RAPID CLIMATIC CHANGES DOCUMENTED FROM THE NORDIC SEAS DURING THE LAST GLACIAL CYCLE: TIMING AND POSSIBLE MECHANISMS INVOLVED.
Dokken, Trondpresent adress: Massachusetts Institute of technology, Dept. of Earth, Atm. and Planetary Science, Cambridge, MA 02139, USA. Jansen, Eystein University of Bergen, Dept. of Geology, 5007 Bergen, Norway. Adkins, Jess Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964, USA. Duplessy, Jean C. Centre des Faibles Radioactivites mixte CNRS-CEA, Gif sur Yvette, France.
Early results from both the GISP and GRIP ice core projects show pronounced millennial climate oscillations (D-O events) during the last glacial period. Recently, the marine record has been proven to show an exactly similar oscillations all over the North Atlantic, most pronounced in the sedimentary record reflected in changes in lithic grain/g, in spectrophotometric reflectance, and in the magnetic susceptibility. The synchronous climatic shifts indicated by the MS- and sedimentary record in the Nordic Seas and D-O events enable a close comparison to ice-core age determinations and conventional dating techniques (AMS 14C-ages converted to calendar ages and SPECMAP isotope ages) in the marine record throughout the last glacial cycle. Refinement of the age model will further be discussed in relation to ongoing U-series measurements
Changes in the strength of, or cessation of the thermohaline overturning have been implicated as a main driver for these rapid changes observed in the North Atlantic region, at least during glacial forcing conditions. Although many model studies have documented the possibility of the thermohaline circulation to switch between on and off modes, or change the strength and location of the overturning cell, or even the depth of overturning, observations in the form of proxy records of water mass distribution are ambiguous. Here we discuss a possible mechanism that support a constant export of deep/intermediate water generated in the Nordic Seas, despite periods within the last glacial period and especially within the Heinrich layers, clearly indicate a break down or weakening of the thermohaline circulation. We propose a scenario where the ocean circulation operated in two different modes depending on the present state of climate. During stadial periods and extreme cold periods the ocean operated within a mode associated to present day processes surrounding Antarctica, and especially in the Weddell Sea. Here intensive brine formation take place to form Antarctic Bottom Water (AABW) within coastal polynyas created by strong winds blowing off the Antarctic continent to keep newly formed sea ice away from the shoreline. Similar conditions may have been characteristic for the high latitude northern areas during cold periods. Here, huge shelf areas, especially in the Barents Sea and the shelf surrounding the Arctic Ocean, and as well the shelf along the coast of Greenland, Iceland and Norway, may have been source areas for extensive brine formation which strongly affected the deep- and intermediate water within the Nordic Seas and the North Atlantic. Our evidence for identifying brine generated deep- and intermediate water, is found in the benthic d18O values which become lighter during cold periods, which can be explained by isotopic fractionation processes which deviate from normal 18O-Salinity relationship within areas whit extensive sea ice formation, similar to processes active in the Weddell Sea today. The situation for the interstadial (warm) periods looks more like present day circulation dominated by open water convection (thermohaline circulation), however, not necessary in the same strength as today.