Geol 135 Sedimentation
J Bret Bennington
Eustatic Sea Level Change
As we have learned from our discussion of sequence stratigraphy, sea level change exerts an important control on the deposition of sedimentary strata. In addition, the major discontinuities used to define allostratigraphic units are caused by sea level changes:
Changes in sea level depend on both local and global controls. Global controls are particularly important because they may create allostratigraphic boundaries that are globally correlatable - providing an important chronostratigraphic tool.
Global or eustatic sea level changes controlled by two factors:
Each of these mechanisms operates within a characteristic time scale and produces a characteristic range of sea level change. Operating in concert, the effect is to produce a wide range of overlapping cycles of sea level change, each with a characteristic order (time scale) and magnitude (depth).
Five Orders of Cycles
Beginning in the 1970's, researchers at Exxon used seismic survey data to define unconformity bounded units in cross section. By correlating these units from region to region and by comparing patterns of stratigraphic relationships, Exxon researchers were able to derive a continuous curve of eustatic sea level for the Phanerozoic. The original Vail sea level curve and updated Exxon curves, although not accepted as accurate by all geologists, do suggest that sea level has fluctuated through the Phanerozoic at a variety of cycle orders.
1st Order: Two first order cycles are recognized, lasting from 200-400 million years. These cycles have been related to changes in the volume of the ocean basins caused by continental dispersal and collision. High rates of seafloor spreading during continental dispersal reduce ocean basin volume, leading to high sea level. During supercontinent formation, sea floor spreading is reduced, the mid-ocean ridges subside, and ocean volume increases, lowering global sea level.
2nd Order: In the 1950's Sloss defined six unconformity bound stratigraphic sequences in the Phanerozoic rocks of North America spanning 10s-100 m.y., which he named after North American indian tribes. These sequences were later correlated with those described from the Russian platform, demonstrating their eustatic nature.
Second order cycles have now been correlated between four different continents, demonstrating their eustatic nature. It is now widely accepted that these cycles are caused by changes in the volume of oceanic ridges, related to changes in spreading rates.
3rd Order: These cycles have durations of 1 to 10 m.y. and are typically about 2.5 m.y. in length. The cause of third order cycles is controversial. They are too short to attribute to most tectonic events affecting the volume of the ocean basins, but too long to attribute to glacio-eustacy. Other tectonic events, such as changes in plate stress and regional tectonism, are of the correct duration, but would not generate sufficient change in water depth to account for the magnitude of the third order cycles.
4th Order and 5th Order: 500 ky - 200 ky and 200 ky - 10 ky cycles that are widely documented in many periods of the Phanerozoic in both shallow marine and pelagic strata. These cycles are most likely the result of changes in global climate driven by cycles in the Earth's orbital parameters and tilt, collectively referred to as Milankovich Cycles.
Climate changes generate sea level changes by altering the temperature and salinity of the oceans and by changing the balance of freshwater stored on land as lakewater, groundwater, and glacial ice.
Tidal cycles: Local sediment cycles have been noted in the rock record that can be related to ocean and lake tides generated by the orbital cycles of the earth-sun-moon system. These cycles have periods ranging from daily to annual.
To understand some of the important causes of eustatic sea level change on a wide variety of scales we need to look beyond the surface of the Earth and into the solar system.
Planetary orbits are elliptical, having a semimajor axis, semiminor axis, and two foci. The Sun occupies one focus of the ellipse. When the planet is at the point in its orbit where it is closest to the sun it is at perihelion. When it is at the farthest point from the Sun in its orbit it is a aphelion. For the Moon, the analogous points are called perigee and apogee.
The Earth rotates on its N-S axis as it revolves around the Sun. The imaginary plane formed by the path of the Earths orbit is called the ecliptic of the solar system.
Earth - Moon relationships
The moon orbits the Earth every 28 days. This results in the phases of the moon as seen from the Earth:
On the Earth the Moon causes tides. Because of the interaction between gravity and centripedal acceleration there are two tidal bulges that the Earth rotates through each day. This produces a diurnal tidal cycle with two high and two low tides each day.
The size of the daily tides is affected by a number of variables in the Earth-Sun-Moon system:
New Moon - Sun - Moon - Earth alignment = syzygy of the new moon
Full Moon - Sun - Earth - Moon alignment = syzygy of the full moon
Apogee and Perigee - The Earth - Moon distance changes as the Moon traverses its elliptical orbit. During each month there is an apogee point and a perigee point.
Lunar Declination - The plane of the Moons orbit is at an angle of 5° from the Earths orbit. This causes the Moon to reach a maximum north and a maximum south position relative to the Earths equator once each month. The two points where the Moons orbital plane intersects the Earths form the Lunar Node. As viewed from above, the lunar nodes rotate counterclockwise, taking 18.6 years to return to their original positions.
Diurnal - The Earth experiences two tidal cycles every 24 hours. The two high tides will be of unequal amplitude (as will the two low tides) because lunar declination affects the position of the tidal bulges, moving them north and south relative to the equator on opposite sides of the Earth. This is called diurnal inequality. Diurnal inequality disappears when lunar declination is zero - at the lunar nodes.
Spring-Neap - Tidal amplitudes are highest when the Moon is new or full (in syzygy), causing bimonthly spring tides. During first and last quarter the amplitudes are lowest when the Earth, Sun, and Moon form a right angle, causing bimonthly neap tides.
Syzygy-Perigee - Exceptionally high tidal amplitudes occur when perigee coincides with syzygy. This occurs every 7 months, producing high spring tides.
Lunar nodal cycle -The lunar node rotates once every 18.6 years in the nodal cycle. This means that once every nine years the lunar node is oriented toward the Sun, causing syzygy to coincide with zero declination of the Moon.
Earth - Sun orbital relationships
The equator of the Earth is tilted relative to the ecliptic. This axial tilt is currently about 23°. The axial tilt is primarily responsible for the Earths seasons. Thus, northern hemisphere winter is caused by the fact that at this point in the Earths orbit the northern hemisphere is tilted away from the sun and receives indirect sunlight over a shorter day. This causes cooling in the northern hemisphere in spite of the fact that the Earth is near perihelion at this time.
The various orbital characteristics of the Earth - Sun pair are not stable over long periods of time due to the influence of the other planets in the solar system.
Change in the ellipticity of the Earths orbit - The ellipticity of the Earths orbit changes from nearly circular to about 5%. The length of time this change takes is about 93,408 years.
Progression of the longitude of perihelion - This is caused by the semimajor axis of the Earths orbit pivoting around the plane of the ecliptic. It takes 93,408 years for perihelion to return to its original point.
Change in the Earths axial tilt - The degree of axial tilt of the Earth varies from 22° 29 36" to 23° 50 30" on a cycle of 17,280 years.
Precession - The rotating Earth wobbles much like a childs top such that the point in space toward which the polar axis points changes over time. Currently the pole points toward Polaris - the so called north star. However, in 12,000 years another star, Vega, will be the north star. In 24,000 years it will be Polaris again. The time needed for the Earth to complete one precession cycle is 25,920 years.
Precession and the axial tilt cycle interact such that the Earths pole returns to the same position and degree of tilt every 52,000 years.
But what, if anything, do these Earth - Sun orbital parameters have to do with sea level change? A possible answer to this question was formulated in the 1930s by a Serbian physicist named Milutin Milankovitch. Milankovitch calculated how the interactions of the various orbital cycles would affect the amount of sunlight reaching the Northern and Southern hemispheres. What he discovered was a periodicity of warming and cooling that seemed to match the timing of glacial episodes known to have occurred back through the Pleistocene. Milankovitch proposed that orbital cycles were, in effect, beating out the rhythm of the ice ages.
Milankovitchs ideas were initially rejected for two reasons:
1. His predictions indicated that warming and cooling alternated between northern and southern hemispheres. Yet no Pleistocene glaciations occurred south of the equator. Pleistocene glaciations are a Northern Hemisphere phenomenon.
2. 10,000 years bp is predicted by Milankovitch to be a peak warm period. Yet sea level at this time is known to have been low, indicating the presence of large amounts of glacial ice.
These problems have still not been completely resolved, but instead are now seen as less of a problem for Milankovitchs ideas because other data have been gathered that strongly suggest that he was at least mostly correct.
Deep sea sediment cores from the Pleistocene
Studies over the last two decades of cores taken from the sediments of the deep sea have given geologists a very accurate record of climate change over the last 200,000 years. The volume of glacial ice can be estimated based on the oxygen isotope record in shells of benthic foraminifera in deep sea sediments. This record has demonstrated that climate oscillations have occurred through the Pleistocene at periods that match those of the Milankovitch orbital parameters. This correlation has convinced many geologists that orbital parameters do indeed control the pulse of the ice ages.
Not all oxygen atoms are the same. In nature, two isotopes of oxygen exist - O18 and O16 - which differ from one another only in their atomic mass. Water contains a constant ratio of the the two oxygen isotopes, with O16 being much more abundant than O18 . When water evaporates, the light isotope, O16 , is more readily able to escape from the sea as vapor, so that evaporaton leaves the sea enriched in the heavier isotope. Because water vapor soon returns to sea as rainfall or runoff and mixes back in, the normal ratio of O16 to O18 is maintained. However, if the evaporating water falls as snow and becomes glacial ice, it does not return to the sea, leaving the oceans enriched in O18 . The more glacial ice that forms, the more enriched in the heavier oxygen isotope the oceans become.
Foraminifera are tiny one-celled animals that live throughout the oceans. Some forams live on the bottom of the ocean, others float around near the surface in the plankton. Forams construct tiny shells of calcium carbonate, using oxygen dissolved in the oceans. Their shells trap both oxygen isotopes in whatever ratio exists in the water, preserving the isotope ratio in the shell. When forams die, their shells become buried and preserved in the layers of mud accumulation on the ocean floor.
4 major Milankovitch cycles
There are 4 major climate cycles that recur with periods between 20,000 and 400,000 years - the so called "Milankovitch Bandwith"- which are collectively referred to as Milankovitch Cycles. These climate cycles result from the actions and interactions of the Earth's orbital cycles. The strongest control on climate is probably the summer length and temperature in which ever hemisphere is the most land-dominated, because this has the greatest impact on the buildup of glacial ice from season to season.
Over the last 700 ka major glaciations have waxed and waned with a periodicity of about 100 ka, reflecting the dominant influence of the eccentricity cycle. Prior to 700 ka, glacial cycles occurred with a periodicity of about 40 ka, reflecting the dominant influence of the obliquity cycle. It is not clear what caused the change in the dominant climate cycle.
Milankovitch period cycles prior to the Quaternary
Major episodes of continental glaciation have been documented from the geologic record for the Late Proterozoic, Ordovician, Pennsylvanian, and Quaternary. For all of these times, transgressive-regressive cycles have been documented with inferred Milankovitch periodicities. However, Milankovitch cycles have also been documented in strata from times when the earth is thought to have been mostly ice-free (the Triassic, the Cretaceous) and it is not clear what mechanism is generating these cycles. It may be that the groundwater reservoir is capable of storing enough water during intervals of wet climate to cause significant reductions in the volume of water in the world's oceans. It is also possible that assumptions about the ice-free nature of the earth at these times are in error.
Why Milankovitch cannot explain it all
Many other factors besides changes in insolation (total amount of solar heat and its distribution across the globe) must affect climate cycles. Changes in CO2 levels in the atmosphere, changes in oceanic circulation, changes in continental positions, changes in albeido, and others must interact with insolation in ways that are only beginning to be understood.
It should also be noted that Milankovitch band periodicities in sea level change have been noted in rocks deposited during times of warm global climate and NO GLACIATION. Thus, there must be other mechanisms besides ice volume that can link climate to sea level. Dave Jacobs and Dork Sagian have proposed that Milankovitch band sea level changes might be caused by variations in monsoonal precipitation that fill and empty lake basins. It is also possible that long term changes in precipitation levels create cycles by altering the amounts of sediments delivered to depositional basins.