Sea Level Processes and Indicators
129
when sea level rises globally. In a very simplified way, one might say that climate is
mild during times of high sea level, and harsh during times of retreat of the oceans
into its basins. Sea level fluctuations, then, are closely tied to paleoclimatic evolution.
Global changes in sea level also have important bearing on the production of
hydrocarbons and coal. Sediments rich in organic matter, which have the potential to
deliver petroleum when heated under pressure (Sect. 10.2) accumulate especially
during periods when the continents are extensively flooded. The black shales of the
lower Jurassic and the middle Cretaceous derive from such widespread transgressions. Coal deposits typically form during more regressive periods, when lowlying coastal plains are available for growth of vegetation in swamps and moors.
Thus, sea level fluctuations are of interest in economic geology.
Quite generally, the rates of erosion of the continents and the sites of deposition in
the ocean depend much on continental relief and position of sea level with regard to
the shelves. How sediment is transported to the deep ocean is also controlled by sea
level. During high stands, the transport of sediment by turbidity currents will be
reduced. Turbidity currents depend on supply of mud to the outer edge of the shelf.
This supply is highest when sea level is low, and is greatly reduced during high
stands when submerged shelves and estuaries trap the material delivered by rivers.
Thus, the types of sediment bodies found at the base of the ocean margins should
depend on the history of sea level fluctuation.
Changes of sea level positions through time, then, are of fundamental importance
in the geologic sciences. The dynamics of coastal morphology, the mapping of sealevel indicators, the interaction of sea level and climate, and the origin of sediment
bodies in continental margins are topics which must be considered. However, sea
level is also of the utmost interest to people living at the coast: their livelihood and
their very survival depend on various manifestations of sea level change. This is true
for both short-term fluctuation (tides, storm waves, tsunamis) and long-term trends,
such as the isostatic uplift in Scandinavia or Canada, resulting from the removal of
the ice caps between 15 000 and 8000 years ago, or the steady sinking of major delta
regions (Mississippi, Nile, Ganges, Rhine). Also, the current general rise in sea level
due to global warming over the last 100 years which results in expansion of seawater,
is of concern. The rise is of the order of 2 cm per decade (Fig. 5.22).
We shall next review the evidence for sea level processes and fluctuations, starting
with the short-scale and the obvious, and proceeding to the Pleistocene and pre-Pleistocene changes. At the end we append a story on Venice: the old city is slowly but
surely sinking below the sea.
5.2 Sea Level Processes and Indicators
5.2.1 Wave Action. The most conspicuous indicators of sea level are those associated
with wave action. Familiar examples are wave-cut terraces, and all kinds of beach
deposits. We have already discussed some of the pertinent processes (Sects. 4.2.1 and
4.2.2).
Wave action leaves both erosional and depositional witnesses in the record. Many
coastlines are marked by a steep escarpment where the land ends and the sea begins.
129
when sea level rises globally. In a very simplified way, one might say that climate is
mild during times of high sea level, and harsh during times of retreat of the oceans
into its basins. Sea level fluctuations, then, are closely tied to paleoclimatic evolution.
Global changes in sea level also have important bearing on the production of
hydrocarbons and coal. Sediments rich in organic matter, which have the potential to
deliver petroleum when heated under pressure (Sect. 10.2) accumulate especially
during periods when the continents are extensively flooded. The black shales of the
lower Jurassic and the middle Cretaceous derive from such widespread transgressions. Coal deposits typically form during more regressive periods, when lowlying coastal plains are available for growth of vegetation in swamps and moors.
Thus, sea level fluctuations are of interest in economic geology.
Quite generally, the rates of erosion of the continents and the sites of deposition in
the ocean depend much on continental relief and position of sea level with regard to
the shelves. How sediment is transported to the deep ocean is also controlled by sea
level. During high stands, the transport of sediment by turbidity currents will be
reduced. Turbidity currents depend on supply of mud to the outer edge of the shelf.
This supply is highest when sea level is low, and is greatly reduced during high
stands when submerged shelves and estuaries trap the material delivered by rivers.
Thus, the types of sediment bodies found at the base of the ocean margins should
depend on the history of sea level fluctuation.
Changes of sea level positions through time, then, are of fundamental importance
in the geologic sciences. The dynamics of coastal morphology, the mapping of sealevel indicators, the interaction of sea level and climate, and the origin of sediment
bodies in continental margins are topics which must be considered. However, sea
level is also of the utmost interest to people living at the coast: their livelihood and
their very survival depend on various manifestations of sea level change. This is true
for both short-term fluctuation (tides, storm waves, tsunamis) and long-term trends,
such as the isostatic uplift in Scandinavia or Canada, resulting from the removal of
the ice caps between 15 000 and 8000 years ago, or the steady sinking of major delta
regions (Mississippi, Nile, Ganges, Rhine). Also, the current general rise in sea level
due to global warming over the last 100 years which results in expansion of seawater,
is of concern. The rise is of the order of 2 cm per decade (Fig. 5.22).
We shall next review the evidence for sea level processes and fluctuations, starting
with the short-scale and the obvious, and proceeding to the Pleistocene and pre-Pleistocene changes. At the end we append a story on Venice: the old city is slowly but
surely sinking below the sea.
5.2 Sea Level Processes and Indicators
5.2.1 Wave Action. The most conspicuous indicators of sea level are those associated
with wave action. Familiar examples are wave-cut terraces, and all kinds of beach
deposits. We have already discussed some of the pertinent processes (Sects. 4.2.1 and
4.2.2).
Wave action leaves both erosional and depositional witnesses in the record. Many
coastlines are marked by a steep escarpment where the land ends and the sea begins.