Sea Oxygen-18, and also that the ocean is

Sea level changes
have occurred throughout Earth’s history, with the causes of the changes varying.  Locally, sea level may change if
tectonic forces cause the land to shift up or down.  However, in terms of global changes
in sea level, the variations must be due to either changes in the volume of
water in the oceans or changes in the volume of the ocean basins (Kominz,
2001). 

Changes in volume of water in the oceans

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            The
two largest reservoirs of Earth’s water are its oceans (97%), and its glaciers
(2.7%).  Throughout the last three
billion years, the main variable influencing these percentages was the amount
of water that was being held in glaciers on the continents.  When ice ages occur, and glaciers remove a
large amount of water from the oceans, continental shelves can be exposed.  Inversely, as these glaciers recede, melt,
and release their previously landlocked water back into the oceans, sea level
will rise (Kominz, 2001).

Several different
methods have been used to determine the magnitude and timing of historical sea
level change such as dredging of continental shelves, studying ancient corals,
and carbon dating.  These methods have
shown that human activity used to occur near the present shelf-slop boundary, and
that fossils of wildlife only found in shallow water are now covered by over
100 meters of water.  The conclusions
made based on the aforementioned methods explain that in the near distant past,
sea level was substantially lower than it is currently (Kominz, 2001). 

When looking further
back into the record of sea level change, the stable isotope, Oxygen-18, is a
valuable tool in estimating actual sea level changes.  Water in the atmosphere typically has a lower
Oxygen-18 to Oxygen-16 ratio because evaporation of the lighter isotope
requires less energy.  This is
significant because it means that the snow that accumulates in glaciers is low
in Oxygen-18, and also that the ocean is proportionally enriched with the
heavier isotope.  Since marine organisms
make their shells out of the calcium, carbon, and oxygen that are present in
the sea water at that specific time, the ratios of Oxygen-18 to Oxygen-16 found
in the organisms’ shells differ based on changing oceanic conditions.  When a marine organism dies, it sinks to the
bottom of the seafloor where it decays and leaves behind its shell to become
part of the sedimentary record.  By using
this varying isotopic ratio and applying it to oceanic sediments and fossils,
scientists can estimate when sea level was rising or falling due to
glaciation.  Unfortunately, the amount of
Oxygen-18 present in an organism’s shell is not solely due to the amount of
Oxygen-18 present at the time of shell formation, but can also be attribut