Subarctic Front migration at the Reykjanes Ridge during the mid‐ to late Holocene: evidence from planktic foraminifera

Abstract

Prins, M. A. 2018 (January): Subarctic Front migration at the Reykjanes Ridge during the mid-to late Holocene: evidence from planktic foraminifera. Boreas, Vol. 47, pp. 175-188. https://doi.org/10.1111/bor.12263. ISSN 0300-9483.Expansion of fresh and sea-ice loaded surface waters from the Arctic Ocean into the sub-polar North Atlantic is suggested to modulate the northward heat transport within the North Atlantic Current (NAC). The Reykjanes Ridge south of Iceland is a suitable area to reconstruct changes in the mid-to late Holocene fresh and sea-ice loaded surface water expansion, which is marked by the Subarctic Front (SAF). Here, shifts in the location of the SAF result from the interaction of freshwater expansion and inflow of warmer and saline (NAC) waters to the Ridge. Using planktic foraminiferal assemblage and concentration data from a marine sediment core on the eastern Reykjanes Ridge elucidates SAF location changes and thus, changes in the water-mass composition (upper~200 m) during the last c. 5.8 ka BP. Our foraminifer data highlight a late Holocene shift (at c. 3.0 ka BP) in water-mass composition at the Reykjanes Ridge, which reflects the occurrence of cooler and fresher surface waters when compared to the midHolocene. We document two phases of SAF presence at the study site: from (i) c. 5.5 to 5.0 ka BP and (ii) c. 2.7 to 1.5 ka BP. Both phases are characterized by marked increases in the planktic foraminiferal concentration, which coincides with freshwater expansions and warm subsurface water conditions within the sub-polar North Atlantic. We link the SAF changes, from c. 2.7 to 1.5 ka BP, to a strengthening of the East Greenland Current and awarming in the NAC, as identified by various studies underlying these two currents. From c. 1.5 ka BP onwards, we record a prominent subsurface cooling and continued occurrence of fresh and sea-ice loaded surface waters at the study site. This implies that the SAF migrated to the southeast of our core site during the last millennium.

Exceptional 20th Century Ocean Circulation in the Northeast Atlantic

“…The dramatic 20th century planktic foraminiferal faunal changes seen in our records are not found in other sites located either in the SPG interior (RAPID‐21‐3K and Perner et al, 2017, Figure S6) nor within the main body of the warm Atlantic water flowing into the Nordic Seas (Figure 3; Andersson et al, 2010; Mary et al, 2015; Staines‐Urías et al, 2013). While these records tend to be of lower resolution than those presented here, the absence of similar trends in these regions highlights that the 20th century trends we observe were not predominantly caused by the mean effects of global warming, but instead reflect a northwestward expansion of the warm conditions in the Iceland Basin due to a change in ocean circulation.…”

“…Age models for each core are based on a combination of 210 Pb and 14 C ( Figure S7, Text S1). Sediment ages falling within the mid-20th century were verified by the presence of radiogenic 137 Cs (e.g., Perner et al, 2017) and spheroidal carbonaceous particles (SCPs; Rose, 2008Rose, , 2015.…”

Understanding Arctic Ocean Circulation: A Review of Ocean Dynamics in a Changing Climate

“…As mentioned in section , the α ‐ β stratification boundary is of importance to climate in that it establishes the southern extent of winter sea‐ice cover. Sediment core proxy data suggest significant changes in the position of the subarctic front over the Holocene (Moros et al, ; Perner et al, ), and much further back in the climate record, where the shifting influence of Atlantic and Polar Water types is related to changes in sea‐ice extent (e.g., Stein et al, ). During the last major interglacial period (∼130,000 and 80,000 years ago, characterized by conditions warmer than today), Arctic sea ice biomarker proxy records and modeling suggest that the Barents Sea was ice free for much of the year under the strong influence of inflowing Atlantic Water (Stein et al, ).…”

Variability in the Northern North Atlantic and Arctic Oceans Across the Last Two Millennia: A Review

“…Open ocean records along the NAC pathways are entirely based on marine sediment cores found in high‐deposition sediment environments, which allow for high temporal (subdecadal to decadal) resolution proxy reconstructions (Figure ). On the Reykjanes Ridge, foraminiferal assemblage reconstructions record gradual cooling over the Common Era potentially involving increased freshwater and decreased subpolar frontal influence at the site (Perner et al, ). This is in line with previous low‐resolution reconstructions farther north (Thornalley et al, ).…”