Holocene multidecadal‐ to millennial‐scale variations in Iceland‐Scotland overflow and their relationship to climate

Abstract

The Nordic Seas overflows are an important part of the Atlantic thermohaline circulation. While there is growing evidence that the overflow of dense water changed on orbital time scales during the Holocene, less is known about the variability on shorter time scales beyond the instrumental record. Here we reconstruct the relative changes in flow strength of Iceland-Scotland Overflow Water (ISOW), the eastern branch of the overflows, on multidecadal-millennial time scales. The reconstruction is based on mean sortable silt (SS) from a sediment core on the Gardar Drift (60°19′N, 23°58′W, 2081 m). Our SS record reveals that the main variance in ISOW vigor occurred on millennial time scales (1-2 kyr) with particularly prominent fluctuations after 8 kyr. Superimposed on the millennial variability, there were multidecadal-centennial flow speed fluctuations during the early Holocene (10-9 kyr) and one prominent minimum at 0.9 kyr. We find a broad agreement between reconstructed ISOW and regional North Atlantic climate, where a strong (weak) ISOW is generally associated with warm (cold) climate. We further identify the possible contribution of anomalous heat and freshwater forcing, respectively, related to reconstructed overflow variability. We infer that ocean poleward heat transport can explain the relationship between regional climate and ISOW during the middle to late Holocene, whereas freshwater input provides a possible explanation for the reduced overflow during early Holocene (8-10 kyr).

Constraining the Variability of the Atlantic Meridional Overturning Circulation During the Holocene

“…Using the prominent AMOC perturbations characterizing the Younger Dryas (YD) and Heinrich Stadial 1 (HS1) as test cases, our model faithfully reproduces the amplitude of both (Andersen et al, 2004). (b) Sortable silt records of ISOW strength from (right axis) Core GS06-144 08GC (Mjell et al, 2015) and an (left axis) Iceland slope stack (Thornalley et al, 2013). (c) Relative abundance of the polar foraminifer N. pachyderma from the Core MD99-2251 (Ellison et al, 2006).…”

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

“…Several bottom flow speed reconstructions (Table ) exist across the pathway of the deep overflow waters that cross the Iceland‐Scotland Ridge. The subdecadal sortable silt records closer to the sill (2,300–2,080‐m depth) reveal broad similarities on multicentennial scales with an increase in flow speed from 0 years CE until 600–800 CE followed by a gradual decline of ISOW flow vigor toward the modern (Mjell et al, ; Moffa‐Sanchez et al, ; Mjell et al, ; Figures a and b). Farther downstream, and at similar depths, the SSrecords also coincide with the slow ISOW between 0 and 600 CE but the rest of the records diverge from the upstream records by showing a steady increase toward the present with several step changes including an increase in the flow at ~1400 CE (Moffa‐Sánchez & Hall, ; Figure c).…”

“…Bathymetric base map is made using Ocean Data View (ODV; (Schlitzer, ). The graph is ordered from the ISOW from north to south including (a) RAPiD‐17‐5P, which is south of Iceland and close to the Greenland‐Scotland Ridge (Moffa‐Sanchez et al, ; note that the SS from the DSOW, also shown in (e) has been plotted in blue in reverse to show the antiphasing); (b) GS06‐144‐09MC (dark red; (Mjell et al, ) and GS06‐144‐08GC (red; Mjell et al, ; note the discrepancy when splicing); (c) Southern Gardar drift SS from RAPiD‐21‐3K (Moffa‐Sánchez & Hall, ) and magnetic susceptibility from MD99‐2251 (Kissel et al, ); (d) magnetic susceptibility from the deep Charles Gibbs Fracture Zone (CGFZ) CH77‐02 and DD08‐3182C1 (Kissel et al, ); (e) SS record from the from RAPiD‐35‐COM in the Eirik Drift off the southern tip of Greenland in the path of the DSOW (Moffa‐Sanchez et al, ); and (e) sortable silt record from KNR‐178‐48JPC/56JPC in the pathway of lower LSW/upper DWBC (uDWBC; Thornalley et al, ). The record in grey denotes three‐point smoothed benthic foraminiferal Mg/Ca temperatures at 1,845 m on the Scotian Shelf interpreted as deep LSW (Marchitto & de Menocal, ; note that the data from the top 23.5 cm of this record were plotted on the updated age model for KNR‐158‐10MC from Thornalley et al, ).…”

Climate variability in the subarctic area for the last 2 millennia

“…However, the intense volcanic activity of the 19th century (1809, 1815 and around 1840; Sigl et al, 2015) may also explain the apparent early warming trend, suggesting that it may have been recovery from an exceptionally cool phase. On the scale of the Holocene, internal fluctuations occurring on a millennial scale have been identified in the subarctic North Atlantic area and were tentatively related to ocean dynamics (Debret et al, 2007;Mjell et al, 2015). Therefore, to better understand the cooling trend of the last 2 millennia in a larger temporal context, taking into account the role of oceanic variability in the long-term temperature variations, a longer time series encompassing the entire Holocene would be useful.…”