the florida state university college of arts and sciences evolution of ...

ecent glacial era occurred about 3 million years ago with the formation of permanent ice sheets
at high latitudes (Lambeck et al., 2002). As the ice sheets grew and retreated, sea levels
fluctuated in response. Major sea-level cycles have occurred at approximately 100,000 year
intervals over the past 4 million years with amplitudes of 120-140 m (Lambeck et al., 2002).
Superimposed on these cycles are lesser cycles of a few tens of thousands of years (Lambeck et
al., 2002). This sea level history is reflected in the benthic oxygen isotope record in Figure 1.4.
Directly datable records of past sea levels exist only for the last glacial cycle from about 130,000
years to present (Lambeck et al., 2002). Any records prior to this time were either destroyed by
sea-level rise at the onset of deglaciation or by the advance of ice sheets during the lead up to
maximum glaciation (Lambeck and Chappell, 2001). Figure 1.8 illustrates the changes in sea
level that have occurred over the past several glacial cycles. Sea-level fall from 130,000 to
75,000 years (Marine Isotope Stage (MIS) 5) to the Last Glacial Maximum (LGM) was not
uniform, but oscillated rapidly with amplitudes of 10-15 m approximately every 4,000 years
(Lambeck et al., 2002). Five positive excursions of sea level have been identified within MIS 3,
which covered a period from 60,000-25,000 years ago (32,000; 36,000; 44,000; 49,000-52,000;
60,000 years B.P.) (Lambeck et al., 2002). The lowest sea levels during the last glacial cycle
occurred from 30,000 to 19,000 years ago (Lambeck et al., 2002). The onset of the LGM was
rapid, with sea level falling 30-40 m within 1,000-2,000 years (Lambeck et al., 2002). Post-
LGM sea-level rise was not temporally uniform.
An attempt has been made to relate sea-level fluctuations to oscillations in solar
insolation. Subtle changes in the sun’s brightness may have triggered drastic climate change
(Kerr, 2001). Solar activity acts on cycles of varying duration, including a 200-year cycle (Kerr,
2001). The mechanisms that control solar activity are not well understood. Solar activity could
be related to changes in the chronosphere that affect the UV region of the solar spectrum. This
affects ozone production and the stratospheric temperature structure (Hodell et al., 2001).
Another mechanism may be the effect of cosmic ray intensity on cloud formation and
precipitation. Changes in solar output may affect global mean temperature, humidity, convection
and intensity of Hadley circulation in the tropics (Hodell et al., 2001). Sea-level fluctuations
during the last glacial cycle may have responded to dominant oscillations in insolation with
periodicities of 40,000 and 20,000 years (Lambeck et al., 2002). Insolation minima at 140,000
and 20,000 years correspond to two associated glacial maxima.
11