Long-term variations in Iceland–Scotland overflow strength during the Holocene

Abstract

The overflow of deep water from the Nordic seas into the North Atlantic plays a critical role in global ocean circulation and climate. Approximately half of this overflow occurs via the Iceland–Scotland (I–S) overflow, yet the history of its strength throughout the Holocene (~ 0–11 700 yr ago, ka) is poorly constrained, with previous studies presenting apparently contradictory evidence regarding its long-term variability. Here, we provide a comprehensive reconstruction of I–S overflow strength throughout the Holocene using sediment grain size data from a depth transect of 13 cores from the Iceland Basin. Our data are consistent with the hypothesis that the main axis of the I–S overflow on the Iceland slope was shallower during the early Holocene, deepening to its present depth by ~ 7 ka. Our results also reveal weaker I–S overflow during the early and late Holocene, with maximum overflow strength occurring at ~ 7 ka, the time of a regional climate thermal maximum. Climate model simulations suggest a shoaling of deep convection in the Nordic seas during the early and late Holocene, consistent with our evidence for weaker I–S overflow during these intervals. Whereas the reduction in I–S overflow strength during the early Holocene likely resulted from melting remnant glacial ice sheets, the decline throughout the last 7000 yr was caused by an orbitally induced increase in the amount of Arctic sea ice entering the Nordic seas. Although the flux of Arctic sea ice to the Nordic seas is expected to decrease throughout the next century, model simulations predict that under high emissions scenarios, competing effects, such as warmer sea surface temperatures in the Nordic seas, will result in reduced deep convection, likely driving a weaker I–S overflow

Evolution of the central Nordic Seas over the last 20 thousand years

“…The following interval (7e6 until 3 ka) was characterized by high convection rates as shown by high carbon isotope ratios, both planktic and benthic, in most areas of the Nordic Seas (e.g., Bauch et al, 2001a;Sarnthein et al, 2003). Sediment grain size data and modeling results also seem to support this interpretation (Thornalley et al, 2013). The fact that the convected water influenced the bottom environment not only of the Greenland Basin (Telesi nski et al, 2014a) but also of the Lofoten Basin indicates a broad spreading of the well ventilated deepwater on both sides of the oceanic ridge separating the Greenland and Lofoten basins before flowing southward across the GreenlandeScotland Ridge and into the North Atlantic (cf.…”

“…Renssen et al, 2006), also coincided with an increase in sea-ice occurrence in the northern Nordic Seas (Müller et al, 2012) and a weakening of deep convection in the Greenland Sea (Telesi nski et al, 2014a). Sediment grain size records and modeling results (Thornalley et al, 2013) suggest a rather gradual shoaling of deep convection in the Nordic Seas through the middleelate Holocene. Our results, however, show that there was a stepwise decrease in the convection strength around 3 ka (Telesi nski et al, 2014a), perhaps superimposed on a longer-term trend starting in the middle Holocene (Fig.…”

Holocene evolution of the North Atlantic subsurface transport

“…Reconstructions of MOW strength based on contourite grain size data from the Gulf of Cádiz indicate that MOW was sluggish during the early Holocene and strengthened after 8 ka BP. During the last 2 millennia, MOW strength decreased again (Rogerson et al, 2012;Toucanne et al, 2007). Thus, the timing of the early Holocene MOW strengthening roughly coincides with a northward movement of the AF-AC and a strengthening of the subsurface warm water transport.…”

Reconstructing extreme AMOC events through nudging of the ocean surface: a perfect model approach

“…First, the use of proxy records from sedimentary sources, usually sensitive to the deep ocean currents (e.g. McManus et al 2004;Kissel et al 2013;Thornalley et al 2013). These estimates are, however, localised and challenging to interpret.…”