Climate Science
Lecture 17 – Northern Hemisphere Icesheets 2.75
MYA to Today
1) North Atlantic Sediment Core
a) Texture – Ice Rafted Sediment indicated
when lots of icebergs were calving
b) Oxygen isotope data
i) A slow “linear” longterm trend to colder
conditions – more ice sheet volume
ii) 41,000 cycle dominates from 2.75 mya
until 0.9 mya
iii) 100,000 cycle dominates from 600 kya to
today
(1) The 23,000 and 41,000 periods also
present, but not dominate
iv) Transition period exists between the two
v) Since oxygen isotope data is also
sensitive to temperature, not just ice
volume, more data is needed to constrain
the ice sheet record
2) Data from Coral Reefs
a) Coral reefs have annual growth bands and are
therefore dateable
b) Coral reefs grow very near sea level
i) Therefore sea level changes can be tracks
by study of coral reefs
c) With dates and sea level history they can
confirm the oxygen isotope data regarding ice
sheets
i) Formation on extensive ice sheets lowers
sea level and visa versa
3) Milankovitch cycles and matching of data
a) Predictions
i) 23 K year cycle should dominate
ii) 41 K year cycle is present but only ½ as
strong
iii) 100 K cycles insignificant
b) Observations
i) Before 2.75 mya the glacial threshold line
was below the variation of summer
insolation minimums
ii) By 2.75 mya the threshold line began to
intersect the insolation curve cause short
pulses of glaciation
(1) Glacial cycles show major 41,000
cycle, but theory predicts stronger
23,000 cycle for changes in summer
isolation– slight mismatch
iii) About 1 mya things changed
(1) Threshold for glaciation line and the
insolation curve overlap far more
(2) Meaning long periods of ice cover and
larger ice sheets
(3) By 600ka the cycle became
dominated by 100 k year glacial cycle
– also not predicted by Milankovitch
iv) The nature of the build-up of ice sheets
and melting is also not explained by this
theory
(1) Slow build up and fast melting –
sawtooth curve