Historical Geology/Sea level variations

Changes in global sea level are important both as a symptom and as a cause of climate change. In this article we shall look at what causes changes in global sea level, and how they can be detected in the geological record.

Causes of sea-level changes

One cause of sea level variations is the formation or melting of continental glaciers. It is estimated, for example, that if the ice sheets currently covering Greenland and Antarctica were to melt, the global sea-level would rise by approximately 70 meters.

Another factor that affects global sea level is the mid-ocean ridges. The more active they are, the larger they are, and the more water they displace. The ridges can also change in total length: when a supercontinent rifts apart, a mid-ocean ridge will be produced where it rifts; the production of the mid-Atlantic ridge by the rifting of Pangaea is the most recent example. As the ridge is in effect a mountain range 40,000 miles long, its formation clearly displaced a massive amount of water. Hotspots producing islands such as Hawaii have a similar though lesser effect.

The collision of continents can reduce sea-level by reducing the area of the continents as they compress into one another. For example, it has been estimated that the collision of India with south Asia and the compression resulting in the raising of the Himalayas would have lowered the sea-level by about 10 meters.

Local variations

The position of the shoreline can change as a result of the movement of the land, as a result of tectonic events or of isostatic rebound after depression caused by glacial cover. This means that there's no use in just looking at one or two places to estimate sea levels in the past, because what looks like a global fall in sea levels might in fact be a local rise in the level of the land.

Such local effects did in fact confuse the 18^th century scientist Anders Celsius, whose measurements of the sea level along the Scandinavian coast convinced him that the seas must be shrinking. He was not to know that in fact Scandinavia was slowly rising out of the sea as a result of isostatic rebound.

To identify global changes in sea-level, it is necessary to look beyond these localized changes and see the bigger picture.

The Vail and Hallam curves

The Vail curve representing past global sea-levels is based on the work of Peter Vail and his associates. It is based on the study of unconformities in the geological record representing times of marine regression. An important part of Vail's work was establishing that these unconformities could be identified underground by the use of reflection seismology, a method which relies on studying the reflections of seismic waves artificially generated by producing small explosions.

The Vail curve has one major problem, from a methodological point of view: it is based on proprietary data of the Exxon-Mobil corporation which the general public is not allowed to see. This doesn't necessarily mean that the Vail curve is wrong, but it does mean that if some aspect of it is wrong, if the data on which it's based are incorrect in some way, or if Vail has somehow misinterpreted the data, then no-one would be able to tell. This makes it different from other results in geology, the quality of which is insured by the ability of scientists to cross-check each other's work.

The same problem does not arise with the Hallam curve, the brainchild of Anthony Hallam. This was constructed on the basis of a different methodology: Hallam calculated the area of the continents covered by the sea over the course of time by seeing where marine sediments were and were not deposited at various times, allowing him to sketch out a series of shorelines for the various continents over time; clearly when the land area shrinks globally, this corresponds to a global rise in sea level.

The Vail and Hallam curves.

The Hallam and Vail curves are presented in the chart to the right. Note that these are "first order" curves which only show broad trends: they are not on a fine enough scale to show the fluctuations caused by recent advances and retreats of sheet glaciers.

The reader will note the broad similarities between the two curves, which suggest that Hallam and Vail are basically on the right track, especially since this agreement is produced by two different methods. But the reader will also note the differences in detail, which suggest that one curve or the other, or probably both, are not completely accurate.

Their broad accuracy is confirmed by the fact that we see in them the features that we would expect to see; for example, we see a fall in sea level accompanying the assembly of Pangaea, and a rise in sea-level corresponding to the fracturing of Pangaea and the formation of the Mid-Atlantic Ridge. We can also see falls in sea-level corresponding to known times of glaciation.