Strong and deep Atlantic meridional overturning circulation during the last glacial cycle

Abstract

Extreme, abrupt Northern Hemisphere climate oscillations during the last glacial cycle (140,000 years ago to present) were modulated by changes in ocean circulation and atmospheric forcing. However, the variability of the Atlantic meridional overturning circulation (AMOC), which has a role in controlling heat transport from low to high latitudes and in ocean CO2 storage, is still poorly constrained beyond the Last Glacial Maximum. Here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle, dominating ocean circulation in the Atlantic, whereas a shallower glacial mode with southern-sourced waters filling the deep western North Atlantic prevailed during glacial maxima. Our results are based on a reconstruction of both the strength and the direction of the AMOC during the last glacial cycle from a highly resolved marine sedimentary record in the deep western North Atlantic. Parallel measurements of two independent chemical water tracers (the isotope ratios of (231)Pa/(230)Th and (143)Nd/(144)Nd), which are not directly affected by changes in the global cycle, reveal consistent responses of the AMOC during the last two glacial terminations. Any significant deviations from this configuration, resulting in slowdowns of the AMOC, were restricted to centennial-scale excursions during catastrophic iceberg discharges of the Heinrich stadials. Severe and multicentennial weakening of North Atlantic Deep Water formation occurred only during Heinrich stadials close to glacial maxima with increased ice coverage, probably as a result of increased fresh-water input. In contrast, the AMOC was relatively insensitive to submillennial meltwater pulses during warmer climate states, and an active AMOC prevailed during Dansgaard-Oeschger interstadials (Greenland warm periods).

Abrupt climate and vegetation variability of eastern Anatolia during the last glacial

“…3d) point to a strong aridification and cooling in eastern Anatolia after 70 ka BP. The observation is consistent with low summer insolation (Berger, 1978;Berger et al, 2007), increased global ice volume (Shackleton, 1987), and cooler sea surface temperature (SST; Cacho et al, 2000), which, combined with the atmospheric effects of a weakening AMOC (Atlantic Meridional Overturning Circulation; Böhm et al, 2015;Bond et al, 1993) contributed to a widespread aridity across the Mediterranean region (e.g., Fletcher et al, 2010;Kwiecien et al, 2009;Sánchez Goñi et al, 2002).…”

Impact of meltwater on high-latitude early Last Interglacial climate

“…However, marine sediment core evidence suggests that the AMOC was temporarily weaker at this time (e.g. Böhm et al, 2015). Furthermore, this cooling persists at 125 ka when the data show an overall warming compared with the present day (see Fig.…”

“…The contribution of the Greenland ice sheet to global LIG sea level rise has recently been quantified (Born and Nisancioglu, 2012;Colville et al, 2011;Helsen et al, 2013;NEEM community members, 2013;Quiquet et al, 2013;Stone et al, 2013), with the IPCC Fifth Assessment Report stating a range very likely between 1.4 and 4.3 m of equivalent sea level height . Taking contributions from thermal expansion and mountain glaciers into account and that global sea level was at least 6 m higher than today (Dutton et al, 2015), this implies that a contribution is likely also required from the WAIS (noted specifically by Colville et al, 2011) and/or other parts of the Antarctic ice sheet.…”

“…Enhanced insolation forcing in the NH during the penultimate deglaciation resulted in rapid ice sheet retreat and an increase in freshwater input to the North Atlantic and a suppression of the Atlantic Meridional Overturning Circulation (AMOC) near the end of the deglaciation (Carlson, 2008). In addition, marine sediment core evidence shows that North Atlantic Deep Water (NADW) production was reduced compared with the present day but recovered to present-day values by 125 ka (Böhm et al, 2015;Lototskaya and Ganssen, 1999;Oppo et al, 1997).…”

Carbon isotope evidence for a northern source of deep water in the glacial western North Atlantic

“…This finding is at variance with the prevailing view that there was little or no northern deep water present in the glacial North Atlantic (18,19), but a recent synthesis of Nd isotopes in the western Atlantic has also been interpreted as evidence for a glacial source of NADW (20). Furthermore, Böhm et al (21) found that some interstadials and the last interglacial on BR had lower eNd than in the modern ocean. This very low eNd could indicate there were both warm and cold climate processes that formed deep water in the North Atlantic that are inactive today.…”