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Sunday, 6 June 2010

On postglacial rebound and relative sea-level




I'm often asked where sea-level (ie relative sea-level) was in the Neolithic and the Bronze Age of Southern Britain. Well, we are still not entirely sure. Two factors come into play -- isostatic rebound following the melting of the "glacier ice load" over parts of Britain and Ireland, and eustatic sea-level rise consequent upon the return of vast amounts of meltwater to the oceans after c 20,000 years ago. So sea-level was rising, and so was the land -- and both were jerky rather than smooth. Both eustatic and isostatic rises were initially very fast, gradually tailing off to near zero. So shoreline sequences are often very difficult to interpret. The maps above show some of the complexity in Scandinavia. The highest shorelines are not necessarily in the areas of greatest rebound -- since many of those areas were covered with ice until later stages. There was a minimum rebound of over 275m near the head of the Gulf of Bothnia -- that translates into an ice load which was c 900m thick. In the same area the current rate of uplift is c 9mm per year.

But actually the rebound was much more than that, since sea-level at the time of initial isostatic recovery was around 120m lower than it is today. That means the TOTAL amount of recovery over the past 20,000 years has been at least 400m and more likely 500m. That equates to an ice load over the Gulf of Bothnia which was 1.5 km thick. That's a lot of ice.

Then things get REALLY complicated, because water that floods in to replace ice also has an isostatic effect, depressing the land surface over which it floods -- or at least slowing down the rate of ice-melt-induced uplift. There is also an elastic effect, which means that land surfaces up to 150 km beyond an ice edge can also be depressed by ice loading, and then affected by rebound when the glacial episode comes to an end. And there is a compensatory effect too, with UPLIFT beyond an ice edge caused by compensatory flow within the Earth's mantle when a glaciated area is depressed. When post-glacial uplift begins, this compensatory flow is reversed, so that areas beyond the ice edge may actually sink. Between these two zones there may be a hinge-line along which isostatic effects are zero. Add to that the complicating factors of rock type, fissures and faulting, and even tectonic movements that have nothing to do with glaciation, and confusion begins to set in. And by the way, vast loads of glacial and fluvio-glacial sediments transferred from an area of glacial erosion to an area of glacial deposition (for example, from the uplands of Britain into the Irish Sea and Celtic Sea) will also trigger isostatic effects.

In the old days, when I was a geomorphologist, I had to try, in places like Greenland, Antarctica and Iceland, to unravel the history of raised shorelines, ice edge positions, isostatic uplift and depression, and eustatic oscillations -- it was the closest I have ever been to four-dimensional puzzle solving!

Confused? So am I.......



Now to Britain. The map above shows that there is still slight uplift going on in the northern parts of the British Isles, but that there is sinking in the south. This sinking might be partly a compensatory isostatic effect, partly tectonic, and partly because of the massive sediment load carried by rivers into the southern North Sea and English Channel. The rate of uplift in the Highlands of Scotland is only about one third of that of the Gulf of Bothnia, since the British-Irish ice mass was that much smaller and since isostatic recovery is now more or less complete.

The highest post-glacial or Holocene shorelines in Scotland are nothing like as high as the shorelines I have studied in the Arctic and Antarctic. There are some at c 45m in south-west Scotland and Northern Ireland, although most of those identified have been beneath 30m. In West Wales there is no evidence of post-glacial shorelines above present sea-level; this means that the amount of isostatic recovery following the removal of the ice load has been less than the 120m or so of eustatic sea-level rise.

If we assume that isostatic uplift in South-west Britain is more or less complete, we would expect to find some evidence of prehistoric shoreline occupation BELOW present sea-level. This is just what we do find. During the Palaeolithic sea-level was more than 20m lower than it is today. During the Mesolithic c 7,000 years ago, sea-level was about 10m lower than today, and we find evidence of the Mesolithic sea-level rise in the submerged forests around the coasts of Pembrokeshire. In the Neolithic, sea-level was at c -6m, and in the Bronze Age it was at -4m. Around the time of Christ, it was about 1.5m lower than at present.

This all means that in many locations (for example, on low coastal forelands or in estuarine environments) HWMST would have been located out beyond the position of the present coastline. This is something that does need to be borne in mind by those who argue for the human transport of bluestones by land and sea. To my mind, the degree of difficulty would have been greatly increased -- with extensive boggy and heavily-wooded tracts to be negotiated in places where there is now sea.

Click on the maps to enlarge them.

4 comments:

Kostas said...

Brian,

What accounts for the increase in sea level 'uplift' going further north, from -1 mm to 9 mm per year in your 'diagram'? It can't be due to Earth's rotation, since you would expect the opposite effect. Could it be that instead of the sea levels rising the land mass is sinking? Land masses can sink or rise unevenly but hard to imagine sea levels rising unevenly. If so, that would indicate the Earth is getting 'flatter' the closer we are to the poles. Any studies on that? Just curious.

Constantinos

Brian said...

You are mixing up shorelines with sea-level. There is LAND uplift as high as 9 mm per year in northern Scandinavia -- and that is down to isostatic rebound.

As far as sea-level is concerned, there is a slight equatorial bulge and that is down to the centrifugal effect of the spinning earth.

Kostas said...

Thanks for clarifying this Brian. I was misreading the diagrams. I acknowledge my mistake!

Constantinos

Brian said...

You're not the only one to be confused, Kostas! This is fiendishly difficult territory. As I mentioned, you need four-dimensional thinking to come to terms with which shoreline was which, and where, and when......

Maybe sorting out the giant erratics on the coasts of Southern England will be easier -- but I somehow doubt it.