Instability in upper-ocean structure and its implications for Deep Water formation during marine isotope stage 11

The marine isotope stage (MIS) 11 interglacial, which occurred approximately 424 to 374 thousand years ago, is a period of significant climatological interest due to its unusual duration and intensity of warm conditions under relatively subdued orbital forcing, a phenomenon often referred to as the “MIS 11 paradox”. This study focuses on understanding the factors behind this paradox and its implications for the formation of Deep Water in the North Atlantic.

We examined the upper-ocean structure in the Iceland Basin during MIS 11, a key region for modern deep-water formation. By analyzing geochemical measurements, including stable nitrogen isotopic ratios and carbon and oxygen isotopic ratios of planktic foraminifera, we reconstructed the upper-ocean structure and its potential role in driving the Atlantic Meridional Overturning Circulation (AMOC) during MIS 11.

The findings reveal that MIS 11 experienced an initial AMOC intensification, followed by a secondary strengthening prior to the onset of the climatic optimum. The secondary intensification was attributed to the gradual reduction of northern-hemisphere sea ice, allowing for a northward extension of surface-ocean currents. This resulted in the maintenance of an anomalously deep summer mixed layer in the polar Nordic Seas during MIS 11 compared to the Holocene. The deep-water formation in the Nordic Seas played a crucial role in extending the enhanced warming of the northern hemisphere and delaying the onset of the next glacial interval.

While the contemporary Atlantic Ocean primarily relies on deep-water formation in the eastern subpolar region, the study suggests that the relative importance of deep-water formation in polar regions may increase under extreme scenarios of anthropogenic warming. By studying MIS 11 as a potential analog for Earth's contemporary climate system, we provide valuable insights into the long-term fate of the AMOC and its implications for global climate.

This study also highlights the significance of understanding the convective behavior of the subpolar Atlantic for a comprehensive understanding of the AMOC during MIS 11. We present new geochemical measurements and reconstructions of upper-ocean structure in the Iceland Basin, shedding light on the potential link between summer mixed-layer depth and deep-water formation.