Climate forcing factors.
Tectonic forcings
Climate History & Paleoclimate - October 8, 2010
Climate history and paleoclimate - HS 2010
Schematic climate system
 Today’s focus: Changes in Earth’s orbit
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Changes in the tectonic setting
 Last lecture: Orbital forcing
 Important: geological time scale!!!
Climate history and paleoclimate - HS 2010
Long-term climate evolution
 Warm periods, especially in the
Cretaceous. -> extreme
greenhouse climate
 3 major glaciations in the last
500 Myr:
 440 Myr: in the Silurian
 325-240 Myr: in the Permian
 since 35 Myr: late Cenozoic
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Cretaceous world - Greenhouse climate
 Large areas that were under water.
Climate history and paleoclimate - HS 2010
Cretaceous world - Greenhouse climate
 Large deposition of limestone, also in the Alps
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Cretaceous world - Greenhouse climate
 Tropical plants
found in the polar
region
Climate history and paleoclimate - HS 2010
Cretaceous world - Greenhouse climate
 Climate scientist have used
geologic data (fauna, flora and
geochemical) to compile an
estimate of temperatures 100
Myr ago
 Temperatures were warmer than
they are today at all latitudes,
especially in polar region
->equable warm climate
 20ºC warmer at the North Pole
than today
40ºC warmer at the South Pole
than today
Climate history and paleoclimate - HS 2010
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High sea level
 During low sea level: sediment
deposition on the continental
slope
 During high sea level: sediment
deposition on the shelf
 These sediments can be used to
reconstruct the global (eustatic)
sea level.
Climate history and paleoclimate - HS 2010
Sea level variations
 The sea level was 100 - 300 m
higher in the Cretaceous
 Why?
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Reasons for sea level changes
 Changes in the volume of the
ocean basin
 Warm, young crust is thicker,
therefore fast spreading causes
removes more water than slow
spreading ridges
Ridge depth = 2500m + 350 (crustal age)1/2
(m)
(t=0)
(in Myr)
Climate history and paleoclimate - HS 2010
Reasons for sea level changes
 Water stored on ice sheets
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Reasons for sea level changes
 Tectonic factors
 Climatic factors
Climate history and paleoclimate - HS 2010
Effect of changes in sea level on climate
 Effects of sea level changes on
climate are linked to the very
different thermal responses of
land and water.
 Tend to moderate continental
climate extremes by producing
milder winters and cooler
summers
 “Maritime climate inland”
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Greenhouse climate - High CO2 level
 High temperature need high
atmospheric CO2 levels
 CO2 concentrations are 3-4
times higher than modern
levels
Climate history and paleoclimate - HS 2010
Black shales - Oceanic anoxic events
Skelton P. 2003: „The Cretaceous World“. Gorge near Gubbio in Northern Italy, showing the „livello
Bonarelli“ as a thin dark band.
 Layer rich in organic matter
 Debate about the formation: Likely the result of high primary production
after a CO2 pulse triggered by volcanic events
 Interestingly, coincide with carbonate platform crisis; acidic ocean; low pH
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Long-term climate evolution
 Warm periods, especially in the
Cretaceous. -> extreme
greenhouse climate
 3 major glaciations in the last
500 Myr:
 440 Myr: in the Silurian
 325-240 Myr: in the
Carboniferous/Permian
 since 35 Myr: late Cenozoic
Climate history and paleoclimate - HS 2010
Icehouse interval
 3 major glaciations in the last
500 Myr:
 440 Myr: in the Silurian
 325-240 Myr: in the
Carboniferous/Permian
 since 35 Myr: late Cenozoic
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Polar position hypothesis
 An early hypothesis of long-term climate change focused
on latitudinal position as a likely cause of glaciations of
continents.
 Polar position hypothesis:
Ice sheets should appear on continents that were located
at polar or near -polar latitudes, but no ice should appear if
the continents were located outside polar region
Climate history and paleoclimate - HS 2010
Moving continents
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Gondwana glaciation and the South Pole
Glaciations:
 440 Myr: in the Silurian
 325-240 Myr: in the
Carboniferous/Permian
Climate history and paleoclimate - HS 2010
Testing of the hypothesis
 The polar position hypothesis is supported by the observed
glaciations, but the polar position can not be the only factor
controlling glaciations. -> Other factors: greenhouse gases!?
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Long-term controls on atmospheric CO2
 Changes in Earth’s geography can not explain the large
climatic variations of the last 500 Myr.
 Another likely factor in these climatic changes is variations
in the CO2 concentrations of the atmosphere.
 Was controls the CO2 levels on long time scales?
What are the sources?
What are the sinks?
How easy can the size of the different CO2 reservoir being
change?
Climate history and paleoclimate - HS 2010
Major carbon reservoirs
 Different sizes of carbon
reservoirs
 Different rates of carbon
exchange
 Inverse relationship
between reservoir size and
exchange
 Carbon cycles through the
smaller reservoir within a
few year, but moves much
more slowly through the
larger and deeper
reservoirs.
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Input of CO2 from volcanoes
 CO2 is released from volcanoes and hot springs
 Is not constant, because vol. explosions are irregular in time
Climate history and paleoclimate - HS 2010
Influence of the volcanoes
 If the volcanic outgasing is
zero, what would be the
effect on the atmosphere?
Calculate!
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How is CO2 removed from the atmosphere on
long timescales?
 Through chemical
weathering!
Climate history and paleoclimate - HS 2010
Chemical weathering removes atmospheric CO2
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Climate controls on chemical weathering
 Temperature: Weathering rates roughly double for each
10ºC increase in temperature.
(-> factor 8 over 30ºC)
 Precipitation: Increased rainfall raises the groundwater
level, and the water combines with CO2 to form carbonic
acid.
 Vegetation: Plants extract CO2 from the atmosphere
(photosynthesis) and deliver it to soils (carbonic acid->
enhanced weathering).
Climate history and paleoclimate - HS 2010
Negative feedback from chemical weathering
 Chemical weathering acts
as a negative climate
feedback.
 Mechanism that act as
Earth’s thermostat and
moderate long-term climate
 Chemical weathering
thermostat
 Why is it not working today
with the current raise in
CO2 concentrations?
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How is CO2 level lowered in the atmosphere on
long timescales?
 How changed the input?
 How changed the output?
Climate history and paleoclimate - HS 2010
Testing of the hypothesis
 The polar position hypothesis is supported by the observed
glaciations, but the polar position can not be the only factor
controlling glaciations. -> greenhouse gases!
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Control of CO2 input - Changes in seafloor
spreading
 Spreading rate hypothesis (also
BLAG hypothesis after Berner,
Lasaga und Garrels):
Changes in the rate of seafloor
spreading over millions of years
have controlled the rate of
delivery of CO2 to the
atmosphere.
-> faster spreading ->more
frequent release of magma ->
greater delivery of CO2
but also -> more rapid
subduction -> more melting ->
more CO2
Climate history and paleoclimate - HS 2010
Age of the seafloor - Testing the BLAG hypothesis
 Oldest ocean crust: 175 Myr
 Cannot be directly tested with the old
glaciations!!
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Evaluation of the BLAG hypothesis
 Indication of faster spreading rates around 100 Myr ago than at present
-> Would support the BLAG hypothesis
Climate history and paleoclimate - HS 2010
Control of CO2 removal
 Climate-related factors of weathering: temperature, precipitation,
vegetation
 But these are not the only factors controlling weathering
 Surface area is very important for the rate of weathering
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Weathering and exposure time
 Rate of Weathering is high on fresh/young soils because:
 Vulnerable minerals are removed through time
 Finer particle disappear earlier as weathering consumes them
Climate history and paleoclimate - HS 2010
When is the rock fragmentation increased?
 Exposure of freshly fragmented
rock is enhanced in regions of
tectonic uplift
 Uplift weathering hypothesis:
Uplift accelerate chemical
weathering through the
processes displayed in the left
figure
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Weathering in the Amazon Basin
 Modern weathering in the
Amazon Basin is dominated by
the Andes!
Climate history and paleoclimate - HS 2010
What causes uplift?
 Subduction of oceanic crust (e.g. Andes)
“Continuous process”
 Collision of continents (e.g. Himalaya)
Episodic event
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Evaluation of the uplift weathering hypothesis
 325-240 Myr: Formation of the supercontinent “Pangaea”
 Since 55 Myr: Formation of the Tibetian Plateau
 Both, the spreading rate hypothesis and the uplift hypothesis
seems to provide plausible explanations of major icehousegreenhouse changes of climate.
Climate history and paleoclimate - HS 2010
Going towards today’s ‘icehouse’ climate
Let’s have a closer look at the Cenozoic cooling
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What are the reasons for the cooling?
- Polar position hypothesis
Antarctica is located since
100 Myr at the South Pole!
Climate history and paleoclimate - HS 2010
What are the reasons for the cooling?
- Polar position hypothesis
We already saw that this hypothesis can not sufficantly
explain the climate evolution of the last 100 Myr.
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What about the other hypotheses?
 Gateway hypothesis
 BLAG ocean spreading rate hypothesis
 Uplift weathering hypothesis
Climate history and paleoclimate - HS 2010
Gateway hypothesis
 Climate effect due to the opening or closing of ocean
gateways
Ocean gateways which
have opened, closed, or be
strongly modified during the
Neogene
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Opening of Drake’s Passage
 Opening of the ocean gap
between South America and
Antarctica allowed a strong
Antarctic circumpolar current
(ACC) to flow uninterrupted
around the Antarctic continent
 But when was it happening?
Climate history and paleoclimate - HS 2010
Neodymium (Nd) isotopes as a water mass tracer

143Nd/144Nd
in fish teeth is distinct for the Pacific and the Atlantic
Climate history and paleoclimate - HS 2010
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Neodymium (Nd) isotopes as a water mass tracer
 The two sites in the southern
Atlantic show an increase around
41 Myr
 This is earlier then originally
thought (20 to 25 Myr)
 The start of the Antarctic
glaciation is around 37 to 33 Myr
 Sufficient timing?
Climate history and paleoclimate - HS 2010
The seafloor spreading hypothesis
 But have now a closer look at the spreading rates over time
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The seafloor spreading hypothesis
 A general decreasing trend, but
increasing spreading rates in the
last 15 Myr years
 This reversed trend would not
support the seafloor spreading
hypothesis, but before!
Climate history and paleoclimate - HS 2010
Uplift weathering hypothesis
Uplift:
 Collision between India and Asia
since 55 Myr forming a large
Tibetan Plateau.
 No continent collision occurred
from 100 to 65 Myr
 The existence of the massive
Tibetan Plateau makes the
modern topography unusual,
consistent with the uplift
weathering hypothesis
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Uplift weathering hypothesis
Weathering:
 The rate of influx of sediments
from the Himalayas and Tibet to
the deep Indian Ocean has
increased almost tenfold since
40 Myr ago
 Reason for this increase:
 Creation of steep terrain
 Changes in the climate pattern
(stronger South Asian monsoon)
Climate history and paleoclimate - HS 2010
Uplift weathering hypothesis
Weathering:
 Further indication of increased
weathering: Strontium isotopes
 87Sr/86Sr has two sources:
 River input
 Hydrothermal input
 Sr isotopes indicate an increased
river influence in the last 40 Myr
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Uplift weathering hypothesis
 Core analysis in the Bengal Fan:
 CO2 consumption from the net
burial of organic carbon during
Himalayan sediment deposition
was 2-3 times that resulting from
the weathering of Himalayan
silicates
(from France-Lanord&Derry, 1997, Nature)
Climate history and paleoclimate - HS 2010
Conclusion
 The exact cause of global cooling during the last 50 Myr
remain uncertain.
 The Drake Passage opening was not enough to initiate
glaciations in Antarctica
 The decrease in atmospheric CO2 is likely to crucial factor,
but it remains unclear to what extent the seafloor spreading
or the increases removal through weathering is responsible
for the CO2 decrease.
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