Rapid early Holocene deglaciation of the Laurentide ice sheet
2008-07-24,
Carlson, Anders E.,
LeGrande, Allegra N.,
Oppo, Delia W.,
Came, Rosemarie E.,
Schmidt, Gavin A.,
Anslow, Faron S.,
Licciardi, Joseph M.,
Obbink, Elizabeth A.
The early Holocene deglaciation of the Laurentide Ice Sheet (LIS) is the most recent
and best constrained disappearance of a large Northern Hemisphere ice sheet. Its
demise is a natural experiment for assessing rates of ice sheet decay and attendant
contributions to sea level rise. Here we demonstrate with terrestrial and marine
records that the final LIS demise occurred in two stages of rapid melting from ~9.0-
8.5 and 7.6-6.8 kyr BP with the LIS contributing ~1.3 and 0.7 cm yr-1 to sea level
rise, respectively. Simulations using a fully coupled atmosphere-ocean general
circulation model suggest that increased ablation from enhanced early Holocene
boreal summer insolation may have been the predominant cause of the LIS
contributions to sea level rise. Although the boreal summer surface radiative
forcing of early Holocene LIS retreat is twice that of projections for 2100 C.E.
greenhouse gas radiative forcing, the associated summer surface air temperature
increase is the same. The geologic evidence for rapid LIS retreat under a
comparable forcing provides a prehistoric precedent for a possible large negative
mass balance response of the Greenland Ice Sheet by the end of the coming century.
Geochemical proxies of North American freshwater routing during the Younger Dryas cold event
2006-12-19,
Carlson, Anders E.,
Clark, Peter U.,
Haley, Brian A.,
Klinkhammer, Gary P.,
Simmons, Kathleen,
Brook, Edward J.,
Meissner, Katrin J.
The Younger Dryas cold interval represents a time when much of the Northern
Hemisphere cooled from ~12.9 to 11.5 kiloyears before present. The cause of this event,
which has long been viewed as the canonical example of abrupt climate change, was
initially attributed to the routing of freshwater to the St. Lawrence River with an attendant
reduction in Atlantic meridional overturning circulation. However, this mechanism has
recently been questioned because current proxies and dating techniques have been unable
to confirm that eastward routing with an increase in freshwater flux occurred during the
Younger Dryas. Here we use new geochemical proxies (ΔMg/Ca, U/Ca & 87Sr/86Sr)
measured in planktonic foraminifera at the mouth of the St. Lawrence Estuary as tracers
of freshwater sources to further evaluate this question. Our proxies, combined with
planktonic δ18Oseawater and δ13C, confirm that routing of runoff from western Canada to the
St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase
in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup (Sv) = 106 m3 s-1). This base
discharge increase is sufficient to have reduced Atlantic meridional overturning circulation
and caused the Younger Dryas cold interval. In addition, our data indicate subsequent
fluctuations in the freshwater flux to the St. Lawrence River of ~0.06 to 0.12 Sv, thus
explaining the variability in the overturning circulation and climate during the Younger
Dryas.