Orbital Forcing of Pleistocene Climate

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Orbital Forcing of Pleistocene Climate
Dominance of the 41kyr Cycle Sarah
Mar-Gerrison
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Evidence of a 41kyr Cycle

• Between 3Ma and 0.8 Ma, benthic d18O records
  (fig. 1) show that
• global ice volumes varied almost exclusively in
  response to a 41kyr cycle (corresponding to the
  obliquity cycle), with little/no correlation to
  the 23kyr and 19kyr cycles (precession cycles)
• total ice volumes were less and/or global
  temperature was higher
• models must be used to try to establish cause of
  this effect

Fig. 1 Imbrie, Berger Boyle et al. 1993
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East Antarctic Ice Sheet Margin Model Raymo,
Lisiecki Nisancioglu, 2006

• Pre 3Ma
• d18O was depleted by gt0.5, from which, Earth is
  estimated to have been 3C warmer (Mid-Pliocene
  Thermal Maximum around 3.3-3.0 Ma)
• Ice volumes were also thought to be less due to
  evidence for higher sea level coastal terraces
  indicate 3518m higher water and Pacific atolls
  indicate up to 25m higher water
• 3Ma1Ma
• In their model, it is hypothesised that between
  roughly 3-1Ma, the East Antarctica Ice Sheet
  (EAIS) behaved more dynamically, similar to the
  Greenland or West Antarctic Ice Sheet (WAIS)
  today, where temperature straddles the region
  over which accumulation ablation (fig. 2)

Fig. 2 Raymo, Lisiecki Nisancioglu, 2006
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East Antarctic Ice Sheet Margin Model Raymo,
Lisiecki Nisancioglu, 2006

• 3Ma-1Ma contd
• At this time, due to the lower volume of ice,
  ablation was dominated by terrestrial melting, as
  opposed to the marine calving of icebergs. Such
  margins are controlled by summer melting and are
  therefore sensitive to orbitally driven changes
  in local summer insolation i.e. sensitive to
  precession and obliquity cycles.
• The northern and southern hemispheres are both
  sensitive to precession cycles, but the opposite
  effect is happening in each hemisphere (i.e.
  ablation at one pole, while the other is
  accumulating, or, out-of-phase accumulation)
  and so their effect cancel out - this is their
  reason for the lack of a 19kyr and 23kyr cycle in
  d18O records.
• This leaves obliquity as the only remaining
  factor seen in isotope records.

Fig. 3 http//calgary.rasc.ca/images/radec_earth_
precession.gif
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East Antarctic Ice Sheet Margin Model Raymo,
Lisiecki Nisancioglu, 2006

• Mid Pleistocene Transition
• A decrease in temperature and an increase in ice
  volume, in particular in the N.hemisphere is
  predicted over this period (e.g. d18O records and
  coral reefs). This created a sea level lowering,
  which increases land area. With lower
  temperatures and more land, accumulation exceeds
  ablation and the EAIS grows, eventually forming a
  margin at the sea. High latitude temperatures are
  now low enough to prevent summer melting an the
  EAIS becomes stable.

Fig. 4 Raymo, Lisiecki Nisancioglu, 2006

• Now, in the N.hemisphere winter there is
  accumulation in the north this creates a
  lowering of sea level, increasing land area in
  Antarctica allowing accumulation there too. Now,
  changes in ice volume due to precession are no
  longer cancelled out and 19kyr and 23kyr cycles
  can be seen in d18O records
• Their model found that a decrease in ice
  equivalent to 80m of sea level in the
  N.hemisphere and at least 20m in the S.hemisphere
  created a 41kyr world (fig. 4)

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Integrated Summer Insolation Huybers, 2006

• According to Huybers, summer insolation is a
  better measure of the influence of insolation on
  ablation than mean annual insolation, as summer
  is the ablation season.
• The duration of summertime and the intensity of
  summer insolation are primarily controlled by
  precession, however, they are anti-correlated
  i.e. when summertime is shortest, summer
  insolation is strongest (fig. 5)
• When total summertime insolation is summed,
  these two factor nearly balance, eliminating
  precession from the benthic d18O record, leaving
  only the obliquity component.

Fig. 5 Huybers, 2006
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Insolation Gradient Model Raymo Nisancioglu,
2003

• Changes in the difference between insolation at
  25and 70 during summertime (the insolation
  gradient in summer) is driven by obliquity (fig.
  6) e.g. an increase in obliquity from 22.1 to
  24.5 increases summer insolation by 24 W/m²,
  increases summertime from 133 to 137 days,
  increasing total summer energy from 4.9 to 5.3
  GJ/m² (Huybers, 2006), which will then affect the
  insolation gradient
• The temperature gradient this creates drives heat
  and moisture pole-ward, thus, changes in the
  insolation gradient drive changes in heat and
  moisture supply to the poles

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Insolation Gradient Model Raymo Nisancioglu,
2003
Fig. 7 Raymo Nisancioglu, 2003

• They found that summer insolation gradients
  correlate with benthic d18O, perhaps indicating
  that an increase in gradient promotes ice sheet
  growth (fig. 7)
• N.b. the d18O changes lag behind insolation
  gradient changes due to the time taken for it to
  take effect and so it has been overprinted to
  have an 8000yr lag as this is what seems
  reasonable (and it also fits the data!)

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Insolation Gradient Model Raymo Nisancioglu,
2003
Fig 8 http//www.youtube.com/watch?vCdB_p7dmAwU
featurerelated

• As overall global temperature decreases, high
  latitude cooling occurs, increasing the
  insolation gradient and increasing snow cover.
  Additionally, the albedo is increased due to the
  change from forest/grassland to ice cover, which
  further increases the insolation gradient. This
  does two things

• pole-ward transport of heat and moisture is
  increased, which feeds ice-sheet growth (positive
  feedback)
• local cooling decreases precipitation, therefore
  decreasing ice growth (negative feedback)

• These authors believe a balance between the two
  feedbacks determines the overall size of the ice
  sheets.

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Summary

• A dominant 41kyr cycle dominated benthic d18O
  between, about 3.0 and 0.8Ma, which is thought to
  correspond to the obliquity cycle being the
  primary control in ice sheet growth
• Two primary theories have been hypothesised to
  explain this

• Ice Sheet Margin Model
• Insolation Gradient Model

• Precession causes out-of-phase ablation and
  accumulation in each hemisphere in the warmer
  late Pliocene/early Pleistocene, so that only
  obliquity is observed in d18O records.
• gradual global cooling increases N. hemisphere
  ice sheets and decreases sea level, leading to
  transfer from a terrestrial to a marine EAIS
  margin
• Precession now causes in-phase ablation and
  accumulation in each hemisphere, therefore both
  precession and obliquity are observed in isotope
  record

• the anti-correlation of summer duration and
  summer insolation intensity caused by precession
  balance out, eliminating precession from the
  isotope record
• summer insolation gradient changes are caused
  mostly by changes in obliquity, therefore
  obliquity dominated the isotope record.
• as global temperature decreased, ice covered
  more area and so albedo became the factor
  determining ice volume