Thursday, 16 April 2014 Back to News

Seabed Holds The Key To Understanding Our Icy Past: Part 1

Understanding Our Icy Past From The Seabed

Dr. Tom Bradwell, of the British Geological Survey, shares how discoveries about the glacial geomorphology of the British seabed have been a revelation, and offers a detailed insight into some of the scientific breakthroughs that have been made as part of MAREMAP. Over to you, Tom...

How big was the last ice sheet to cover the British Isles, and when was Britain in in its icy grip?
What is the landscape legacy of the ice age on- and offshore and how have sea levels changed over this time?
What is glacial geomorphology?

Geomorphology is the science of the Earth's landscapes and landforms, the processes that formed them and their links to environmental change. Glacial geomorphology, simply put, is the study of landforms and processes specifically relating to glaciation.

Usually glacial geomorphologists work on land, where the evidence is clearest, the landscapes are most striking and the processes are easily visible and accessible. For instance, some geomorphologists monitor how glaciers respond to daily, seasonal or annual changes, and examine the sediments and landforms they leave behind (the BGS is doing this at present in Iceland, where we have set up a glacier observatory with instruments continuously measuring different aspects of the glacier's health over the past 5 years).

A Drowned Glacial Landscape

The seabed around much of the UK is one of the finest examples of a drowned glacial landscape. Surprisingly, though, relatively few glacial geomorphologists like getting their feet wet!

We know from modern analogues (such as in Greenland and Antarctica) that the last ice sheet to cover the British Isles terminated in the sea. Sea-level histories compiled elsewhere show that much of what is now seabed would have been dry land at the height of the last ice age 25,000-30,000 years ago.

So, to examine how extensive the last ice sheet was, we need to look offshore. To get an idea of its size we need data covering large areas of seabed.

The Age of Ice

Did a large ice sheet really cover the landscape of Britain and its seabed?

Until recently the answer to this question was unclear and frankly, just too difficult to answer.

But not anymore.

Seabed bathymetry data is now routinely collected by all sorts of ocean-going craft. Every boat that puts to sea these days is equipped with a good echosounder and satellite positioning system for accurate navigation and safety-at-sea purposes. Over the years this data has been collected and compiled by companies interested in marine navigation and shipping routes. These ships, surprisingly, were slowly but surely building up an image of the underwater landscape around Britain. And what came as an even bigger surprise was the stunning glacial geomorphology it revealed!

Back in 2008, BGS colleagues and I used this (licensed) dataset to map the glacial landforms on the seabed and make inferences for the first time about the true size, shape and retreat pattern of the last ice sheet to cover the northern half of the British Isles. The results were published in Earth Science Reviews a peer-reviewed scientific journal.

For me, as an Earth scientist, it changed the way I viewed the seabed as a landscape - the fact that the imprint of a melting ice sheet could be perfectly preserved on the seabed over such a large area and for such a long time was fascinating. It was like a priceless archive that had been locked away in a vault for 20,000 years just waiting to be discovered!

Surprisingly - and perhaps counter-intuitively - the glacial landforms we discovered offshore were far more abundant and more complete than the equivalent record onshore (gathered from more than 100 years of field surveys). It wasn't just the number of glacial features revealed on the seabed that surprised us, it was their diversity too! Plus, of course, what it all meant for ice sheet and sea level history in our part of Europe.

In simple terms, the echosounder data allowed us to make sense of what we had only seen glimpses of in the past. Like a giant dot-to dot map - people had already seen fragments of the evidence before - but because of a lack of data, no-one could see the whole picture. Glacially carved tunnel valleys in the North Sea, big moraines on the continental shelf west of Shetland, relict sediment piles and fans built up over successive ice ages were all known about before this. We had no idea, though, as to their true form, full extent or connections with other features. Before these spatially extensive digital bathymetric datasets became available, we knew only what could be gleaned from a coarse grid of widely spaced geophysical (seismic) lines. These were mainly designed for looking deep into the seabed, but not for capturing the subtle geomorphology of the seafloor.

This is why our interpretation of the submarine glacial geomorphology around the UK was always a bit fuzzy. But the detail and geographical extent of this new echosounder data changed all that forever.

Since 2008, things have moved on at pace! The volume of marine data acquisition has increased and multibeam (swath) bathymetry data from UK waters has become more commonplace - especially through the fantastic work of the UKHO/MCA's civil hydrography programme. All this has allowed an even better, more detailed view of the seabed and the secrets it holds.

Look out for Part Two, coming soon...