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Mid-Holocene Ice Shelf Retreat
Hemer, M.A. and Harris, P.T.  2003.  Sediment core from beneath the Amery Ice Shelf, East Antarctica, suggests mid-Holocene ice-shelf retreat.  Geology 31: 127-130.

What was done
Noting that "the past and present circulation pattern beneath floating ice shelves is crucial information for understanding the impact of global warming on present-day ice shelves and the subsequent effect on global ocean circulation and climate," the authors extracted and analyzed a sediment core from beneath the Amery Ice Shelf, East Antarctica, at a point that is currently about 80 km landward of the location of its present edge.

What was learned
The core contained a 0.5-m-thick surface layer of siliceous mud and diatom ooze of marine origin, which the authors say is evidence that "landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ~80 km."  Within this layer, they discovered "peaks in biogenic opal, ADA [absolute diatom abundance], and Fragilariopsis curta," which they say "are associated with increased proximity to an area of primary production, such as the sea-ice zone."  Of even greater interest was their observation that "the relative abundance of F. curta begins to increase ca. 5700 14C yr B.P. toward the peak in F. curta abundance and ADA observed ca 750 14C yr B.P.," which they say "may be attributed to a gradual retreat of the ice front, thus bringing the productive sea-ice zone waters closer to the [core] site over the period from 5700 14C yr B.P. to ca. 750 14C yr B.P."

What it means
The authors say their analysis "suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum."  This observation, in turn, suggests that since the cessation of the ice shelf's retreat some 750 14C yr B.P., there has been a major seaward extension of the ice shelf's edge, possibly driven by the cooling associated with the development of the Little Ice Age, which in turn suggests that we could again witness some retreat of the ice shelf's edge if the Modern Warm Period ever begins to manifest itself in East Antarctica, which it has yet to do.  It also suggests that if this does occur, we need not be unduly concerned; for the Amery Ice Shelf has retreated before - peaking, apparently, during the Medieval Warm Period -- at a time when mean global air temperature must have been considerably higher than it is now, all without any catastrophic increase in sea level.

Reviewed 21 May 2003