Skip to 0 minutes and 7 seconds FRASER STURT: Hello. In this section, we’re going to talk about sea level change and why it’s so important for archaeologists and although the sea may look level, the first important thing to realise is it isn’t really. Sea level always refers to a specific point in space and time. Where you live you may have seen evidence for this. In certain parts of the world, we have what are called raised beaches, where you’ll find sand and gravel platforms which are recognisably a beach but are now miles away from the sea.
Skip to 0 minutes and 34 seconds In other areas, you may have submerged forests - tree stumps and bits of tree trunk lying in the intertidal zone - showing you that this was one a landscape which is now underwater. And although this is a complicated topic, we’re going to explore it through four simple ideas or processes and two key themes as why we, as archaeologists, are interested in it. The first of those processes is the lunar connection to tidal range. This is a semi diurnal pattern of high and low tides but if we think deeper through time, there are some more important processes which have reshaped the globe dramatically.
Skip to 1 minute and 11 seconds The first of these is eustatic change and eustasy relates to the amount of water that are in our world’s oceans and seas. As we go through glacial stages, ice ages, we lock up water on land as ice sheets. This sucks it out of the sea and lowers sea level. However, it also exerts a weight or a force, just like me sitting on a sofa depressing the cushions, on the land and that changes the elevation under the ice sheet, pushing it down but it can also raise up other areas of land and this is the isostatic component; the elevation of the surface of the earth. So that is our third process. Our fourth is tectonics.
Skip to 1 minute and 53 seconds So we can have a dramatic change in relative sea level due to earthquakes or volcanic eruptions. Quite literally, the land can rise up or drop down metres or tens of metres, radically altering the geography. Now these can change the shape of the planet, the distribution of the world’s oceans and seas, to quite a considerable degree. If we look at the last Ice Age, back 23,000 years ago, we will be dropping sea level down 120 metres, exposing vast swathes of the continental shelf as a landscape. As archaeologists, this is really significant for two key reasons.
Skip to 2 minutes and 31 seconds The first of those is that we do have these spaces which are now covered by the sea which were large areas open for prehistoric populations to move over and live in. The significance of Maritime archaeology is that, in these spaces, we often find preserved organic materials that we don’t always find on land. So this is a complementary and different record. Within those sediments, we also find evidence that can help us improve our understanding of the process of sea level and environmental change. What was it like to live in a landscape that was being inundated at the rates that we see in some areas which may be recognisable to people like you or I?
Skip to 3 minutes and 12 seconds The second reason this is so important is if we suck all that water up and place it in an ice sheet or inundate a landscape and create new bodies of water, we change the way the sea behaves; it’s currents, eddies and directions and this changes the way that people can connect with each other. So it affects seafaring behaviour and the technologies we need to operate in those spaces. So it’s not just about submergence of landscapes, it’s about the sea itself and how those changes impacted on people in the past. As such, we can’t really understand the Maritime archaeological material that we excavate and study unless we can know more about the world in which they lived in.
Skip to 3 minutes and 51 seconds Thus, studying sea level change and its impact on people in the past is fundamental for Maritime archaeology.
Our changing seas
In this video, Dr Fraser Sturt discusses the importance to maritime archaeologists of sea-level change (and its impact on people).
Fraser looks at the four key processes that underlie sea-level change:
- The semi-diurnal lunar connnection (high and low tides)
- Eustatic change
- Isostatic change
as well as the two main reasons that this is of significance for maritime archaeology.
Some of the terms used may be new to you, but they are explained as we go along, in the following step and the glossary. If you have any questions please don’t hesitate to ask in the comments.
By the end of this step you should be aware of the nature of the processes that lead to sea-level change and the ways they impact on maritime archaeology.
© University of Southampton, 2017