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

D J R Thornalley,J F Mcmanus,I R Hall,F J Davies,H Kleiven,D W Oppo,M Blaschek,S Praetorius,H Renssen,I N Mccave

doi:10.5194/cp-9-2073-2013

Abstract

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.

Highlights

Oceanic processes ocTcuhrreingCinrythoesNporhdiecrseeas play a critical role in regulating global climate
It is unlikely that the shallow24 curs in the shallow cores (< 1.3 km) during the earliest overflow was caused by a reduced density of entrained sub
This study has demonstrated that grain size analyses in an extensive compilation of cores, forming a 1.2–2.3 km depth transect, can be used to constrain Holocene changes in both the strength and depth of the I–S overflow

Summary

Introduction

Oceanic processes ocTcuhrreingCinrythoesNporhdiecrseeas play a critical role in regulating global climate. The cooling of relatively salty water in the Arctic and Nordic seas causes the formation of cold, dense, intermediate and deep waters, which overflow the Greenland–Scotland Ridge (∼ 400–800 m water depth) into the Atlantic Ocean as Denmark Strait Overflow Water (DSOW, ∼ 3 Sv (106 m3 s−1), Greenland–Iceland Ridge) and Iceland–Scotland Overflow Water (ISOW, ∼ 3 Sv, Iceland– Scotland Ridge) (Hansen and Osterhus, 2000) (Fig. 1). The driving force for the overflows is the vertically integrated pressure difference above the sill depth between the upstream (Nordic seas) and downstream (North Atlantic) sides (Hansen et al, 2001). This pressure gradient depends on numerous factors: the formation of cold, dense, intermediate. Thornalley et al.: Long-term variations in Iceland–Scotland overflow strength and deep waters north of the sill, the density of downstream waters at the sill, as well as the sea surface height difference (barotropic pressure gradient) across the sill, which is sensitive to wind-driven forcing (Hansen and Osterhus, 2000; Hansen et al, 2001; Biastoch et al, 2003; Olsen et al, 2008)

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Results

Conclusion