Persistent ISOW formation during MIS 11

The deep water formation in the Labrador and Nordic Seas is crucial for the global thermohaline circulation nowadays and it remains debated whether changing boundary conditions in terms of global warming may influence the deep convecting activity. Deep convection leads to the formation of Iceland Scotland Overflow Water (ISOW), which is an essential part of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). However, surface conditions in the Nordic Seas were unlikely always favorable for the formation of deep water in the past.

During Marine Isotope Stage (MIS) 11, a strong and active AMOC [e.g. 1] was reconstructed, which also contributed to the mass loss of the Greenland Ice Sheet [2]. However, cold and fresh surface conditions prevailed in the central Nordic Seas [3], which have been ascribed to freshwater input from the higher latitudes [4]. Thus, the question arises, whether and where deep water formation took place in the Nordic Seas.

Here, we reconstruct authigenic neodymium isotopes extracted from deep sea sediment from the Gardar Drift from 470 to 374 ka. IODP Site U1304 is located directly in the modern flow path of ISOW and should therefore sensitively track changes of this water mass in the past. Today, it is characterized by a strongly radiogenic neodymium isotopic composition, which markedly differs from other North Atlantic water masses.

Starting right at the onset and for the full length of the interglacial MIS 11c, a radiogenic Nd isotopic composition is switched on and prevailed indicating the presence of ISOW at the core site. More unradiogenic conditions indicate the return to glacial like conditions during a short event in MIS 11b. However, during MIS 11a the radiogenic values point again to a persistent presence of ISOW.

Thus, although the boundary conditions in terms of freshwater fluxes and sea level were significantly differing in the central Nordic Seas, the deep water formation presumably happened in the southern part of the Nordic Seas. This led to the active formation of ISOW, which in turn helped drive the active and strong AMOC during MIS 11.

 

[1] Dickson et al. (2009), Nat. Geosci. 2: 428-433.

[2] Rachmayani et al. (2017), Paleoceanography 32: 1089-1101.

[3] Kandiano et al. (2016), GRL 43: 10929-10937.

[4] Doherty and Thibodeau (2018), Front. Mar. Sci. 5: 251.