Download Oxygen isotopes

Implications of Mollusk Study

If results from Mississippi Embayment are
representative of open ocean
 SST in general and winter SST in
particular higher at low latitudes in Eocene
 Results are consistent with prediction of
GCM models with high atmospheric CO2
 Decrease in atmospheric CO2 and more
significant winter cooling
 Consistent with oxygen isotopic record
from mollusks
Foraminifer Preservation
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Diagenesis in foraminifera difficult to detect
Aggregates of morphologically discontinuous calcite
microgranules that are fused together create smooth
surface
Often display fine features (pores, spines, etc.)
Substantial porosity
A-F show well-preserved
shell textures
G-I show poorly
preserved shell textures
Textural Preservation in Shells

Pearson et al. (2001) high-resolution SEM
observations of Paleogene and Cretaceous
calcareous oozes
 Revealed diagenetic recrystallation on
micrometer scale
 Yet pores, internal wall layering and surface
ornamentation preserved
 Pelagic calcareous oozes and chalks commonly
used for paleoceanographic studies
 Certain localities (impermeable clays) have
texturally well-preserved shells
Isotopic Differences Preserved
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Clay samples from
continental shelf
 Possible fresh
water influence
Diverse fauna display
normal depth habitatisotopic relationships
Vertical red line
indicates maximum
inferred SST
d18O is considerably
more negative (warmer
SST)
Comparison of Clay and Ooze
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Depth relationships retained; d18O different
Convergence of lines towards diagenetic calcite value
 Suggests shells at Site 523 are 50% original, 50%
diagenetic carbonate
 Paleotemperature at 523 is 15°C lower than at RAS
99-17
Site 523 and RAS
99-17 are same age.
Lines connect
species common to
both sites.
Diagenetic calcite
from Site 523.
Conclusions of Foraminifera Data

Eocene-Cretaceous planktonic foraminifera
isotope data from oozes and chalks “suspect”
 Measured paleotemperatures from oozes
and chalks too low
 Isotopic differentials are reduced
 Meridonal temperature gradients reduced
 Bottom water temperatures constant
 Low latitude surface temperature higher
 Diagenetic shifts of tropical fauna more
severe
Equator-to-Pole Gradient Steeper

Gradients constructed
with clay data steeper
 Low latitude SST
warmer
 Consistent with models
including elevated
atmospheric CO2
 Pearson et al. (2001)
suggest that “allows us
to dispose of the 'cool
tropic paradox' that has
bedeviled the study of
past warm climates”
Late Cretaceous
Middle Eocene
Late Eocene
Implications of the Results

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If results are representative of open ocean
 i.e., little affect from freshwater runoff
Strengthen the link between high levels of
atmospheric CO2 and global warmth
 Predicted considerable warming at all latitudes
appears now consistent with paleo-SST data
Considerably more data are required to define
equator-to-pole temperature gradient
 Requires drilling of continental margin sites
 Avoided in past due to hydrocarbon potential
 New era of Ocean Drilling removes restriction
Climate Change During Last 55 my

Global cooling over last
55 my
 Terrestrial records
 Faunal change
 Floral changes
 Evidence for
glaciations
 Marine records
 Oxygen isotopic
composition of
foraminifera
Temperature from Leaf Margins
Leaves in warm climates have smooth margins
 Leaves in cold climates have irregular edges
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Paleotemperaures from Leaves

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Analysis of leaf edge
margins in N. America
indicate cooling
 Irregular but
progressive trend
towards cooling
 Several short-lived
warming events
Climate records from
terrestrial environments
 Incomplete
 Poorly dated
Benthic d18O over last 70 my

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Benthic foraminifera
 Not likely altered
Erratic trend towards
more positive values
 Signal reflects cooling
of deep ocean and
formation of glacial
ice
 Both indicate
transition from
greenhouse to
icehouse world
Benthic d18O: Temperature or dw
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d18O change
 Cooling of deep ocean
 Growth of ice sheets on land
16O-rich water in ice
 Storage of
 Both factors influence global cooling
 De-convolution allow relative importance
of each factor recognized
 Sort out the mechanisms controlling
cooling
Disentangle Temperature and Ice Volume
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No ice on Earth 70-40 mya
 d18O increased from –0.75‰ to +0.75‰from 55-40
mya
 Temperature effect only
• 1.5‰ x 4.2°C ‰-1 = 6°C
Evidence of ice on Earth at 35 mya
18
 Ice volume and d O unknown
 d18O increased from +0.75 to +3.5‰ from 40 mya to
today
 About 1‰ can be attributed to ice volume
 Temperature increased by 1.75‰ x 4.2°C‰-1 =
7°C
Deep ocean increased 13-14°C over last 55 my
Temperature Trends Similar
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Overall terrestrial and marine trends are similar
Detailed records differ – regional effect?
Deep Ocean Circulation
Deep ocean 55 mya 16°C
 Today deep ocean 2°C
 Site of deep water formation warm
 Changes in circulation affected climate
 Changes may have been abrupt
 After some threshold was reached
 May have remained in one mode while
climate changed
 Then abruptly changed once a certain
threshold was reached
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Climate Cooling

Proxy evidence
indicates an erratic
cooling
 Over both poles
and mid latitudes
 Roughly equal
cooling in first and
second half of
interval
Tectonic Scale Cooling Mechanism?
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Lower volcanic CO2 emissions
Increased weathering
Increased ocean heat
transport
Tectonic changes
 Atlantic widened and Pacific
narrowed
 India and Australia
separated from Antarctica
 India and Australia moved to
lower latitudes
 India collided with Eurasia
 Key oceanic gateways open
and closed
BLAG Hypothesis
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Depends of global spreading
rates
55-15 mya general decrease
in spreading
 Produce cooling
15 mya to today spreading
increased
 Produce warming
Consistent with record prior
to 15 mya
Inconsistent with record
from 15 mya to present
Cannot alone explain cooling
Uplift Weathering Hypothesis

To explain cooling, 3 criteria must be met
 High elevation terrain today must be
unusually large
 High terrain must cause unusual amount of
rock fragmentation
 Fragmentation and exposure must enhance
chemical weathering
Extensive High Terrain
Many young mountain ranges with uplifted
Cretaceous sediments
 Tibet and Himalayas of Asia
 South American Andes
 North America Rocky Mountains
 European Alps
 Are these uplifted terrains, uniquely high?
 Uplift associated with subduction common
 Uplift associated with continentalcontinental collision less common

Earth’s High Topography
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Only a few regions with elevations above 1 km
Most young tectonic terrains
 Exception is E. African plateau
India-Asia Collision

Formation of Tibetan
Plateau
 Large geographic
region elevated
 Initial collision about
55 mya
 Uplift continues
today
 No large continental
collisions between
100-65 mya
Elevation on Earth

Most high elevation caused by subduction of
oceanic crust and volcanism
 Mountain ranges associates with subduction
common throughout geologic time
 Deep-seated heating and volcanism
 East African plateau
• Mechanism of uplift not unique to last 55
my
 Existence of uplifted terrains like the Tibetan
Plateau
 Not common through geologic time
 Conclude – amount of high elevation terrain is
unusually large during last 55 my
Physical Weathering High
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Does the amount of
high elevation terrain
result in unusual
physical weathering?
Most likely given 10
fold increase of
sediment to the Indian
Ocean
 Steep terrain along
southern Himalayan
margin
 Presence of
powerful South
Asian monsoon
Physical Weathering Unusual?
Incomplete geologic record does not tell us
if these deposits are unique
 Deltaic deposits can be subducted and/or
reworked and redistributed
 Rapid deposition of sediments in Indian
Ocean indicates rapid influx
 Supports uplift weathering hypothesis
 No way to determine if physical
weathering stronger than in past
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Chemical Weathering
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Global chemical weathering rates difficult to
determine
 Dissolved ions in rivers clue
 Today concentration modified by human activity
 Difficult to distinguish ions from hydrolysis and
dissolution
 Only hydrolysis important on long term
 Lots of rivers contribute ions to ocean
Chemical weathering rates in past very difficult to
quantify
 Need chemical indicator of hydrolysis
 Isotopes of strontium and osmium