H. F. Kleiven

Rapid Reductions in North Atlantic Deep Water During the Peak of the Last Interglacial Period

Limited Stability Deep ocean circulation is thought to be stable during warm, interglacial periods. Galaasen et al. (p. 1129, published online 20 February) constructed a highly resolved record of North Atlantic Deep Water production during the last interglacial period, around 128,000 to 116,000 years ago. The findings reveal large, centennial-scale reductions—in contrast to the prevailing paradigm. These changes occurred in an ocean warmer than that of today, but in a temperature regime similar to that expected because of global warming, raising the possibility that future ocean circulation, regional climate, and CO2 sequestration pathways could be impacted. Deep ocean circulation was less stable during the last interglacial periods than previously supposed. Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal δ13C record, we show that the influence of North Atlantic Deep Water (NADW) was strong at the onset of the last interglacial period and was then interrupted by several prominent centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer and fresher than at present.

Rapid switches in subpolar North Atlantic hydrography and climate during the Last Interglacial (MIS 5e)

plateauing just below early MIS 5e values. A planktonic d 18 O minimum during the cooling event indicates that marked freshening of the surface waters accompanied the cooling. We suggest that switches in the subpolar gyre hydrography occurred during a warmer climate, involving regional changes in freshwater fluxes/balance and East Greenland Current influence in the study area. The nature of these hydrographic transitions suggests that they are most likely related to large-scale circulation dynamics, potentially amplified by GIS meltwater influences.

Coupled deep-water flow and climate variability in the middle Pleistocene North Atlantic

Abrupt changes in the strength of the Atlantic Meridional Overturning Circulation (comprising northward flow of warm water and a cold southward return flow) are implicated in abrupt climate changes in the late Pleistocene. A sensitive place to assess this circulation is in the cold return flow of Deep Western Boundary Currents. Here, in records of flow speed and isotopic composition of surface and bottom waters from a Deep Western Boundary Current location near the northern source of North Atlantic Deep Water, we show both orbital and millennial-scale coupling between deep ocean flow and climate in the middle Pleistocene (0.75–0.87 Ma), when the boundary conditions in terms of the mean state and amplitude of climate change were different from more recent periods. The coupling appears as a phased series of events initiated by reduced vertical density gradients and initial ventilation of deep waters. The occurrence of these events in interglacials during the middle Pleistocene suggests that the millennial-scale climate variability in the North Atlantic was more pronounced at that time than previously thought. This demonstrates that, given the right boundary conditions, rapid climate shifts can also occur during relatively warm climate conditions.

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

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).

Multidecadal changes in Iceland Scotland Overflow Water vigor over the last 600 years and its relationship to climate

Changes in the Atlantic Meridional Overturning Circulation (AMOC) have commonly been invoked to explain the low-frequency climate changes evident over millennial-multidecadal timescales during the Holocene period. While there is growing evidence that deep ocean circulation varied onmillennial timescales, little is known about ocean variability on shorter timescales. Here we use a marine sediment core (GS06-144-09MC-D) recovered from a high accumulation rate site on the Gardar Drift in the Iceland Basin (60°19′N, 23°58′W, 2081m) to reconstruct decadal-centennial variability in the vigor of Iceland-Scotland OverflowWater (ISOW) with the paleocurrent proxy “sortable silt”mean grain size SS . Our SS record reveals that changes in ISOW vigor have occurred on multidecadal-centennial timescales over the past ~600 years; similar timescales as documented in Atlantic Multidecadal Variability observations and reconstructions. Our findings support a link between changes in basin-wide climate and deep ocean circulation.