Strong Atlantic overturning circulation under high-latitude freshwater input during marine isotope stage 11: Insight from modelling and nitrogen isotope

Abstract

The marine isotope stage 11 (MIS 11) is often used as a potential analogue for the Holocene because of its similarities regarding orbital and greenhouse gas forcing (e.g., Droxler et al., 2003). However, recent studies portrayed the surface subpolar North Atlantic to be fresher by about 6 salinity units compared to today (Kandiano et al., 2017). This added to the growing body of evidence that the subpolar North Atlantic was characterized by much fresher and cooler surface waters during MIS 11 peak interglacial conditions (MIS 11 sensu stricto(ss)) compared to the Holocene (Kandiano et al., 2016; Thibodeau et al., 2017). Interestingly, MIS 11ss is often characterized by a strong AMOC (e.g., Vázquez Riveiros et al., 2013; Dickson et al., 2009), which seems at odds with the notion of major fresh and cold water input at high latitudes (e.g., Rahmstorf et al., 2005). Here, I show that the fresh and cold surface layer in the Nordic Seas created a strong salinity gradient in the polar North Atlantic at the beginning of the interglacial concurrent with strong variation in the surface nutrient utilization. I further demonstrate, with the help of a simple box model, that the weakening of this gradient in the peak interglacial allowed for the onset of deep water formation in the Nordic Seas, which contributed to a global intensification of the AMOC. Finally, I highlight that this thermohaline-driven sharp intensification of the AMOC is concomitant with the observed general enrichment in North Atlantic benthic δ13C (Lisiecki, 2010)and could help explain the abnormally long duration of MIS 11 (Dickson et al., 2009), underlining the importance of the North Atlantic freshwater budget in regulating AMOC.