Climate Variability During a Period With Different Orbital Forcings Than the Last Climatic Cycle: Evidence From MIS 10-11

Koc N and Jansen E (Department of Geology, University of Bergen, Bergen, Norway, N-5007)

It is still a challenge to understand the mechanisms behind climatic variability occurring on millennial time scales. The variability observed during the last glacial period has been attributed both to mechanisms internal to the climate system, as well as to mechanisms external to the climate system. The orbital geometry of the last glacial/ interglacial cycle was characterized by high amplitude variations in the precessional parameter. We present evidence from cores DSDP 609 and ODP 643, bearing on the stability of the Laurentide and Scandinavian ice sheets during a period with low amplitude precessional variations, MIS 10 and 11, which may indicate a role of orbital forcing in millennium scale climate variability.

Unlike the last glacial period, where the records of IRD-input both from the Norwegian Sea and the mid-latitude North Atlantic gave evidence for coherent and possibly synchronous high frequency fluctuations of the Scandinavian and the Laurentide ice sheets, MIS 10 IRD records of ODP 643 and DSDP 609 show that these two ice sheets reacted differently under the orbital forcings of MIS 10-11. At ODP Site 643 the IRD record is dominated by variability on sub-Milankovitch time scales with a spacing of about 5 kyr during MIS 10, indicating that the Scandinavian ice sheet, like during the last glacial period, was also unstable during MIS 10. In contrast, the IRD record from DSDP Site 609 shows a general increase in IRD/g through MIS 10 culminating in a maximum during the glacial maximum at the end of MIS 10. This is a totally different pattern than observed for the last glacial period, indicating a more or less stable growth of the Laurentide ice sheet through MIS 10 to a size which provided opportunities for major IRD events only towards the last part of the interval. We suggest that the lack of Heinrich event type of variability in mid North Atlantic during most parts of MIS 10 is connected to the low amplitude insolation variations during MIS 11. We further suggest that a certain/critical size is necessary before an ice sheet is sensitive to forcings that result in millennial scale variability. Ice volume during MIS 10 was much less than during MIS 2, which also indicates a smaller Laurentide ice sheet during MIS 10, and the Laurentide ice sheet did not grow to a significant size early enough within the glacial period to exhibit

unstability during MIS 10. Hence, there appears to be some importance

of external forcings in setting the stage for the size of the ice sheets, and consequently the ability for millennial scale variability of the ice sheets to develop.