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Abrupt climate events are generally attributed as a characteristic of glacial (intermediate-to-large cryosphere) climate states. While a large cryosphere may be a necessary boundary condition for millennial-scale events to persist, it remains unclear whether high-magnitude climate variability is purely a glacial phenomenon requiring cryosphere-driven feedbacks. High-resolution climate records are used to portray North Atlantic climate's progression through low-ice, interglacial boundary conditions of Marine Isotope Stage (MIS) 11c into the glacial inception. We show that this period is marked by rapid shifts in both deep overflow and surface climate. The reorganization between polar and Atlantic waters at subpolar latitudes appears to accompany changes in the flow of deep water emanating from the Nordic Seas, regardless of magnitude or boundary conditions. Further, during both glacial and interglacial boundary conditions, we find that a reduction in deep water precedes surface hydrographic change. The existence of surface and deep ocean events during an interglacial, with similar magnitudes, abruptness, and surface-deep phasing as their glacial counterparts, alters our concept of “warm” climate stability and the requisite cryospheric thresholds and feedbacks for it.

There is emergent evidence that abrupt shifts of the Atlantic Meridional Overturning Circulation (AMOC) have occurred during interglacial periods, with recent observations and model simulations showing that we may have over-estimated its stability during warm climates. In this study, we present a multi-proxy reconstruction of deep-water characteristics from the Rockall Trough in the Eastern North Atlantic to assess the variability of Nordic seas and Labrador Sea deep-water formation during past interglacial periods MIS 1, 5, 11, and 19. To test the warm climate stability hypothesis and to constrain the variability of deep-water formation for past warm climates, we performed geochemical analysis on planktic (Nd isotopes) and benthic foraminifera (δ18O and δ13C) along with sedimentological analysis. This approach allows us to reconstruct paleocurrent flow strength, as well as the origin and contribution of different water masses to one of the deep-water components of the AMOC in the Rockall Trough. We found that deep-water properties varied considerably during each of our chosen periods. For example during the Holocene εNd variability is smaller (1.8 per mil) when compared to variability during MIS 19 (3.3 per mil), an interglacial that experienced very similar orbital boundary conditions. Our results confirm that deep-water variability in the eastern North Atlantic basin was more variable in previous interglacial periods when compared to our current Holocene and provide new insight into the relative contribution of Nordic Seas Deep Water and Labrador Sea Water in the Rockall trough.

Whether or not Arctic regions remain(ed) a carbon sink or source to the atmosphere during rapidly warming climates (in the past) is a fundamental question with regards to future global warming and ocean acidification. The boron isotopic composition of planktonic foraminiferal shell calcite (δ11BCc) can potentially provide valuable information of past seawater pH if information on a second carbonate system parameter, temperature, and salinity is available. However, most applications of palaeoceanographic proxies to the cold polar oceans are limited due to a paucity of calibration data, limited information on the calcification habitat, and secondary effects of the carbonate system on the temperature recorded by Mg/Ca values measured in the dominant Arctic species Neogloboquadrina pachyderma sinistral (NPS). Here we present a new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) δ11B dataset measured on live NPS collected via plankton tows from the Labrador Sea and Baffin Bay. We compare our results with δ11Bborate derived from pH measurements, δ13C DIC seawater values, temperature and salinity collected at the time and depth the foraminifera calcified. To quantify the control of low carbonate ion concentration on Mg/Ca derived temperatures we measured B/Ca alongside Mg/Ca in the calibration dataset. We are thus able to present a new geochemical correction scheme that can isolate non-thermal controls on the Mg/Ca-temperature relationship for NPS, allowing us for the first time the reconstruction of carbonate system parameters in the Arctic Ocean.

Abrupt climate events are generally believed to be characteristic of glacial (intermediate-to-large cryosphere) climate states, requiring either sizeable ice-sheets or large freshwater pulses to act as triggers for abrupt climate changes to occur. Amplification occurs when these triggers bear upon the Atlantic Meridional Overturning Circulation (AMOC). However, the focus on glacial climate states in abrupt climate change research has led to an underrepresentation of research into interglacial periods. It thus remains unclear whether high-magnitude climate variability requires large cryosphere-driven feedbacks or whether it can also occur under low ice conditions. Here we present a high resolution analysis of surface and deep water components of the AMOC spanning the transition from Marine Isotope Stage (MIS) 19c to 19a to test if orbital boundary conditions similar to our current Holocene can accommodate abrupt climate events. Sediment core DSDP 610B (53°13.297N, 18°53.213W), located approximately 700-km west of Ireland, was specifically chosen due to its high sedimentation rate during interglacial periods, excellent core recovery over the Quaternary and its unique geographical location. Above the core site, the dominant oceanographic feature is the North Atlantic Current and at 2417-m water depth, 610B is influenced by Wyville Thomson Overflow Water flowing southwards. A multiproxy approach including paired grain size analysis, planktic foraminifer assemblage counts, and ice-rafted debris counts within the same samples allows us to resolve the timing between both surface and bottom components of the AMOC and their response to abrupt climate events during MIS-19 in the eastern subpolar gyre. This study is societally relevant as future freshwater inputs from a melting Greenland ice sheet may impact ocean circulation, potentially causing shifts in climate for many European countries.