Sea level changes during the Ice ages

1.6.2 Sea level changes during

deluge myths

Places where deluge myths are preserved

© 2011 Hans van der Baan / Ingeborg Scheffers

Fig. 1.57 The Serapeum, a temple from late Roman times near Pozzuoli in Italy, now located at groundwater level. The columns of the left part of the picture show dark rings with marine borings from a time when the sea inundated the temple. As it certainly was built on dry land, it provides evidence of several yo-yo-like earth movements during the last 2,000 years in this region. (Photo credit: D. Kelletat).

Fig. 1.56 Places on earth where deluge myths are preserved.

an irregular transition into the last colder (glacial) period, with the growth of ice sheets and fall of sea level culminating in the Last Glacial Maximum (LGM) 21,000 to 18,000 years ago. At that time, sea level was 121 to 125 metres lower than it is level changes can be observed and deciphered in our coastal landscapes even today (Figs. 1.47–1.55).

With the onset of a warming climate after the LGM, the large continental ice sheets and glaciers began to melt and globally sea level rise continued until around 6000 years ago.

There were, however, intermittent periods of more rapid phases of sea level rise triggered by more rapid temperature change, or by meltwater pulses caused by calving of large ice masses in the Antarctic and in the glacial regions of the -sive natural meltwater reservoirs which had been formed by retreating inland glaciers. Overall, the postglacial sea level rise started slowly, acceler-ated in the mid part of this time span and slowed down again by about 8000 BP. Consider that in a time span of about 8000 years sea level rose by 120 m, which equates to 15 mm/year on average or a rate of approximately 1°/1.5 m per century, with occasional sharp increases. During the two periods of melt water pulses, each lasting around 300 years, sea level rose by 5 metres per cen-tury! Beside the average rates of rise, there have been periods that lasted for centuries during which sea level rose ten to twenty times faster corresponding to a sea level rise of many centi-metres/year. Nearly all coasts have thus experi-enced submergence or “drowning” and evidence for lower glacial sea levels is found seaward of present coastlines out to depths of 100 m or more.

of Applied Sciences by Adrian Meyer and Karl Rege. It shows the earth starting at the last glacial maximum 21000 years ago and ends 1000 years in the future (for reference visit: Blue Marble 3000;

http://radar.zhaw.ch/bluemarble3000_en.html).

-oped cultures such as in Mesopotamia and the deltas of the Nile, Indus and the Yellow River, the postglacial sea-level rise resulted in a pro-gressive inundation of the land which may have encroached many hundreds of metres or even kilometres per year. During a number of

genera-tions areas which were formerly use for coastal living, hunting and the development of primitive agricultures would have thus been drowned. This must have had a profound effect on the Mesolithic and early Neolithic cultures and societies. It is at least a possibility that the widespread myths of a deluge have their roots in this natural process (Fig. 1.56). We can more easily understand these myths if we consider the very large number of submerged or partly drowned relicts of ancient times (Fig. 1.57). Artefacts from those times have areas of Mesolithic settlement.

Although this post-glacial sea level rise has been a topic for research for many years, there are a number of unresolved issues, for exam-ple: (1) During the mid-Holocene, sea level was higher than present in some regions, but the exact timing, duration and magnitude of this high stand remains unclear; (2) the nature of sea level chang-es on millennial scalchang-es – debatchang-es exist between models of a smoothly falling sea level since the mid-Holocene high stand or a step-like recession;

and (3) sea-level variations on shorter time scales, e.g. at decadal to century resolution relevant to human society and usable for risk assessment purpose.

change within seconds or hours during short-period sea level variations caused by waves, tides or storm surges. Irregular but frequently occur-ring variations in sea level are triggered by air pressure: under an atmospheric high pressure system sea level will be lowered, whereas sea level is higher than normal in the centre of a low pressure system like a cyclone. Variations of one hectopascal (hPa) of pressure normally results in 1 cm of sea level (rise or fall). This is true for the open ocean as well as for the coastline, but at the coastline the effect may be stronger and more dangerous. During the passage of a hur-ricane over the shoreline not only low pressure raises sea level, but also wind forces push water towards the coast, producing a storm surge.

During Hurricane Katrina in 2005 the height of the associated storm surge reached around ⁺ 7 m in the Gulf of Mexico along the US south coast.

Tsunami waves, only decimetres high in the open ocean and triggered by earth movements, volcanism or massive slumping, may reach con-siderable run-up levels at the coast of 20 m or today. The signatures and fingerprints of these sea

Northern Hemisphere and by overflow from mas

A terrific visualization is the Google Earth Blue Marble 3000, developed at the Zurich University

been found in fisherman´s nets over many flooded

Let us briefly consider that sea level may also

In flat and low lying coastal areas with devel

even more. The highest run­up during the 2004 Indian Ocean Tsunami was 51 m above sea level at one point in north­western Sumatra,but around 30 m along several hundred kilometres of coast­

line. We will look more closely at marine natural hazards in Chapter 8 and discuss more recent and future sea level rise in Chapter 9.