportunity for coccolithophores to occupy new habitats after glacial conditions and/or by an increased advection and upwelling of nutrient-rich Subantarctic Mode Water (SAMW).Because a more comprehensive knowledge of natural interglacial climate variability is important within the context of future climate change, special emphasis is given to the present and last interglacial period in this thesis. Comparing the results for the present (Holocene) and the last (MIS 5.5) interglacial period reveals a similar pattern of changes in the Azores region, and leads to the following hypothesis: Due to an early northern hemisphere insolation maximum, a first amelioration of climate, characterized by a decreased productivity and a more northern position of the Azores Front is found during the early interglacials. Thereafter, a delayed response of the northern hemisphere ice-sheets caused an increased input of freshwater to the North Atlantic, leading to a reversal towards more glacial conditions. This accelerated melting was terminated by a collapse of the ice-sheets, resulting in an outburst of proglacial lakes, an event that is recognized in the Azores region at least during the Holocene (8.2 kyr event). After the freshwater forcing ceased, full interglacial conditions established during the late interglacials.In general, an increased primary productivity and a more southern position of the Azores Front are found during cold periods, whereas a decreased primary productivity and more northern position of the Azores Front are found during warm periods. As the Azores Front delineates the northern boundary of the North Atlantic subtropical gyre, these results indicate a contracted gyre during cold periods and an expanded gyre during warm periods. Furthermore, within the context of an expected future global warming, these results lead to the assumption that the oligotrophic waters of the North Atlantic subtropical gyre probably would expand northwards in response to a warming. A resulting decreased marine primary productivity, and hence a decreased atmospheric CO2 fixation by marine phytoplankton, would provide a positive feedback to the increasing temperatures. However, taking MIS 5.5 as an analogue scenario for the expected future climate change, the presented results indicate that a future decrease in marine primary productivity, due to an expansion of the oligotrophic gyre, could probably be counteracted by increased coccolithophore productivity." @default.
• W2534451812 created "2016-10-28" @default.
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• W2534451812 date "2013-05-27" @default.
• W2534451812 modified "2023-09-23" @default.
• W2534451812 title "Late Quaternary changes in paleoproductivity and hydrographyin the Azores region deduced from coccolithophore assemblages" @default.
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