Search

Millennial-scale climate oscillations of the last ice age registered in Greenland and Antarctic ice cores did not always vary in unison. A striking example is that the strongest Antarctic warming episodes occurred during Heinrich episodes in the North Atlantic region. Although the bipolar seesaw affords a possible explanation for such anti-phasing, it does not account for the equally striking observation that climate varied in unison between the hemispheres about half the time. Such phasing differences suggest the need for an alternative hypothesis in which the polar regions at times responded in unison to common forcing, and at other times left the impression of a bipolar seesaw. We posit that this impression arose from the effect of warmer-than-usual summers on continental ice sheets adjacent to the North Atlantic Ocean during each Heinrich episode. The relatively warm Heinrich summers produced discharges of meltwater and icebergs of sufficient volume to stimulate very cold winter conditions from widespread sea ice on a freshened ocean surface. The intervals between Heinrich episodes featured relaxation of sea-ice-induced winter severity from reduced summertime influx of meltwater and icebergs, indicating relatively cooler summer conditions. It is postulated that the causative variations in freshwater fluxes were driven by a climate signal most evident in Antarctic ice cores but also recognized in other paleoclimate records in both polar hemispheres. We suggest that this widespread signal arose from changes in the latitude and strength of the austral westerlies and the resulting effect on the western Pacific tropical warm pool, a mechanism dubbed the Zealandia Switch. • Bipolar synchrony of millennial-scale glacier melt during the last glaciation. • Interhemispheric glacier signal inconsistent with 'bipolar seesaw' hypothesis. • Northern 'stadials' attributed to warm summers paired with extremely cold winters. • Enhanced melt-induced northern seasonality obscured global ice-age climatic signature. • Differentiating summer from winter effects key to interpreting paleoclimatic proxies. [ABSTRACT FROM AUTHOR]

The century-long 8.2 ka cold event interrupted early Holocene boreal warmth and may have halted retreat of Greenland ice-sheet margins during the last deglaciation. Here, we synthesize new and existing glacial geological data to assess the behavior of the southwest Greenland ice sheet during the 8.2 ka cold event. In southwest Greenland near the town of Kangerlussuaq, existing and new 10Be surface exposure ages demonstrate that deposition of the Keglen moraines ended at 8.0 ± 0.1 ka (n = 10, 1 outlier), with prior studies arguing that these moraines represented an ice-margin stillstand in response to the 8.2 ka cold event. However, new 10Be ages show that the southwest Greenland ice-sheet margin retreated from the Umîvît moraines, which lie 5–10 km outboard of the Keglen moraines, at 8.2 ± 0.2 ka (n = 11). Accordingly, we suggest that the southwest Greenland ice-sheet margin in the Kangerlussuaq region retreated 5–10 km during the 8.2 ka cold event. Farther south and inland from Nuuk, new 10Be boulder-on-bedrock ages adjacent to the modern ice-sheet margin demonstrate that the ice-margin was retreating with no moraine deposition at 8.2 ± 0.1 ka (n = 4, 1 outlier), with this retreat continuing up to at least ∼7.5 ka according to an existing threshold lake record. Therefore, we propose that the southwest Greenland ice-sheet margin underwent continued retreat during the 8.2 ka cold event in response to elevated early Holocene boreal summer insolation that overwhelmed the impacts from century-scale cooling. Caution should be used in assuming climatic causation for moraine deposition based on temporal correlation. • New 10Be ages in SW Greenland show ice retreat during the 8.2 ka cold event. • Elevated shortwave radiation may have offset the impacts of 8.2 ka cooling. • Moraine-age correlation with a cold period is not necessarily causation. [ABSTRACT FROM AUTHOR]

During the last deglaciation, North Atlantic climate abruptly warmed at the Bølling (∼14.7 ka), cooled into the Younger Dryas (∼12.9 ka) and abruptly warmed again into the Holocene (∼11.7 ka). While these events are defined by Greenland ice cores, there is still considerable uncertainty on Greenland ice-sheet margin responses to abrupt climate change. To refine the ice sheet's deglacial history, we present new cosmogenic nuclide surface exposure ages from boulders on bedrock at five sites in southernmost Greenland fjords located midway between the coast and inland ice margin. We find ice-sheet thinning below three local topographic highs at 12.7 ± 0.3 ka (n = 3), 13.1 ± 0.4 ka (n = 1, 2 outliers), and 12.3 ± 0.2 ka (n = 3), with up-fjord retreat at 12.5 ± 0.3 ka (n = 3) and 12.7 ± 0.2 ka (n = 4) based on two sites just above the mid-fjord marine limit. These mid-fjord 10Be ages therefore show southernmost Greenland ice-sheet thinning and retreat during the Younger Dryas. We hypothesize that this thinning and retreat was a response to ocean warming prior to the Holocene and/or summer shortwave radiative forcing during the Younger Dryas due to peak boreal summer insolation. Our results also support a previously hypothesized winter bias in proxy records of Younger Dryas atmospheric cooling, since a large summer cooling during the Younger Dryas could have counteracted the effects of ocean warming and direct radiative forcing, inhibiting ice-sheet retreat. • Southernmost Greenland ice-sheet margin thinned during the Younger Dryas. • Southernmost Greenland ice-sheet margin retreated during the Younger Dryas. • Greenland ice retreat was likely driven by summer insolation and ocean warming. [ABSTRACT FROM AUTHOR]