PICKS · Past Interglacial Climates at Key Sites: Sea Surface Temperature and Salinity Reconstructions from the Northern North Atlantic during MIS 5e and MIS 11

Past Interglacial Climates at Key Sites: Sea Surface Temperature and Salinity Reconstructions from the Northern North Atlantic during MIS 5e and MIS 11

1. FINAL SUMMARY REPORT Reconstructing temperatures and salinities is one of the crucial goals in paleoceanography, as it allows drawing conclusions about configuration and intensity of the ocean circulation which in turn is the main amplifier of climate changes. Knowledge of the boundary conditions of different climate states also gives a solid basis for a future climate prediction which is the main task of modern climate research. This project was aimed to reconstruct temperatures and salinities in the northern North Atlantic during two interglacial periods which were proposed by IPCC as potential future climate analogues, Marine Isotope Stages (MIS) 5e and 11, starting around 125 and 420 ka, respectively. Moreover, MIS 11 was recently indicated as a “super-interglacial”, characterized by long duration (around 18 ky), the small Greenland and Antarctica ice sheets, the comparatively high sea level, and exceptionally warm Arctic environments. The selected core locations were situated under the important elements of the North Atlantic Meridional Circulation (AMOC), the main engine of the global ocean circulation, on one hand, and an important contributor to the warm budget in Europe and the Arctic, on the other hand. One core location was situated under the North Atlantic Current (NAC) in the middle latitudes and two others were in the Nordic Seas, under the Norwegian Current and the northern edge of the Norwegian Sea gyre. These core positions allowed reconstructing dynamics of warm water input into the region. The main emphasis was placed on investigating the intensity and the stability of the warm water input into the North Atlantic and persistence of the post-glacial melt water remnants in the Nordic seas during interglacial times. Relatively new organic geochemical methods were selected for temperature and salinity reconstructions. The temperatures were reconstructed from isoprenoid glycerol dialkyl glycerol tetraether (GDGT) membrane lipids produced by Thaumarchaeota because their relative abundances, converted to TEX86 indices, strongly correlate with temperature. The salinities were reconstructed from hydrogen isotope compositions (δD) of alkenones, long-chain unsaturated ketones, produced by certain species of haptophyte algae. The numerous inorganic records established for each selected core previously provided an opportunity for a better interpretation of the obtained results. On the basis of the obtained results it was concluded that: 1. The elevated temperatures obtained in the middle latitudes of the North Atlantic for MIS 11 imply that this period was characterized by an enhanced warm water input into the region as compared to the Holocene. As a consequence, this might have resulted in an intensification of the Subpolar Gyre. 2. Transient cold events associated with fresh water input were revealed by both temperature and salinity records during period of full interglacial conditions of MIS 11. Our newly produced temperature reconstructions, in combination with other proxies, clearly showed that temperature changes occurred at various water depths It was assumed by us that melt water released from the continuously melting ice sheets caused these transient cold events. Stratigraphical positions of the events implied that their nature might be analogous to the 8.2 ka Holocene cold event. 3. Contrary to the middle latitudes, the Nordic Seas showed comparatively low temperatures across MIS 11 with a temperature optimum occurring during the late phase of the interglacial. This temperature optimum precisely matches with the highest salinity values. However, even then neither temperatures nor salinities reached the modern day values. 4. The temperature record of the Nordic seas obtained for MIS 5e is in accordance with temperature estimates derived from planktic foraminiferal abundances and provides further details for a or the climatic scenario of this period. Salinity reconstructions indicate a freshening of the Nordic seas towards the interglacial demise. The obtained results help to reconstruct details on interglacial dynamics of the selected periods in the northern North Atlantic. They clearly indicate that the stability of interglacial conditions can be interrupted by abrupt cold events when ice sheets (for instance, the Greenland ice sheet) are melting. The obtained results might be used in computer simulations for a future climate prediction. After the data have been published they will be available at PANGEA data library (www.pangaea.de) for future use in paleoclimatic studies and climate model experiments.

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