Evolution of subpolar North Atlantic surface circulation since the early Holocene inferred from planktic foraminifera faunal and stable isotope records

Abstract

Past changes in the surface flow regime of two main eastern North Atlantic warm water pathways toward the Nordic seas were reconstructed based on faunal analyses in combination with carbon and oxygen stable isotope measurements in planktic foraminifera. The investigated sites, in the surroundings of the Faroe Islands, are located in the transitional area where surface waters of subpolar and subtropical origin mix before entering the Arctic Mediterranean. In these areas, large-amplitude millennial variability in the characteristics of the upper-water column appears modulated by changes in the intensity of the Subpolar Gyre circulation. From 7.8 to 6 ka BP, faunal records indicate a deep mixed-layer which, in conjunction with lighter δ18O values, suggest that the inflowing Atlantic waters were dominated by a relatively cooler and fresher water mass, reflecting a strengthening of the Subpolar Gyre under conditions of enhanced positive NAO-like forcing and reduced meltwater input. A shift in the hydrographic conditions occurred during the Mid-Holocene (centered at 5 ka BP). At this time, increasing upper water column stratification and the incipient differentiation of the stable isotopic signal of the Iceland–Faroe and Faroe–Shetland surface water masses, suggest increasing influx of warmer, more saline surface waters from the Subtropical Gyre, as Subpolar Gyre circulation weakened. The mid-Holocene decline in Subpolar Gyre strength is presumably related to a shift toward a low state of the NAO-like forcing associated with decreased solar irradiance. Later in the Holocene, from 4 ka BP to present, the increased frequency and reduced amplitude of the surface hydrographic changes reflect corresponding fluctuations in Subpolar Gyre circulation. These high frequency oscillations in Subpolar Gyre strength suggest increased surface circulation sensitivity to moderate freshwater fluxes to the Labrador–Irminger Sea basin, highlighting the importance of the salinity balance in modulating Subpolar Gyre dynamics, particularly under conditions of low NAO atmospheric forcing.