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Tuesday, 17 January 2023

Ice cap growth on coastal lowlands

 It's always interesting, in glacial geomorphology, when the evidence just doesn't fit your hypothesis and you have to come up with something quite counter-intuitive in order to explain the features on the ground. When my old friend David Sugden and I were working in the South Shetland Islands in 1965-66 we assumed at the outset that all of the major ice streams of the Quaternary must have flowed northwards -- ie away from the West Antarctic Ice Sheet and towards the deeper waters of the Drake Passage, that must have remained largely ice-free. But we kept on encountering major landforms which did not fit the hypothesis, and conversations with Capt John Frost of the RRS "Shackleton" about the presence of deep channels -- in places where one might not expect them -- eventually led us to conclude that the axis of an enlarged South Shetlands Ice cap had not run along the island chain itself, but had been out to sea, over the shallow submerged platform with its skerries and shoals which was very dangerous for shipping.

Bathymetric map of the South Shetlands island chain, showing the shallow platform 
to the north and west.

At the time nobody knew very much about eustatic and isostatic interactions, but we did know that sea-level had been low during the big glacial episodes, and if there had been a sea-level drop of c 120m at a time of extensive and intensive glaciation, then the coastal platform would have been exposed, and available for ice cap growth.  This was the map which we put into the scientific domain:

We postulated in a number of articles that as sea-level fell with the onset of a big glacial episode, more and more of the coastal shelf would have been exposed, leading to a northwards migration of the ice cap axis, in response to high precipitation rates to the north and west of the island ice caps, which must already have been in existence.  We thought that there might have been an intervening ice-shelf phase, but we had no way of determining that from the evidence as we saw it.

We suggested that the big glaciation, with ice streams flowing southwards and south-eastwards into the Bransfield Strait via the sounds between the islands,  was not the Devensian / Weichselian / Wisconsin glaciation but the preceding one -- or two, or three........

Much ice has flowed and melted since 1966, and although our ideas have stood the test of time there has been a huge amount of research in the South Shetlands and on the Antarctic Peninsula, and it's now apparent that the "offset South Shetlands Ice Cap" which we described has existed on multiple occasions, including -- most recently -- the Weichselian.  So the outlet glacier troughs have been occupied by streaming ice over and again, maybe getting deeper with each successive glacial episode.  Bethan Davies has written about the most recent deglacial phases around the Bransfield Strait. Some of her maps seem to show an LGM ice cap axis over the island chain, with a grounding line on the shelf, and some show cold-based ice grounded between the South Shetlands and South Orkneys.  I must check up on what the latest thinking might be.......

There is now a big literature.  

What interests me about all of this is the extent to which we can extrapolate for our studies of "the South Pembrokeshire problem" -- was the area glaciated during the Devensian, or was it not?

If an ice cap could thicken and grow, and if its axis could migrate windwards across a low-lying coastal platform in the South Shetland Islands within the time-frame of a glacial episode, why not in Pembrokeshire too?  I shall ponder further.....

A slice of one of the maps from Bethan Davies and colleagues, 2012.

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