Elke Vogelsang

Glacial North Atlantic: Sea‐surface conditions reconstructed by GLAMAP 2000

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX-28) modern analog technique with 947 modern analog samples and 119 well-dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice-free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45degrees and 60degreesN, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12degreesC. In the subtropical ocean gyres the new reconstruction supports the glacial-to-interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2degrees and 6degreesN suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6degrees-8degreesC during Northern Hemisphere summer. Differences between these SIMMAX-based and published U37(k)- and Mg/Ca-based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.

Fundamental Modes and Abrupt Changes in North Atlantic Circulation and Climate over the last 60 ky — Concepts, Reconstruction and Numerical Modeling

Centennial- to millennial-scale changes in global climate over the last 60 ky were first documented in ice cores from Greenland, with ice sheets around the North Atlantic and its thermohaline circulation (THC) as prime candidates for a potential trigger mechanism. To reach a new quality in understanding the origin and causal links behind these changes, two strategies were intimately tied together in this synthesis, high-resolution 3-D ocean modeling and paleoceanographic reconstructions. Here, five time series with a time resolution of several decades and various time slices of surface and deep-water paleoceanography were established from hundreds of deep-sea cores for the purpose of monitoring rapid changes across the North Atlantic and testing or initiating model results. Three fundamental modes were found to operate Atlantic THC. Today, mode I shows intensive formation of North Atlantic Deep Water (NADW) and strong heat and moisture fluxes to the continents adjacent to the North Atlantic. Peak glacial mode II leads to a reduction in NADW formation by 30-50%, in line with a clear drop in heat flux to Europe. The glacial Nordic Seas, however, remain ice-free during summer and little influenced by meltwater, in contrast to the sea west ofIreland, where iceberg meltwater blocks an eastbound flow into the Norwegian Sea and induces a cold longshore current from Faeroe to the Pyrenees. The subsequent Heinrich 1 (HI) meltwater mode III leads to an entire stop in NADW and intermediate-water production as well as a reversed pattern of THC, stopping any heat advection from the central and South Atlantic to the north. In contrast to earlier views, the Younger Dryas, possibly induced by Siberian meltwater, began with mode I and ended with mode III, continuing into the Preboreal. Modeling the impact of modes I to III on the global carbon budget, we find that the atmosphere has lost 34-54 ppmv CO2 from interglacial to glacial times, but has gained 23-62 ppmv CO2 at the end of HI within a few decades, equivalent to 33-90% of modem, man-made CO2 release. The robust 1500-y Dansgaard- Oeschger (D-O) cycles and their multiples of as much as 7200 years, the Heinrich event cycles, are tied to periodical changes between THC modes I/II and II/III. In the Irminger Sea rapid D-O coolings are in phase with initial meltwater injections from glaciers on East Greenland, here suggesting an internal trigger process in accordance with binge-purge models. Ice rafting from East Greenland and Iceland occurs only 240-280 y later, probably inducing a slight sea-level rise and, in tum, Heinrich ice rafting from the Laurentian ice sheet during H1, H2, H4, H5. At H1 a major surge from the Barents shelf has lagged initial cooling by 1500 y and entails the most prominent and extended reversal in Atlantic THC over the last 60 ky (probably also at the end of glacial stage 4, at H6). Meltwater stratification in the Inninger Sea reaches its maximum only 640 y after initial meltwater injection and induces, via seasonal sea-ice formation, brine-water injections down to 4 km water depth, signals leading the classic D-O jump to maximum warmth by only 125 y. It may be inferred from this short-phase lag that brine water-controlled deep-water formation probably entrains warm water from further south, thereby forming the key trigger mechanism for the final tum-on of the Atlantic THC mode II roughly within a decade (or mode I, in case of favorable Milankovitch forcing).

Paleoceanographic Proxies in the Northern North Atlantic

The advantages and limitations of various proxy indicators of physical boundary conditions, such as sea-surface temperature (SST) and salinity (SSS), circulation patterns, deep-water ventilation, nutrient cycles and the response of the biota (productivity and carbon-flux), have been investigated in the Greenland-Iceland-Norwegian (GIN) Seas. Low sea-surface temperatures, the presence of sea ice, strong seasonality and vigorous thermohaline circulation in this area cause severe difficulties for the application of several classical temperature, salinity, nutrient and flux proxies. Quantitative estimates are provided for temperature, salinity and the seasonal duration of seaice cover, although margins of error are still larger than in the North Atlantic south of Iceland. However, proxies based on benthic foraminifera, reflecting biotic processes at the benthic boundary layer (i. e. assemblage composition and isotopic signals of benthic foraminifera), exhibit potential as indicators of seasonal carbon flux and deep-water ventilation in the Greenland-Iceland-Norwegian Seas.