Eustatic Sea Level Change Mechanisms

Eustatic sea-level changes occur on an oceanic to worldwide scale. They result from either a change in the volume of seawater, or a change in the size of the ocean basin that contains it. Even in these large-scale cases, however, mean sea level can vary from place to place due to local tectonic and hydrographic effects. Several processes that can cause worldwide changes in sea level are listed below.


















Glaciation

For a number of reasons, the volume of glacial ice near the poles waxes and wanes over time. As a result, water is alternately taken from or added to the world oceans. This can result in sea-level oscillations of up to 200 meters. For example, modern continental glaciers are 1.5 to 2.5 km thick and have a total estimated volume of 33 million km3. If we assume the maximum volume of Pleistocene glaciers to have been 71.3 million km3, Flint, 1971 then the difference is 38 million km3. Using the assumption that glacial water volume is 91.7% of the volume of sea water from which it is derived, a sea-level drop of 106 m can be accounted for by Pleistocene glaciation. Melting of the present Greenland and Antarctic glaciers would produce a sea-level rise of approximately 60 meters.

Since 18,000 years ago, sea level has been rising in response to the melting of glacial ice, mainly from the Laurentian and the Scandinavian ice sheets of northern Canada. Rapid changes in the oxygen isotopic composition of skeletal foraminifera and corals (melt water has a low O18/O16 ratio) indicate that this rise was not uniform; two major and several minor pulses of melt water were added to the ocean. Based on gaps in the branching-coral (Acropora palmata) record offshore of Barbados, Fairbanks 1989 proposed major meltwater pulses at about 14,000 and 11,000 yBP. Whatever the actual age or number of meltwater pulses, the main point is that sudden changes in glacial volume have resulted in significant and rapid transients in sea level.

In addition to sea-level changes caused by the transfer of seawater to and from glaciers, sea water volume can change due to thermal expansion and contraction -- for every 1o C decrease in the mean temperature, world-wide sea level will drop by 2 m. Fauna from Pleistocene ocean sediments suggest a 5 degree lower surface temperature at that time, which would have resulted in a 10-meter lower sea level due solely to thermal contraction.

Despite a growing amount of data, we are still have basic questions about glacially driven sea-level change, including the amount of rise and fall and the cause of glacial cycles. The astronomic control of glaciation proposed by Milankovitch 1941 has been generally accepted to explain Pleistocene glacial cycles. The concept is that the nature of earth's orbit around the sun plays a primary role in patterns of glaciation and and, therefore, sea level change. The amount of solar radiation reaching the earth at any latitude or season is determined by three aspects of the position of the earth in its orbit around to the sun. These include:

The earth follows an elliptical orbit around the sun (Fig. 2.3a). The shape of the elliptical orbit, which is measured by its eccentricity, varies from between one and five percent through time. During times of minimum eccentricity, solar radiation is more uniformly distributed through the year, and seasonality is less pronounced than during times of high eccentricity. The period between successive eccentricity maxima and minima is about 96,000 years.

The season when the earth is closest to the sun (the perihelion) changes over time (Fig. 2.3b). Because of this precession, the vernal equinox occurs at perihelion every 22,000 years with the autumnal equinox occurring midway between (i.e., 11,000). If the earth is closest to the sun during the northern hemisphere summer and farthest during the northern winter, then the northern hemisphere climate will have hot summers and cold winters. These conditions are not conducive to glaciation because cold winters are usually drier than mild winters (i.e., less snowfall), and snow will likely melt during the hot summer. In contrast, when the earth is closest to the sun during the northern hemisphere winter and farthest during the summer, milder climates prevail year round. As a result, the wetter winters and cooler summers encourage glacial accumulation.

Finally, the earth's axis of rotation can have a profound effect on global climate patterns. Presently the earth is tilted at 23.5 relative to the orbital plane of earth around the sun (Fig. 2.3c). As a result, latitudes north of 66.5N are totally dark during the northern winter solstice while latitudes south of 66.5S are in total daylight. This tilt varies by about three degrees, from 21.5 to 24.5. This tilt or obliquity, influences the distribution of solar radiation throughout the year and over the 41,000-year cycle from one extreme to the other. During minimum obliquity, there is slightly less seasonal variability in solar radiation that during maxima.

All three elements (orbital eccentricity, its precession and the changing tilt of the earth) influence the distribution of solar radiation reaching the earth's atmosphere, acting to increase or diminish seasonal differences and, therefore, the tendency for glaciation. Milankovitch 1941 mathematically combined these variations to produce a northern hemisphere climate curve that predicted a clear periodicity to the amount of incident radiation and, therefore, glacial cycles. He claimed that astronomic factors could introduce significant cyclic variation into the earth's climate even with no changes in solar activity or the earth's albedo and no changes in composition of the earth's atmosphere. His proposal did not deny that other factors could be important, but suggested that variations in earth's orbital elements alone could trigger significant climatic change.

A "Greenhouse effect" is the behavior of solar radiation when it interacts with gases in the Earth's atmosphere like carbon dioxide, methane and water vapor. The radiation absorbed by atmospheric gasses is infrared (IR) radiation. These atmospheric gases are known as "greenhouse gases." Recent studies have highlighted carbon dioxide and other greenhouse gases as central to our understanding of global climate change. Ice cores tells us about the Earth's past climate by small bubbles of air that were trapped in glaciers, that are actual samples of ancient air.

Deep cores in Antarctic ice show that the amounts of carbon dioxide and methane in the earth's atmosphere have varied for at least the last 160,000 years. These changes correlate closely with changes in global temperature and the elevation of sea level. These variations may have been caused by natural changes in the biosphere, circulation of oceanic deep water or volcanic activity.

More recently, global warming has been tied to the dramatic increase in carbon dioxide and chloroflorocarbons associated with growing industrial pollution, increased automobile emissions and the destruction of large tracts of forest, especially in the tropical regions of the planet. According to the report of the Intergovernmental Panel on Climate Change (IPCC), 1990 levels of carbon dioxide and methane were 26 and 115 percent, respectively above pre-industrial levels less than a century earlier. They continue to increase at rates of 0.5 to 0.9% annually. Because these "greenhouse gasses" tend to trap heat within the earth atmosphere, the expected result is an increase in the rate of sea-level rise. This is discussed in greater detail at the end of this chapter.

In contrast, the level of incident light and resulting global temperatures can be dramatically reduced by increased particulate material in the atmosphere during periods of unusually high volcanic eruptions or other large-scale events. For example, snowfall was recorded in the Bahamas as a result of the eruption of Pinatubo in the Philippines.

Tectonics

Crustal movements may be local or global in scale. For example, tectonic activity along mid-ocean ridges will cause eustatic sea-level changes that are expressed worldwide. Locally the coast may be raised or lowered relative to regional sea level by subsidence or uplift. The 1964 Alaska earthquake raised the coast by more than 5 meters.

The most common tectonic mechanism to impact global sea-level change is the movement of the earth's lithospheric plates. This includes the opening and closing of major ocean basins, the addition of new crust along mid-ocean ridges, and changes in continental or plate margins with subduction and isostatic adjustments to loading by new crustal material. At the same time, volcanic activity along the mid-ocean ridges (as well as subaerial vulcanism) releases juvenal water that is released from the rock during extrusion.

As sea-floor spreading proceeds, basin shape and volume changes. Near the ridge, the new and more buoyant sea floor rises. Near the oceanic margins the older and increasingly dense crust sinks or is subducted beneath adjacent plates. Whether sea-floor spreading causes a sea-level rise or fall is related to the spreading rate. Under conditions of rapid spreading, the buoyant crust has less time to cool (and sink) and sea level rises in response to the shallowing basin. In ocean basins where spreading is slow, crustal sinking dominates and sea level will fall.

While the origin of the Atlantic provided a new ocean basin, ensuing events also affected the bathymetry of existing basins and further changed the volume of the world ocean. The early Atlantic Ocean was shallow until mid to late Cretaceous time, gradually replacing the deeper Pacific Ocean. As the Atlantic Ocean continued to grow, changes in the rate of spreading and the volume of the mid-Atlantic ridge resulted in changes in the total volume of the ocean basins, thereby triggering changes in sea level. Similar tectonic processes have been present over the span of geologic time and probably account for most of the cyclicity of sea-level change in pre-Pleistocene marine sediments.

Tanner 1965 proposed that Pleistocene glaciations were triggered by the continents approaching their present configuration with the south pole over a landmass and the north pole situated in a shallow inland sea. The restricted heat transfer encouraged a much more zoned world climate. Glacial and interstadial episodes were then controlled by the diversion of bottom waters away from the Arctic Sea by features such as the ridge between Greenland and Scotland. More recent studies by George Stanley at Johns Hopkins University suggest an important role for the Isthmus of Panama as it formed 2-3 million years ago. These paleoclimate models highlight the potential importance of continental and oceanic evolution to the extent that they modify large-scale oceanic circulation. As our understanding of the relationship between this "oceanic conveyor belt" of Broecker (Lamont-Doherty Earth Observatory) and global climate grows, so does our appreciation of the complex interplay among tectonic, oceanographic and atmospheric factors.