Cenozoic cooling trend
from Greenhouse to Icehouse
Presentation by Jacqueline Reber & Niklaus Merz
Picture: Hjorth Hill, Antarctica, by Ólafur Ingólfsson
Content
Introduction:
Cenozoic era
Cenozoic cooling trend
Tectonic-scale climate change:
BLAG theory
Uplift weathering hypothesis
Applicability for Cenozoic cooling
Alternative theories for cooling:
Locality of continents
Gateway hypothesis
Cloud coverage hypothesis
Feedback mechanisms
Uncertainties about building up the Icehouse
Conclusion
References
1
Cenozoic era
•
Describes the era of the last 65 million
years
•
Start was marked by the K-T event (mass
extinction)
•
Cenozoic follows the Cretacous hothouse
Figure: USGS, http://geomaps.wr.usgs.gov/socal/geology/geologic_history/index.html
Cenozoic era
What happened ?
• progressive development of the modern world through plate
tectonics movements → changes in oceanic circulation patterns
• Large-scale mountain building (= orogeny) → changes in
atmospheric circulation patterns,
• Formation of sea ice and large ice sheets (first in Southern
Hemisphere, later also in Northern Hemisphere)
• Evolution of the modern biota (Cenozoic often called as age of the
mammals)
• Shift of vegetation (Palms and crocodiles are not expected to live in
Arctic regions nowadays, no Beech forest in Antarctica as well)
2
Cenozoic era
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
Cenozoic era
• Steadily increasing biodiversity
• No mass extinction anymore since the K-T-event
Figure: www.wikipedia.com/biodiversity
3
Cenozoic cooling trend
Figure: Zachos et al., 2001
Cenozoic cooling trend
Zachos et al, 2001:
• Data from Deep-sea foraminifera sediments
• Shift from expansive warmth with ice-free poles to extremes of cold
are not unexpected! → orbital geometry and plate tectonics are in
perpetual motion
Cooling appeared all over the world but most intensive at high latitudes
Evidence for cooling from vegetation, ice and sediments (oxygen
isotope, Mg/Ca-ratios in shells)
4
Cenozoic cooling
• Cenozoic started as a warm period
• Total decrease of temperature was about 10°
Celsius (global mean value)
• Decrease in temperature was strongest in high
latitudes (subtropical conditions → major ice
ages)
• General: Although the significant changes in
climate (from greenhouse to icehouse) the earth
always remained habitable (issue of locality)
Figure:
Cenozoic cooling
• Started with very high sea level
• General trend of lowering sea level
during the Cenozoic era
• Large perturbations in sea level due
to ice ages, but also due to spreading
rates (reminder: last week)
•
Figure: Vail curve of global sea level changes (H. Levin, The earth through time,
2006)
5
Cenozoic cooling trend
Figure: Zachos et al., 2001
Cenozoic cooling trend
• No continuous decreasing → large fluctuations
• Mainly two phases of cooling
• Question of reasons for the cooling trend and these perturbations →
question of scales!
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
6
How to explain the cooling
The Cenozoic cooling happened over many million years → has
to do with tectonics!
Figures: W. Ruddiman, Book “Earth’s Climate”, 2008
How to explain the cooling
• Phenomenon like large-scale glaciations asks for tectonic-scale
climate change
• Expense of icehouse (for example: ice ages in Europe) is also
influenced on the orbital-scale
•
Figure: Vostok ice cores, Antarctica, Petit et al., 1997
7
How to explain the cooling
• High CO2-levels seems to be a major cause of the
cretaceous hothouse
• Evolution of the icehouse is not possible without
decreasing CO2 (Greenhouse gas warming effect)
• pCO2 estimated from
surface ocean pH, which
is derived from the boron
isotope ratios of
planktonic foraminifers
(subtropical sediments)
Figure: Pearson et al, 2000
Key question
• Which tectonic processes led to a strong
decrease in CO2-concentration?
8
Tectonic control of CO2 input
• BLAG spreading rate hypothesis
• Uplift weathering hypothesis
BLAG
Figures: W. Ruddiman, Book “Earth’s Climate”,
2008
9
BLAG
Chemical weathering as negative feedback:
CaSiO3 + CO2
Silicate rock
CaCO3 + SiO2
Atmosphere
CaCO3 + SiO2
Plankton
CaSiO3 + CO2
Ocean sediments
Silicate rock
Atmosphere
Feedback
Figures: W. Ruddiman, Book “Earth’s Climate”,
2008
10
Summary BLAG
•
•
•
•
Long-term stability
Constant amount of CO2 exchange
Climate changes due to delay time
Increasing weathering rate with
temperature rise
Uplift weathering hypothesis
• Direct physical impact:
changes in circulation of atmosphere
and ocean
• Indirect biochemical effects via changes in
pCO2
11
Figure: W. Ruddiman, 1997
Physical impact
Uplift can change:
• Monthly mean longitudinal distribution of
winds and temperature
• surface ocean currents
• precipitation patterns
12
Changes of weathering rate
Availability of fresh rocks
Picture: Rock slide from www.images.google.ch
Picture: Himalaya from www.welt.de
Climate related factors:
temperature
precipitation
vegetation
less important
Fragmentation of rock
Figures: W. Ruddiman, Book “Earth’s Climate”,
2008
13
Increase of exposure
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
Regulation
Uplift in 2% of earth‘s surface
Reduction of CO2
Drier climate in 98% of earth‘s surface
Less CO2 removal
14
Summary uplift weathering
hypothesis
• Change of circulation patterns of
atmosphere and oceans
• Increase of chemical weathering
• Focuses on continent collision
Application of the theory on the last
55 Myr
Figure: Zachos et al., 2001
15
Evaluation of BLAG
• Spreading rate relatively slow over the last 55
Myr.
• 100 Myr ago 50% faster than today
• Since 15 Myr increase of spreading rate
X
BLAG was maybe cause for cooling
before 15 Myr but not after.
Evaluation of uplift weathering
hypothesis
To verify the hypothesis three points must
be checked:
• Amount of high-terrain is unusually large
• Unusual amount of rock fragmentation
• Unusual high rate of chemical weathering
16
Existence of high terrains
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
Figure: Zachos et al., 2001
17
Tibetan plateau: 2 million square kilometers
at an altitude of 5 kilometers.
Figure: www.images.google.ch/tibetan
Other areas with uplift in the Cenozoic:
• Central Altiplano and eastern Andes
• Low plateaus in eastern and southern
Africa
• Large regions in the Rocky Mountains
18
Unusual physical weathering
Largest amount of young sediment on the
seafloor of the Indian Ocean.
But: No sediments older than 150 Myr
Unusual chemical weathering
Chemical weathering rate can be measured
as the total amount of dissolved ions in
river run-off.
Difficult to measure today, almost impossible
for the past.
19
87Sr/86Sr
• Direct proxy for silicate weathering
• Depended on the age of soil in river beds
• Incorporated into CaCO3 shells of marine
plankton
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
20
Figure: Raymo and Ruddiman, 1992
Physical impact of the Tibetan
Plateau
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
21
Drawbacks
Figure: Franc-Lanord et al., 1997
Can cooling cause mountain uplift?
The dynamic of subduction and mountainbuilding can be controlled by the process
of erosion and sediment deposition
and ultimately climate
22
Figure: Lamb et al, 2003
Summary
• BLAG hypothesis can not explain the
cooling trend in the Cenozoic
• The weathering hypothesis combined with
burial hypothesis gives a good explanation
What about other approaches?
23
Locality of continents
Evolution since 200 Myr ago:
• Pangaea split in several continents
• Atlantic ocean has widened
• Pacific ocean became more narrow
• India and Antarctica moved
northwards in higher latitudes
Influence of climate → polar position
hypothesis ?
No → locality itself can not explain
the long-term cooling
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
Locality of continents
• The moving plates also influences ocean
currents
• Important for heat transport from low
latitudes to polar regions
24
Gateway hypothesis
• Until 30 million years ago there were beech trees existing in
Antarctica, similar to one‘s living today at the tip of South America
• Split off of Antarctica as a precondition for glaciation
• → hypothesis: land connections with South America and Australia
conveyed oceanic flow from low latitudes to the south pole area →
major heat transport in high latitudes
• Today: Current wheel (Antarctic Gyre)
around the continent → isolation effect
Figure: South Aris, Irish Antarctic Expedition, 1997
Gateway hypothesis
For comparison:
65 Million years ago South
America and Australia built a
long belt along which the oceans
had to flow
→ separation of Australia
around 37 to 33 Myr ago
→ opening of Drake’s Passage
occurred near 25 to 20 Myr ago
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
25
Gateway hypothesis
Figure: Zachos et al., 2001
Gateway hypothesis
•
•
Proposal that onset for Northern Hemisphere glaciation is related to the
closing of the Isthmus of Panama
Link to the Northern Hemisphere warming (latent heat) – but also initiating
the formation of the deepwater in North Atlantic → evaporative cooling of
surface water → increase in atmospheric moisture content → ice building?!
Figure: W. Ruddiman, Book “Earth’s Climate”, 2008
26
Gateway Hypothesis
•
May opening and closing of ocean gateways explain the glaciation during
the last 50 Million years?
•
→ certainly not the explanation for the general cooling – just altering the
energy distribution but no change in global energy budget like greenhouse
gas concentrations or orbital factors
•
Possible impact on glaciation after general cooling
→ Ruddiman doubts significant role of gateway rearrangements (tested
with models)
→ controversy: case Antarctica ↔ Isthmus of Panama, gateways
rearrangement probably was not a critical factor
→ nevertheless, such tectonically driven major events have huge effect on
the dynamics of the global climate system
•
discussion later?!
Alternative concepts for cooling
• Cooling might be caused by increasing cloud coverage at high
latitudes → main cooling in high latitudes during last 50 Million years
• Assumption: Clouds have cooling effect because of high Albedo –
but how strong is this effect?
• Suggested lower cloud coverage
over Eocene oceans (lower
thickness of clouds over warm
oceans)
Figure: Atmospheric Forcing, IPCC, 2007
27
Additional concepts: positive
feedbacks
• Global cooling and glaciation increased mechanical
erosion– more peaks and steep valleys → positive
feedback for chemical weathering
• Ice / Snow-Albedo feedback
• Ice sheet height / mass balance feedback
• Lower Water vapor concentration → declining
greenhouse effect feedback
• → feedbacks themselves can not explain the cooling!
Ice sheets: surface height-mass
balance feedback
Figure: Alfred-Wenger-Institut, www.awi.de
28
Building up the Icehouse
Figure: Zachos et al., 2001
Building up the Icehouse
Benthic foraminifers show a total δ18O decrease of 0.54 %:
–
–
–
0.31% are related to deep sea cooling
0.12% reflects the growth of the Antarctic ice sheet
Remaining 0.11% for Northern Hemisphere glaciation
Uncertainties about timing of the events
29
Building up the Icehouse
Figure: Stoll et al., 2006
Building up the Icehouse
• CO2 depletion as major cause for ice
sheets
• Orbital factors on a lower timescale
• Second role of gateway rearrangements
30
Conclusion
• Large-scale cooling trend consists with
declining CO2-concentration
• The “one and only” explanation has not
been found, yet
Conclusion
• Probable processes for CO2-reduction are:
- chemical weathering
- carbon burial
• Complementary theories for glaciation entail:
- gateways rearrangements
- cloud coverage
- several feedback-effects
31
References
W. F. Ruddiman, 2008, Earth‘s Climate, Freeman New York
W. F. Ruddiman, 1997, Tectonic Uplift and Climate Change, Plenum Press
M. E. Raymo & W. F. Ruddiman, 1992, Tectonic forcing of late Cenozoic climate,
Nature
J. Zachos et al, 2001, Trends, Rhythms, and Aberrations in Global Climate 65 Ma
To Present, Nature
C. France-Lanord & L. A. Derry, 1997, Organic carbon burial forcing of the
Carbon cycle from Hymalayan erosion, Nature
S. Lamb & P. Davis, 2003, Cenozoic climate change as a possible cause for the
Rise of the Andes, Nature
References
A. Henderson-Sellers, 1979, Clouds and the long-term stability of the Earth’s
atmosphere and climate, Nature
Liou, K.-N. and S.-C. Ou ,1989, The Role of Cloud Microphysical Processes in
Climate, J. Geophys. Res.
R.M DeConto and D. Pollard, 2003, Rapid Cenozoic glaciation of Antarctica
induced by declining CO2
32