HOLOCENE CENTENNIAL TO MILLENNIAL-SCALE CLIMATIC VARIABILITY: EVIDENCE FROM HIGH-RESOLUTION MAGNETIC ANALYSES OF THE LAST 10 CAL KYR OFF NORTH ICELAND (CORE MD99-2275)

Abstract

High-resolution magnetic analysis has been performed on the top ~25 m of core MD99-2275 (66°33.06′N, 17°41.59′W; 440 m water depth) from the mid-outer shelf off North Iceland. This is a key boundary region for climate changes related to atmospheric and oceanographic variations influenced by the moving limit between the cold and warm water masses at the Polar Front and the low depression track with its associated westerlies situated across Iceland. An age model based on tephrochronology was previously reported for this core and has allowed precise timing of significant variations in the magnetic mineral concentration and grain size over the last 10 cal kyr. The 'Holocene climatic optimum' identified in North Iceland between ~10 and 6 cal kyr BP is characterized by minor variations in the magnetic record, while there is a clear increase in oceanographic instability from ~6 cal kyr BP to the present. At large scale, a decreasing trend in the magnetic mineral content toward the present is presumably associated with a change in the circulation pattern and coincides in time with decreasing North Atlantic Deep Water formation. Short-term intervals of decreased magnetic mineral content are also identified at 5.2-4.9, 3.77-3.41, 3.01-2.7 and 0.93-0.62 cal kyr BP, the youngest one (AD 1020 to 1330) corresponding in time to the Medieval Warm Period. These events are interpreted to reflect periods of increased activity of the relatively warm, high salinity surface Irminger Current related to stronger input of North Atlantic waters into the Nordic Seas. In addition, spectral analysis of selected magnetic parameters demonstrates centennial-scale periodicities of ~715, 240, 170 and 100 yr, which are more clearly expressed in the last 6 cal kyr BP than in the previous period. These periodicities might be attributed to intervals of persistence in the North Atlantic Oscillation (NAO) mode at centennial timescales. This record, therefore, indicates that the Holocene climate was more unstable than is often assumed, and it demonstrates the importance of high-resolution climatic studies for this recent time period to improve climate model predictions. © 2005 Elsevier B.V. All rights reserved.