John Glacier -- all quiet on the southern front

 

I had my attention drawn to this rather interesting article written by a team including three staff members from my old Geography Department in Durham. It's a detailed and very fascinating examination of the geomorphology and glaciology of the ice free area that includes Nelson Peak and the Miller valley in the Pensacola Mountains in Antarctica.  I have a proprietary interest in that neck of the woods, since the glacier that bears my name (thanks to the Antarctic Placenames Committee) lies just to the north of the field study area!  I have located my little glacier on the map above, which comes from the article.  

John Glacier is about 8 km long, and in its upper section 2 km wide.  It's one of a series of outlet glaciers which tumble over the Washington Escarpment, carrying ice from the East Antarctic Ice Sheet towards the Ronne Ice Shelf and the Weddell Sea.

Measured ice velocities in the Weddell Sea Embayment

Small et al describe the context as follows:

The Neptune Range is a massif within the Pensacola Mountains in the south-east portion of the Weddell Sea embayment.  The Pensacola Mountains are dominated by folded sandstones, mudstones, conglomerates and limestones, with minor igneous rocks exposed in their western part (Schmidt et al. 1978). The Neptune Range consists of the Washington Escarpment with associated ridges and valleys, the Iroquois Plateau, the Schmidt Hills and the Williams Hills. These latter two massifs are located on the eastern margin of the Foundation Ice Stream (FIS) and are separated from the Washington Escarpment by the ice-filled Roderick Valley. The Washington Escarpment trends north-west to south-east for > 100 km and, at its southern end, the Academy Glacier flows from the polar plateau and is confluent with the FIS. To the east of the Washington Escarpment the Iroquois Plateau comprises a large snowfield at an elevation of ∼1400m. Outlet glaciers from this snowfield overtop the Washington Escarpment in numerous places and flow west into Roderick Valley and, via Childs Glacier, into the trunk of the FIS.  In several places, peaks on the Washington Escarpment are sufficiently high to prevent overriding by ice from the Iroquois Plateau.   In the lee of these highest peaks are several ice-free valleys that are blocked at their entrances by ice lobes that spill into the lower valleys from outlet glaciers flowing from the plateau towards Roderick Valley. 

Source:
Antarctic Science pp 1- 28 (2021) Cambridge University Press on behalf of Antarctic Science Ltd. doi:10.1017/S0954102021000237

https://www.cambridge.org/core/journals/antarctic-science/article/icefree-valleys-in-the-neptune-range-of-the-pensacola-mountains-antarctica-glacial-geomorphology-geochronology-and-potential-as-palaeoenvironmental-archives/A7CFE9724599C1055E62EAE0812B20C0

Ice-free valleys in the Neptune Range of the Pensacola Mountains, Antarctica: glacial geomorphology, geochronology and potential as palaeoenvironmental archives
by DAVID SMALL, MICHAEL J. BENTLEY , DAVID J.A. EVANS, ANDREW S. HEIN  and STEWART P.H.T. FREEMAN

Abstract:

We describe the glacial geomorphology and initial geochronology of two ice-free valley systems within the Neptune Range of the Pensacola Mountains, Antarctica. These valleys are characterized by landforms associated with formerly more expanded ice sheet(s) that were at least 200 m thicker than at present. The most conspicuous features are areas of supraglacial debris, discrete debris accumulations separated from modern-day ice and curvilinear ridges and mounds. The landsystem bears similarities to debris-rich cold-based glacial landsystems described elsewhere in Antarctica and the Arctic where buried ice is prevalent. Geochronological data demonstrate multiple phases of ice expansion. The oldest, occurring > 3 Ma, overtopped much of the landscape. Subsequent, less expansive advances into the valleys occurred > 2 Ma and > ∼1 Ma. An expansion of some local glaciers occurred < 250 ka. This sequence of glacial stages is similar to that described from the northernmost massif of the Pensacola Mountains (Dufek Massif), suggesting that it represents a regional signal of ice-sheet evolution over the Plio-Pleistocene. The geomorphological record and its evolution over millions of years makes the Neptune Range valleys an area worthy of future research and we highlight potential avenues for this.

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Debris covered surface in the Miller Valley -- from the paper by Small et al 2021.

The fieldwork done by David Small and his colleagues was concentrated in the ice-free nunatak areas near Nelson Peak and Mount Hawkes, and the article contains a number of excellent illustrations which show what the landscape looks like.  To me, it looks very old indeed -- and this is confirmed by the dating work undertaken.

It looks as if John Glacier is fed by two icefalls on the edge of the Washington Escarpment, with the southern one most active.  In the photos a rocky crag is visible between the two icefalls.  Like the unnamed glacier to the south, the lower part of the glacier appears to have retreated substantially, since it is flanked by wide areas of moraine and accumulated slope debris. On the USGS 1:250,000 map of this area, extensive areas of water are shown beyond the snout as frozen meltwater lakes; but I suggest there is a mistake in the reading of the satellite imagery, and it is more likely that these areas are exposures of very ancient blue ice.  Whatever the truth of the matter, it looks as if the snout has lost contact with the ice that fills the depression to the west.  This fits with the regional interpretations of Small et al, who suggest that the general ice surface in this area has dropped by at least 200m since the time of maximum glaciation.  That would not be surprising, since we are after all in an interglacial episode right now, and as a result of global warming, too many ice surfaces are wasting too quickly.  

If I manage to find out anything more about my little glacier, I'll do another post!

Some of the geomorphological features of the Miller Valley - Nelson Peak area. The glacier at the top edge of this illustration is similar to John Glacier, which lies a few km to the NE, on the other side of Hannah Ridge.