Multiproxy fossil comparison reveals contrasting surface ocean conditions in the western Iceland Sea for the last two interglacials

Abstract

Dinoflagellate cyst (dinocyst), coccolith and planktic foraminiferal assemblages from a core in the western Iceland Sea were used to reconstruct and compare the surface ocean developments of the Holocene and the Last Interglacial (Marine Isotopic Stage or MIS 5e). While increasing subpolar planktic foraminifers from ~10ka indicate subsurface warming peaking around 7.5ka, the dinocyst data suggest that the uppermost ocean remained dominated by cold waters until ~6.5ka. A reduced advection of cold polar waters through the East Greenland/East Icelandic Current thereafter entailed warmest and most saline Holocene conditions between 6.5 and 5ka, in turn followed by a general cooling trend. By contrast, both planktic foraminifers and dinocysts show an increased presence of Atlantic (-type) waters from ~122ka onward resulting in a MIS 5e thermal optimum around 120.5ka. Nonetheless, occasional freshwater input from melting drift ice created stratified but also seasonally variable conditions during this first half of MIS 5e. This stratification signature disappeared at ~120ka when a marked repositioning of the oceanographic fronts occurred. Slightly colder conditions are indicated by both phyto- and zooplankton from there on until the end of MIS 5e around ~117ka. A late MIS 5e cooling at the Iceland Plateau is opposite to a late MIS 5e optimum observed in the eastern Nordic seas. This regional difference is likely explained by various feedback mechanisms following the major reorganisation of the oceanic fronts at ~120ka. The Holocene and MIS 5e interglacial variability is not reflected in the (quasi-monospecific) coccolith assemblages and illustrates the low sensitivity of living coccolithophore communities to subtle temperature changes in the low-temperature regions of the Nordic seas. Overall, quite different surface water properties appear to have characterised both interglacial intervals, with a higher share of warm, Atlantic elements in the planktic communities during MIS 5e. This suggests a higher contribution of Atlantic waters in the southwestern Nordic seas probably due to a more northward expansion of the Irminger Current under weakened polar East Greenland/East Icelandic currents. Such a reduced influence of polar waters in the southwestern Nordic seas may thus explain other evidence for relatively warm conditions in MIS 5e all around southern Greenland.