A Sluggish Ocean Conveyer in the Norwegian-Greenland Sea During the Stage 12 Deglaciation: Geomorphological Evidence from Buried Iceberg Scours on the mid-Norwegian Shelf

Abstract:

To confidently project future environmental changes as the climate warms it is imperative that we are able to quantify and explain changes in the geological past. Such analyses are important for reducing the uncertainty and range of future projections of climate change. The ocean system is a key component of all numerical models because of the role that currents have in distributing heat across the globe. Grain-size measurements, provenance studies and palaeocurrent directions from iceberg scours are among some of the traditional proxies used for investigating the strengths and directions of ancient currents. Many authors have shown that deep-water production and northward transport of heat in the North Atlantic was reduced during the Last Glacial Maximum, but our knowledge of past climatic cycles is comparatively limited.

In this work we show new evidence for a similar reduction during the third last glacial, for which only very limited palaeocurrent information was previously available. This is based on the discovery of an exquisitely well-preserved set of buried iceberg scours seen in 3D seismic reflection data on the mid-Norwegian shelf. These scours are dated to ~429 ka based on a spike of ice-rafted detritus on the Voring Plateau. This marks the culmination of the stage 12 glaciation when the Northern Hemisphere ice cover was rapidly melting before the onset of the stage 11 interglacial. This time period has long been seen as a crucial analogue for understanding the Holocene interglacial and contemporary climate change. The geometry of the scours shows that the icebergs were influenced by tidal and translatory ocean currents. The ratio of these current velocities compared to present shows that the translatory current was 50% slower than today. This suggests that northward movement of heat was reduced due to a more sluggish meridional overturning circulation during the deglacial. This was likely in response to large volumes of deglacial freshwater and icebergs, as is also indicated by the abundance of other deep-draft iceberg scours on the same stratigraphic level. Further work, particularly numerical modelling, is required to fully elucidate the oceanographic implications of these results.