Understanding the changing geography of our planet is fundamental for maritime archaeological research. Over the 2.5 million years we are considering in this course, sea-level has fluctuated by as much as 120m on multiple occasions over thousands of years. This not only shaped the geography of our planet in terms of landmasses, but also changed the behavioural characteristics of the sea and nature of coastlines (e.g. shifts from cliffs to shallowly shelving beaches and vice versa).
The figure at the top of this page shows a graph of changing sea-levels over the last 500,000 years, created from data from the Red Sea. Cores in this area allow researchers to calculate fluctuations in the amount of water in the world’s oceans and seas and correspondingly estimate changing relative sea-levels.
The driving force behind these large-scale changes is ‘orbital forcing’. The sun is the major source of energy that reaches the Earth and largely dictates the temperature of the planet. However, the Earth does not orbit the sun on a vertical axis or follow a uniform path. Instead the earth is tilted away from the sun, and the angle of the tilt changes through time (what we call obliquity). In addition, the orbit of the Earth oscillates around the sun, sometimes passing closer to it, and at other points further away, in a process referred to as ‘eccentricity’. Finally there is a ‘wobble’ in the Earth’s rotation, known as ‘precession’. These three factors operate at different time scales, with eccentricity changing over a c.100,000 – 400,000 year period, obliquity over 41,000 years and precession c.26,000 years. Together they affect how much energy reaches the earth and how it is distributed over the surface of the planet. You can watch a short video about this on the National Geographic website*. (*N.B. This video will not play on iOS and Android mobile devices - apologies).
The significance of changes in orbital pattern on the earth’s climate was first noted by Adhemar and Croll in 1842, but it was the Serbian scientist Milutin Milanković working in the 1920s who recognised the three separate elements and considered their periodicity, giving his name to this phenomenon. These apparently subtle shifts in distance and angle to the sun can lead to the melting of polar ice caps and increases in sea-level, or, reduction of heat at the poles, an increasing ice volume and a corresponding fall in sea-level.
Eustasy can be broadly defined as changes in the total volume of water in the world’s oceans and seas. One of the principal factors that affects Eustasy, and drives the changes shown in figure 1, is how much water is locked up on land in ice sheets.
Isostasy refers to the vertical elevation of the earth’s surface. In many ways you can think of it as a concept like buoyancy. The Earth’s crust floats on a liquid like layer called the asthenosphere. If a great weight presses down on it the surface will depress at the centre and bulge out at the margins. If that weight is taken off, the surface will bounce back. It is for this reason that in order to understand sea-level change we need to understand both Eustasy and Isostacy as although locking water up on land in glaciers will reduce the amount of water in the world’s oceans and seas (and thus lower sea-level) it will also depress the areas under the ice sheets, potentially raising relative sea-level locally.
Tectonics in this context refers to changes in the altitude of the earth surface caused by movements of the earth’s crust. For example, an earthquake along a faultline might see an area of land suddenly drop in elevation. This can very rapidly alter localised sea-level (either raising or lowering it). A good example of this is Port Royal in Jamaica. On the 7th June 1692 an earthquake led to the port site dropping into the sea. Fortunately for archaeologists this also led to the preservation of aspects of the 17th century port that might have been lost to subsequent development, with the Institute of Nautical Archaeology carrying out excellent excavations at the site in the 1980s through to 1990.
Thus the changes we are discussing can occur over very long periods of time, or in a matter of seconds. However the impact that changes in relative sea-level have had on the world people have lived in has been profound. In the last step of this section you will engage with a basic model of palaeogeographic change at a global level over the last 23,000 years to identify how this story played out in your part of the world.
© University of Southampton, 2016