Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 1
Pliocene, El Niño and Northern /
Southern Hemisphere Glaciation
Mirjam Kosch
Florian Suter
Alexander Umbricht
Andy Eigenmann
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 2
Questions



Will the present warm conditions terminate soon, to be
followed by the next ice age?
Will the onset of the next ice age be inhibited by the
current rise in the atmospheric concentration of GHG
induced by humans?
Will that rise restore the warm conditions of the early
Pliocene?
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 3
Outline
1. Methods
2. Southern Hemisphere glaciation
a. Opening of the Drake Passage
3. Northern Hemisphere glaciation
a. Pliocene paradoxon
b. Hypotheses for Greenland glaciation
4. Questions and Discussion
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 4
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 5
1. Methods
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 6
d18O
 Terrestrial water enriched
in light 16O
 Seawater enriched in
heavier 18O
 Compare 18O/16O of
sample with 18O/16O of
standard (fossils)
University of Vermont
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 7
Neodymium Isotopes
 eNd (143Nd/144Nd)
 Oceanic basins with
distinct eNd -values
 Crusts and fossil fish
teeth preserve eNd -values
of bottom water
 Bassin connections
Wikipedia, 2009
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 8
Ice Rafted Debris (IRD)
Objects deposited
on/within ice shield
(Debris)
2. Iceberg calves
3. Drifting and melting
4. Debris deposited onto
bottom of the water
body
1.
Wikipedia, 2009
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 9
2. Southern Hemisphere Glaciation
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
6.11.09
Slide 10
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 11
The Drake Passage
 Separation of South
America and Antarctica
 Estimated opening:
49 – 17 Ma
 Enables Antarctic
Circumpolar Current
(ACC)
Wikipedia, 2009
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 12
The Drake Passage – Indications
 Tracking Pacific Seawater
 Neodymium ratio eNd
 Pacific eNd : -3 to -5
 Atlantic eNd : approximately -9
 Fossil fish teeth gained from sediments
 High resolution of benthic Nd signal
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Slide 13
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Slide 14
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Slide 15
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Slide 16
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 17
The Drake Passage – other possible Influences
 Weathering
 In general not high enough
 Exception: volcanic ash
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 18
The Drake Passage – other possible Influences
Scotia Sea
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 19
The Drake Passage – other possible Influences
 Weathering
 In general not high enough
 Exception: volcanic ashes
 Water influx from the Indian Ocean
 Water influx from the Panama Seaway
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 20
The Drake Passage – other possible Influences
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 21
The Drake Passage – other possible Influences
 Weather
 In general not high enough
 Exception: volcanic ashes
 Water influx from the Indian Ocean
 Water influx from the Panama Seaway
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Slide 22
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 23
The Drake Passage – Feedbacks
 Enhancement of nutrition upwelling
 Stimulation of the biological pump
 Lowering of the atmospheric CO2-concentration
 Possible reduction of oceanic heat flux
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
3. Northern Hemisphere Glaciation
6.11.09
Slide 24
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
6.11.09
Slide 25
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 26
Pliocene Paradox
 Early Pliocene (5-3 Ma) showed a very different climate
state even though the external forcings were practically
the same as today.
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 27
Climate in early Pliocene (5-3 Ma)
 Similarities with today’s
climate:
 Intensity of sunlight
incident on Earth
 Global geography
 Atmospheric
concentration of CO2
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
 Differences with today’s
climate
 Higher globally
averaged
temperatures
 No continental glaciers
on Northern
Hemisphere
 Sea level 25 m higher
 Permanent El Niño
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 28
Climate in late Pliocene (after 3 Ma)
 Cooling in the globally averaged temperatures
 Collapse of permanent El Niño
 Decreasing in atmospheric concentration of CO2
 Appearance of continental glaciers in the Northern
Hemisphere
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
6.11.09
Slide 29
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 30
Hypotheses for the Climate Change
 Panama Seaway hypothesis
 ENSO hypothesis
 Uplift hypothesis
 CO2 hypothesis
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 31
Panama Seaway Hypothesis I
 Tectonically driven closure between 13 Ma and 2.5 Ma
 Change in salinity gradient
 Increase of northward heat transport
 Warmer, more evaporative surface water
 Increased atmospheric moisture
 More snowfall
 Increased ice volume in Greenland
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 32
Panama Seaway Hypothesis II
 Increased northward
transport of warm water
 Increased summer
temperature
 Increased evaporation
 Significantly increased
snowfall
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 33
Panama Seaway Hypothesis III
Open Panama Seaway
 Reduction of ice volume
 Warmer summer
temperatures lead to an
increased ablation
 Increased snowfall only
leads to a small increase
in ice volume
Closed Panama Seaway
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 34
ENSO Hypothesis
 Permanent El Niño state during early Pliocene retarded
the onset of Northern Hemisphere glaciation
 Loss of permanent El Niño state acted as positive forcing
for the onset of glaciation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 35
Key Parameters of El Niño
Normal Pacific pattern
El Niño conditions
NOAA
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 36
Today’s SST patterns
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 37
Conditions during early Pliocene I
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 38
Conditions during early Pliocene II
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 39
Correlation between precipitation and SST
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 40
El Niño as contributor to warm conditions
 More convective clouds over EEP
 Increasing of atmospheric water vapour
 Reduced area covered by stratus clouds
 Decreasing albedo of the planet
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 41
Tropical-Extratropical Coupling
 In equilibrium the loss of heat in high latitudes balances
the gain of heat in lowlatitude upwelling regions
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 42
Changes in Oceanic Heat Transport
 Increase in high latitude
heat loss
 Increase in equatorial
heat gain
 Shallower equatorial
thermocline
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
 Decrease in high latitude
heat loss
 Decrease in equatorial
heat gain
 Deeper equatorial
thermocline
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 43
Collapse of Permanent El Niño I
 Due to global cooling during Cenozoic the temperature
of deep ocean decreased
 Shoaling of the thermocline
 At 3 Ma BP the thermocline became so shallow that wind
could bring cold water to the surface in upwelling zones
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 44
Collapse of Permanent El Niño II
 Only small changes in
summer temperature
 Northward shift of the
Atlantic storm track
 Small increase in
precipitation, especially in
the south
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 45
Collapse of Permanent El Niño III
Permanent El Niño
 Small reduction of ice
volume
Fluctuation El Niño
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 46
Uplift Hypothesis I
 Uplift of the Rocky
Mountains and the
Himalaya
 Larger Rossby Wave
amplitude
 Jet-stream deflection
 Cooler air masses
www.daukas.com
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 47
Uplift Hypothesis II
 Cooling in Canada, North
Atlantic and Greenland
 Generally drier climate
over Greenland
 Northward deflection of
Atlantic storm track leads
to greater precipitation
over southern Greenland
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 48
Uplift Hypothesis III
Before tectonic uplift
 Increase in snow cover
 Cooler summer
temperatures lead to less
ablation
 More precipitation over
southern Greenland
After tectonic uplift
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 49
CO2 Hypothesis I
 Decreased radiative forcing
 Lowered concentration of atmospheric CO2
 Cooler melt-season temperature
 Decreased ablation
 Net annual accumulation
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 50
CO2 Hypothesis II
 Global cooling of 1.3 °C
 Decrease in Greenland
summer temperatures
 Great temperature
response at high latitudes,
due to albedo feedbacks
 Decrease in precipitation
(globally and in
Greenland)
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 51
CO2 Hypothesis III
Before decrease in CO2
 Large increase in ice
volume
 Cooler summer
temperatures lead to less
ablation
 Decreased ablation
dominates over decrease
in accumulation
After decrease in CO2
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Slide 52
CO2 Hypothesis
Eocene 55.8 – 33.9 Ma
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Summary and Questions
6.11.09
Slide 53
Paleoclimatology and Climate History – HS 2009
6.11.09
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
When
 25 to 12 Ma BP
Closure of
Panama Seaway
54
Effects
Assumed Result
 Increased temperature
 More atmospheric
moisture
 more precipitation
 5 to 3 Ma BP
Termination of
Permanent
El-Niño
 Stop of warming the
high latitudes
 slightly higher
temperature
 More precipitation
 Started 65 Ma ago
Tectonic Uplift
 Important after the
termination of perm.
El Niño
 Less then 3 Ma BP
Decrease in CO2
Eocene 55.8 – 33.9 Ma
 Increased Rossby
Waves  deflected
Jet-Stream  cooler
 Less precipitation
 Clearly decreased
temperatures
 Less evaporation
 less precipitation
Oligocene 33.9 – 23 Ma
Miocene 23 – 5.33 Ma
Pliocene 5.33 – 2.59 Ma
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
Questions



Will the present warm conditions terminate soon, to be
followed by the next ice age?
Will the onset of the next ice age be inhibited by the
current rise in the atmospheric concentration of GHG
induced by humans?
Will that rise restore the warm conditions of the early
Pliocene?
6.11.09
Slide 55
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
6.11.09
Slide 56
Some Ideas …
 Solar forcing is in a (long time(?)) minimum
 Response to Milankovitch cycles (e.g. oscillation




between ice-ages and interglacials) was weaker in times
with higher GHG concentrations.
Today we are not in an equilibrium state.
Today‘s GHG concentrations, geological conditions and
solar forcings are similar to those in early Pliocene.
The equilibrium state in early Pliocene included warmer
temperatures, permanent el Niño and a sealevel that
was 25 m higher.
What do we expect for the future?
Paleoclimatology and Climate History – HS 2009
Pliocene, El Niño and Northern / Southern Hemisphere Glaciation
6.11.09
Slide 57
Sources

Wara, M. W.; Ravelo, A. C. & Delaney, M. L. (2005), 'Permanent El Nino-Like Conditions
During the Pliocene Warm Period', Science 309(5735), 758–761.

DeConto, R. M.; Pollard, D.; Wilson, P. A.; Palike, H.; Lear, C. H. & Pagani, M. (2008),
'Thresholds for Cenozoic bipolar glaciation', Nature 455(7213), 652–656.

Fedorov, A. V.; Dekens, P. S.; McCarthy, M.; Ravelo, A. C.; deMenocal, P. B.; Barreiro, M.;
Pacanowski, R. C. & Philander, S. G. (2006), 'The Pliocene Paradox (Mechanisms for a
Permanent El Nino)', Science 312(5779), 1485–1489.

Lunt, D. J.; Foster, G. L.; Haywood, A. M. & Stone, E. J. (2008), 'Late Pliocene Greenland
glaciation controlled by a decline in atmospheric CO2 levels', Nature 454(7208), 1102–
1105.

Scher, H. D. & Martin, E. E. (2006), 'Timing and Climatic Consequences of the Opening of
Drake Passage', Science 312(5772), 428–430.

Sigman, D. M.; Jaccard, S. L. & Haug, G. H. (2004), 'Polar ocean stratification in a cold
climate', Nature 428(6978), 59–63.