Eustatic

The term Eustasie ( alt -gr. Εὖ (eu) good, genuine and στάσις ( stasis ) Stand ) describes in modern geosciences variations in sea level on a global scale.

History and Definition of Terms

The term " eustatic movements " comes from the Austrian geologist Eduard Suess (1888), the order for the advance and the retreat of the shorelines in geological time periods indicated. The cause of the retreat of the coast lines (negative eustatic movement ) associated with gaps or the widespread continental sediments in the geological tradition of the relevant period, he led to the sinking of the ocean basins. The advance of the coastlines, coupled with the spread of marine sediments in the geological tradition of the relevant period ( transgression or eustatic positive movement ), he explained, with the gradual recovery of previously sunken ocean basins by sediment and the resulting displacement.

Today Eustasie or eustatic sea-level fluctuations are defined as variations in sea level relative to a fixed point of geological processes is not affected, where it is the center of the Earth In the latter usually.

Causes

As causes of eustatic sea level changes two key factors are:

It should be added that in principle any of the acts hereinafter enumerated mechanisms on their own but that they overlap each other, which can lead to amplification or attenuation effects occur.

Changes in the sea water volume

The most important mechanism for this is the binding or release of water into and out of continental ice sheets, as Glazialeustasie or glacio - Eustasie called. This can lead to variations in sea level of the order of 10 to 100 meters within 1000-100000 years.

During the last global glaciation in the Pleistocene large quantities of water of the hydrosphere were bound in the ice in the northern hemisphere. Thus, the sea level was located up to 130 m lower than today. The release of the Pleistocene glaciation in the recent ( named in reference to North and northern Central Europe Weichsel glaciation ) bound in ice water masses led, inter alia, in present-day North Sea basin, which was dry during the Ice Age, the sea level rise. This regional event with glazioeustatischem background is known as the Flanders transgression. Even today, the enormous amounts of water in the ice sheets that cover Greenland and the Antarctic continent. A complete melting of ice caps this would lead to a eustatic rise of about 73 meters.

Less dramatic fluctuations in call volume expansion or shrinkage of the sea water body by changing the water temperature ( has the warming of the entire water body of the world's oceans by 1 ° C a eustatic sea-level rise of 70 cm to the sequence) and the binding or release of water on the continents in and from groundwater reservoirs and surface water out ( a few meters in periods of 100 to 10,000 or 100,000 years )

Changes in ocean basin volume

The most important reasons for this are variations in seafloor spreading rate. Any increase in the rate of seafloor spreading, that is, created at the mid-ocean ridge (MOR ) more new oceanic crust or lithosphere rather in a shorter time, then increases the percentage of the seabed, which is occupied by young oceanic lithosphere. Since young lithosphere has a lower density than old and thus is relatively easy, it is higher on the asthenosphere as old lithosphere, which displaced much sea water from the ocean basins and is pressed onto the continents. In periods of lower sea floor spreading rate, there are fewer young oceanic lithosphere. The total volume of the ocean basins is then larger and provides the global ocean body more space. Variations in seafloor spreading rate occur in orders of magnitude 100000-10 million years and can lead to eustatic sea-level changes of several 10 to several 100 meters. The relationship of seafloor spreading rate, volume of the ocean basins and Eustasie is also called tectono - Eustasie.

The entry of sediment into the ocean basins, nor suspected of E. Suess as the main factor of the rise of eustatic sea level, is actually responsible for the change in volume of the ocean basins, only a very small extent. His influence on the Eustasie is a maximum of every 10 meters sea level fluctuation over a period of some 10 million to some 100 million years.

Over the past 500 million years the Earth had most of the time no large continental ice sheets, which is why tectono - Eustasie likely to be the dominant factor for geologically significant global sea level changes. One of the geological ages with the highest global sea was the early Cretaceous ( in the literature often mid- Cretaceous, " means chalk ", called ). It is believed that the eustatic sea level was 100 meters or more above the present level, which can not be explained solely by the melting of continental ice sheets, especially since there was the earth throughout the Mesozoic in a greenhouse climate phase, larger ice sheets So anyway not available may have been. Instead, an increased seafloor spreading rate is suspected as the cause.

Geoidal - Eustasie

This run also under the term Eustasie phenomenon does not affect the global sea level but the distribution of sea water in the ocean basins as a function of gravity anomalies of the Earth, which in areas with a positive gravity anomaly slightly higher and in areas with a negative gravity anomaly has slightly lower sea level resulted. Since the gravitational field of the earth is not only regional variations but also changes for various reasons over geological time, also the distribution of positive and negative gravity anomalies at the surface and be in accordance with the oceans geoidal - eustatic sea level fluctuations changes, in which case it also regional differences are, if the sea level rises or falls. This in turn implies that, for example glacio - eustatic sea level fluctuations and tectono - not actually can be globally uniform but regionally, depending on how the regional gravity field developed in the period, to be stronger or weaker.

Thus, the rapid melting of continental ice sheets, while resulting in an increase in the volume of the water body of the oceans, but it results in that region, which is freed from the ice, and to a decrease of the geoid (ie, a negative gravity anomaly ), as the isostatic crustal movements is compensated by the loss of mass ( the so-called post-glacial uplift ), clearly be slower than the melting. This in turn weakens a regional geoidal - eustatic sea-level waste to a result of the increase glazialeustatischen.

With increasing the amount of water in the oceans, which is caused glazialeustatisch essentially, it comes to an isostatic lowering of the seabed, an effect which is referred to as hydro - isostasy. In turn, raise the continents, namely by twice the amount of the reduction of the ocean floors. This can result in variations in the sediment tradition of oceanic islands and the continental shelves, since the former hydro- isostatic lowering of ocean floors to participate, but latter experienced a rise.

Relative sea level

The eustatic, globally changing sea level stands in relation to the so-called relative sea level. The term refers only to sea-level changes in a regional or local scale and plays in particular in the sequence stratigraphy an important role, where he alternatively as accommodation space: is called (English accomodation space). It is not relative to geologically stable fixed points " measured " but relative to an arbitrarily given area within the earth's crust (English: local date ), which may be the surface of the basement of a sedimentary basin, for example. It is the in the regional or local sediment tradition of direct reading sea level, which is influenced by the various forms of Eustasie as well as by regional tectonics (mainly the raising or lowering of the depositional environment ) ( see also → transgression → ingression and → regression). True eustatic fluctuations can be identified only by a cross-regional comparison of sedimentary sequences of a particular geological time period. Another significant difference for Eustasie is that the sedimentation rate has a very significant influence on the relative sea level, although, in contrast to Eustasie, excessive sedimentation not, here an increase but to a drop in sea level that is leads to a retreat of the shoreline or at least the decrease in water depth in the observed depositional environment.

The investigation " middle Late Cretaceous " sedimentary sequences revealed that the relative sea level at that time was locally up to 250 m above the present value. It is therefore assumed that played in the flooding continues continental areas in the early Cretaceous, a generally enhanced subsidence of the continents during the last phase of the breakup of Pangaea a certain role.

The sea level variations in the Baltic Sea over the past 10,000 years are changes in the relative sea level and not solely due eustatisch. So here play the sediment supply by rivers leading- and neotectonics ( tectonic movements that are generated by the currently prevailing crustal stress field in the corresponding crust ), plus other factors play an important role. Furthermore, Scandinavia raises after melting of the ice sheets of the last glacial period ( post-glacial uplift, see → Geoidal - Eustasie ) a few millimeters per year, resulting in a retreat of the sea on the northern Baltic Sea coast.