Abstract. The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations and can, thus, provide important constraints on the stability of the AMOC. However, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice sheets during the deglaciation, as well as its response to these perturbations remain debated. Here, we investigate the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait and the effect of increased tidal dissipation as a result of the altered bathymetry due to the lower glacial sea level stand. The vigorous and deep AMOC under these glacial boundary conditions, consistent with previous simulations with different models, reacts more strongly to North Atlantic freshwater forcings than under preindustrial conditions. This increased sensitivity is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thereby facilitating faster accumulation of freshwater in the North Atlantic and halting deep-water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, which is in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM. Simulations with reduced North Atlantic deep-water formation, as a consequence of potentially increased continental runoff from ice sheet melt and imposed changes in the hydrological cycle, more closely resemble the overturning circulation inferred from proxies. These circulation states also show bistable behavior, where the AMOC does not recover after North Atlantic freshwater hosing. However, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed based on paleoceanographic data.

This study aims to provide a more detailed understanding of the behavior of 231 Pa/ 230 Th under varying ocean circulation regimes. The North Atlantic provides a unique sedimentary setting with its ice-rafted detritus (IRD) layers deposited during glacial times. These layers have been found north of 40°N (Ruddiman Belt) and are most pronounced during Heinrich Stadials. Most of these sediments have been recovered from the deep North Atlantic basin typically below 3000 m water depth. This study reports sedimentological and sediment geochemical data from one of the few sites at intermediate depth of the open North Atlantic (core SU90-I02, 45°N 39°W, 1965 m water depth) within the Ruddiman Belt. The time periods of Heinrich Stadials 1 and 2 of this core were identified with the help of the major element composition by XRF scanning and by IRD counting. Along the core profile, the sedimentary 231 Pa/ 230 Th activity ratio has been measured as a kinematic proxy for the circulation strength. The 231 Pa/ 230 Th record shows highest values during the Holocene and Last Glacial Maximum, above the natural production ratio of these isotopes. During Heinrich Stadials 1 and 2, when Atlantic meridional overturning circulation was most reduced, the 231 Pa/ 230 Th record shows overall lowest values below the production ratio. This behavior is contrary to classical findings of 231 Pa/ 230 Th from the northwestern Atlantic where a strong Holocene circulation is associated with low values. However, this behavior at the presented location is in agreement with results from simulations of the 231 Pa/ 230 Th-enabled Bern3D Earth system model.

The neodymium (Nd) isotopic composition of seawater is a valuable tool for the reconstruction of past water mass provenance and hence deep water geometry. A meaningful interpretation of Nd isotope down-core records requires knowledge of potential variations of water mass end member characteristics. While often assumed temporally constant, recent investigations revealed glacial-interglacial variability of the northern and southern Nd isotope end members in the Atlantic. These new constraints have a strong influence on the interpretation of the Atlantic deep water mass evolution, yet the processes responsible for the end member shifts remain uncertain. Here we combine a new compilation of Atlantic Nd isotope reconstructions of the early Holocene with the Nd-enabled Bern3D model to quantify the recently proposed hypothesis of a northern Nd isotope end member shift during the early Holocene. We achieve the best model-data fit with a strong increase of the Nd flux in the northern high latitudes by a factor of 3 to 4, which lowers the northern end member signature by about 1 ε-unit. Our findings thus agree with the rationale that glacially weathered material entered the northern Northwest Atlantic after the ice sheets retreated late in the deglaciation and released substantial amounts of unradiogenic Nd as suggested previously. Further, we find that variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC) cannot reproduce the observed Nd isotope excursions of the compiled data, ruling out an early Holocene AMOC "overshoot." Water (AAIW) and at abyssal depth Antarctic Bottom Water (AABW), both exhibiting values between −8 and −9 (Stichel et al., 2012). For paleoceanographic investigations, knowledge about these end member constraints is imperative to faithfully reconstruct past water mass mixing. Recent studies showed that both northern and southern end members have varied considerably since the Last Glacial Maximum (LGM)