If you take a walk along a North Sea beach, you will find lots of different things left behind in the wash margin by the tide: plastic rubbish, seaweed, shells, sediment – it depends what was in the water. And over the centuries, layers have also been deposited in the coastal soil. The content of these layers is dictated by the conditions at the time. Using this “historical message in a bottle” we are reconstructing the rise in sea level along our coasts.
We are doing so using new methods developed at the KlimaCampus especially for the North Sea coast. They involve using particular microorganisms called Foraminifera. On the North Sea Coast most species of this unicellular organism are the size of a grain of sand. They have tests composed of calcium carbonate or sediment particles, which are deposited and survive as fossils – often over thousands of years.
Only a few highly specialised species are able to live at the junction of land and sea. Here, only those that have adapted to the extreme conditions – sometimes fresh and sometimes salt water, sometimes wet and sometimes dry – are able to survive. So in the salt marshes and intertidal zone there are only a dozen or so species of Foraminifera, each with its own “favorite spot.”
If a species likes, for example, regularly being covered by salt water, it will prefer to live at sea level. Another species may tolerate this less often and so its optimal habitat is perhaps 60 to 80 centimetres above sea level, and so on. We can accurately determine these zones for each of the few species, and so we can obtain a statistical vertical cross section: If species X occurs often, species Y occasionally, and species Z only rarely, a sample can only originate, for example, from approximately 40 centimetres above sea level.
The second step is to take bore samples a few metres deep from the foreshore. Here we find deposits from past centuries layered on top of one another. Millimetre for millimetre, we remove the layers and by analyzing the radioactive decay products we can accurately date them. Finally we look at which Foraminifera colonies are present and so we can see how far above sea level the sample was in that particular year. Now we just have to “subtract out” factors such as storm tides and local movements in the earth’s crust; since the ice sheets disappeared 18,000 years ago, the earth’s crust has been rising around Norway, while it has been sinking around Holland.
In this way, we are able to determine to an accuracy of a few centimetres the sea level in the past. The first results show that the water level along the North Sea coast has risen by an average of 25 cm over a period of 150 years. In places, this figure could be 40 cm. This confirms the historical measurements taken and shows that our methods are reliable.
Good news, since in the long term we want to determine the sea level over the last 10,000 years and there are no measurements for this period. Data going back this far are vital if we are to make conclusive statements about the extent to which these variations are natural, and to what extent they are due to manmade climate changes.
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