Signature of the last termination in Maine

Hall, Brenda; Lowell, Thomas; Bromley, Gordon

From the introduction and statement of problem: "The origin of Pleistocene ice ages remains a major unsolved problem of earth sciences. The glacial/interglacial transition at the end of the last ice age, known as a termination, is particularly important, because it represents the greatest natural warming of recent geological history. Yet, the mechanisms by which the earth exits an ice age are not fully understood. The current consensus is that terminations are characterized by out-of-phase climate behavior between the Northern and Southern Hemispheres. Intense winter cooling and spread of sea ice in the North Atlantic is thought to be key in rapidly propagating the effects of North Atlantic cold periods, such as Heinrich Stadial I (HS-1; 14.5-18 ka), into the Southern Hemisphere by causing southward shift of the Intertropical Convergence Zone. This shift at the start of HS-1 may have caused Southern Hemisphere oceanic fronts and westerly winds to migrate poleward, producing rapid warming of southern latitudes and release of carbon dioxide from the Southern Ocean that set in motion the termination. By this hypothesis, a variety of 'bipolar seesaw', cold periods in the North Atlantic region lead to warming in the south, which kick starts the end of the ice age. Recent work, however, has begun to cast doubt on this simple conception of a bipolar seesaw in climate during the termination. Emerging glacial chronologic data seem to indicate that the timing and pattern of climate change during the termination may have been identical in both hemispheres, at least during summer, and that deglaciation (summer warming) in the north occurred during what are traditionally thought of as Northern Hemisphere cold periods. For example, recent data from the Alps suggest rapid deglaciation during HS-1, similar to that seen in the Southern Hemisphere. In addition, new work suggests that glacier fluctuations in later stages of the termination, during the late-glacial period (11.7-14.5 ka) also may be the same in both hemispheres. Thus, a limited amount of emerging data hints at the possibility that significant Northern Hemisphere deglaciation may have occurred during what is traditionally thought of as a cold period, HS-1. One explanation for this apparent paradox is that the termination of the ice age in the Northern Hemisphere was characterized by warming summers, which caused ice melt, but extremely cold winters, which produced the cold mean-annual temperatures seen in the majority of North Atlantic region climate proxies. Resolving the timing of Northern Hemisphere deglaciation during the beginning of the termination and, in particular, whether any of it occurred during HS-1, is key for assessing the mechanisms that ended the ice age."