Changing Sea Levels

between the highest and lowest tides. Rising or falling sea level can reshape the world’s coastlines

-lated areas on Earth. They include major agricul-tural, economic and important natural zones and cultural heritage sites. Sea-level rise is one of the most serious consequences of climate change and is predicted to rise dramatically before the end of the century. Not surprisingly, scientists want to understand sea level as thoroughly as possible.

Some causes and consequences of sea level varia-tions are well known and easy to detect, to meas-ure and explain, but others are rather complex and need sophisticated ongoing research.

Fig. 1.39 Worldwide, the highest spring tides of more than 15–20 m occur in Fundy Bay (eastern Canada). These images show a situation near high water and near low water, which in the inner part of the bay differs for 15 m or more in elevation. Length of the bay is more than 30 km. (Photo credit:

NASA ASTER Science Team).

classified by Davies (1980) into three groups:

mic-significance.

Sea level is defined as the mean height of water

and may influence some of the most densely popu

Sea level rises and falls as the temperature and salinity of the water column varies, which is known as steric sea level change. Sea level also changes as water is added by precipitation, ice melting, or river runoff, or if it is removed by evaporation or conversion to ice (Milne et al., 2009; Pirazzoli, 1991, 1996). The greatest changes in sea level occur over long geologic time inter-vals when changes in the balance between ice and water on Earth cause changes in the water volume of the oceans, as for example when glaciers and continental ice sheets wax and wane. Over the last 2.6 million years (the Quaternary Period), to the ice ages: during the colder (glacial) peri-ods, large continental ice sheets formed at higher latitudes, withdrawing water from the oceans, and sea level decreased dramatically all over the world. During the warmer (interglacial) periods, the continental ice caps and glaciers melted and sea level rose substantially again.

Plate tectonic movements also cause the vol-ume of ocean basins to change over Earth’s his-tory. Scientists refer to these changes that affect the ocean globally as eustatic sea level variations.

Over the span of a human life, these changes are imperceptible but on geologic time spans they are key processes in the evolution of coastlines. Sea level is also affected by isostasy, a principle that of wood or an iceberg on top of the astheno-of the object depends on its mass. The three to four kilometres thick ice sheets that formed

dur-12 7

7 15,6

6 6

6

12 10

5 5,4

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11 7

7

7 7 11

World spring tide range

Spring tide range

range in meters 5 m

3 m 1 m

© 2011 Hans van der Baan / Ingeborg Scheffers

Fig. 1.41 A beach ridge sequence deposited during a continuous glacio-isostatic uplift in northern Norway (Photo credit:

D. Kelletat).

Fig. 1.40 Spring tide range around the coastlines of the world (modified from Davies, 1977).

fluctuations in the water-ice balance occurred due

envisages the lithosphere “floating” like a piece sphere. Isostasy implies that the flotational height

ing the ice ages provide one great example. Due to their great weight on the land masses the conti-nental ice caps caused the Earth’s crust to sink for several hundred metres resulting in the

underly

-quence sea level rose relative to the land. When the ice melted, the land mass rose once more the underlying areas, resulting in recent uplift of these formerly glaciated regions.

Fig. 1.42 Map of glacio-isostatic uplifted pebble beach ridges from the Varanger peninsula, northernmost Norway with ages of 11,000 to about 8,800 BP (Kelletat, 1985).

Mapping of glacio-isostatic uplifted systems of pebble beach ridges from the Varanger peninsula, Norway

higher peneplain rocky outcrops steep rocky slopes frost debris cones fluvial terraces

sand and pebble on valley floors lakes and rivers

bogs and swamps pebble beach ridges inactive / active cliff boulder / pebble beach Store Molvic

Store Molvic

100 500

km

ing asthenosphere flowing outward. As a conse

and the asthenosphere began to flow back into

This phenomenon of isostasy may last over many thousands of years after melting of the ice sheets and can still be observed occurring in some areas of the Northern Hemisphere today. For example shorelines around the Northern Baltic Sea, the Scandinavian land mass or the shorelines around

Hudson Bay, North America are still rising up to 1 cm/y in the regions of the thickest former ice load.

Shorelines deposited about 10000 years ago, when global sea level was about 50 m lower, can now be found up to 200 m above sea level (Figs. 1.41–1.43, see also Kelletat, 1985; Pirazzoli, 1996).

Fig. 1.43 At Hudson Bay in Canada numerous glacio-isostatic uplifted beach ridges have been formed over time. Width of scene is about 30 km at 58° N and 93° W. (Image credit: © Google Earth 2010)

Fig. 1.44 Uplifted dead coral in Simeulue, Indonesia, from the Boxing Day earthquake on 26 December 2004. (Photo credit: Craig Shuman, Reef Check / Marine Photobank).

While the effect of

formerly glaciated areas and their vicinity, isostat-ic movements of the Earth’s crust are also caused by changes of mass due to the wearing down of mountain ranges and the deposition of sediment transport particles from land into the sea over tens of millions of years. On a smaller scale, regional

takes place in nearly all large deltas of the world.

Here, large thicknesses of sediment accumulate in a rather short time (some thousands of years) on a small part of the ocean crust resulting in coastal submergence. The associated relative sea level rise may reach many meters within a thousand years.

Tectonic movements of the land may cause uplift or subsidence and thus local emergence or submergence of the coastline. For example,

Fig. 1.46 The uplift of western Crete in 365 AD has exposed these cup-shaped micro-atolls of vermetids and calcareous algae (Photo credit: D. Kelletat).

Fig. 1.47 A similar phenomenon of seve-ral bio-erosive notches in limestone on the east coast of Rhodes island (Greece) (Photo credit: D. Kelletat).

Fig. 1.45 Notch and double algal rim at ⁺7 m asl in western Crete, Greece, up lifted on July 21st, 365 AD (Photo credit: D. Kelletat).

glacio-isostasy is confined to

loads on the ocean floor as weathering and erosion

subsidence due to sediment load on the sea floor

Fig. 1.48 The typical saw-tooth curve of glacio-eustatic sea-level movement between ice ages and warm phases during the last 2 million years (acc. to Shackleton, 1995).

main isotope stages

(high sea-levels and temperatures of the main interglacial periods)

0 3 3,5

4

5

6 12 16

7 9 11 13 15 17 19 21 25

4,5 5 5,5

0,5

million years 1

million years

©2011 Hans van der Baan & Ingeborg Scheffers

younger pleistocene

older pleistocene

brunhes m agnetic period

middle pleistocene

m atuya m agnetic period

Sea-level curve of the Pleistocene

glacial periods with low sea-levels

m/b: ca 0,78 Ma

earthquakes may be accompanied by a vertical motion as either uplift or subsidence. Two of the largest earthquakes ever measured in the instru­

mental record have been the Sumatra­Andaman earthquake of 26/12/2004 which had a magnitude of 9.1 on the Richter scale and the Japan Tohoku earthquake on 11/3/2011 with a magnitude of 9.0.

(The largest measured earthquake occurred in the subduction zone off the coast of Chile in 1960 and had a magnitude of 9.5 while the Great Alaska earthquake in 1964 had a magnitude of 9.2).

During the Andaman­Sumatra earthquake islands west of Sumatra were uplifted by 1 – 1.5 m, and former flourishing fringing coral reefs are now exposed above sea level (Fig. 1.44). The devastat­

ing earthquake off Japan’s coast in March 2011 caused subsidence along the East coast of Honshu (Japan’s main island) with a maximum subsidence of 75 cm, while the horizontal displacement has been of up to ca. 4.4 m eastwards.

Another well­studied historical example is uplift of the western parts of Crete and Rhodes

Island (Greece) during a massive earthquake on July 21, 365 AD. Within seconds the coastline of Western Crete was uplifted for as much as 9 m (Figs. 1.45 to 1.47). Evidence for this cataclysmic event are visi ble today in the coastal landscape in the form of notches or micro­atolls which formed at sea level and are now well above the present day shoreline.

Beside these sudden events, vertical tectonic movements along the margins of converging plates over longer geologic time spans may have uplift­

ed former beaches or coral reefs to positions far above modern sea level. This is the case for the Huon Peninsula in Papua New Guinea, Barbados, Haiti or parts of Cuba where flights of uplifted coral reef terraces occur many hundreds of metres above sea level. Because tectonic movements and eustatic sea level fluctuations may occur in the same or opposite directions, at different rates or simultaneously, it can be a very challenging exer­

cise for scientists to understand the sea level his­

tory of a certain coast.

Fig. 1.49 Evidence of a former lower sea level is present in this fresh water karstic spring (smooth place in the water) in the Bay of Argolis, Peloponnese, Greece. Karst solution forms were developed down to sea levels of glacial times, which were more than 100 m lower than today. (Photo credit: D. Kelletat).

Fig. 1.51 A notch has been carved by pebbles moving in the surf about 125,000 years ago on the south coast of Crete, Greece. (Photo credit: A. Scheffers).

Fig. 1.53 The horizontal cover on the sloping strata on Ibiza Island are beach deposits from a former interglacial (warm) sea level phase. (Photo credit: D. Kelletat).

Fig. 1.52 A sharply incised bio-erosive notch in coral limestone from a former sea level highstand (warm phase 125,000 years ago), Bonaire, southern Caribbean. (Photo credit: S. Scheffers).

Fig. 1.50 Caves in eastern Sardinia (Italy) show two smooth carved notches as evidence of formerly higher relative sea levels.

(Photo credit: D. Kelletat).

1.6.2 Sea level changes during

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