The overdeepening hypothesis: How erosional modification of the marine-scape during the early Pliocene altered glacial dynamics on the Antarctic Peninsula's Pacific margin

A new synthesis of diatom assemblage data from Ocean Drilling Program (ODP) Leg 178 suggests that the Pacific margin of the Antarctic Peninsula underwent a transition from a shallow shelf to an overdeepened shelf in the early Pliocene. This modification of the marine-scape was due to a relatively brief interval of erosion begun at 5.2 Ma. The erosion was caused by high frequency advances of a super-inflated Antarctic Peninsula Ice Sheet (APIS). The frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the Polar Front migrated southward. By 5.12 Ma, ice streams incised foredeepened glacial troughs into basement on the inner shelf. Sediment eroded from the inner shelf was transported through cross-shelf troughs and deposited in large trough-mouth-fan depocenters on the upper slope. Overdeepened shelf conditions became widespread as troughs widened and intra-trough banks beveled. By 4.25 Ma, trough-mouth-fan construction ceased and subsequent advances of the APIS have been infrequent. We propose that the reduced frequency of grounding events signaled the transition to a modern foredeepened and overdeepened shelf. We hypothesize that a new glacial dynamic emerged in the early Pliocene because overdeepening led to accelerated heat exchange between the ocean and APIS in two ways. Firstly, the overdeepened shelf required that a larger area of the grounded ice sheet's marine terminus be in contact with the ocean. Secondly, erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region. The resultant accelerated melting at the APIS marine terminus, caused the super-inflated APIS to deflate on the mainland, which further decreased the possibility that grounded ice could advance on the overdeepened shelf.

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► We propose the overdeepening hypothesis.
► The hypothesis states that overdeepening of the shelf altered glacial dynamics.
► Diatom data suggests that the peninsula overdeepened at 4.25 Ma.
► The frequency of ice sheet advance reduced after the margin overdeepened.