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Transcript
A Better Example of d18O data
Compilation of paleotemperature
estimates from oxygen stable
isotope ratios for
• surface-living planktonic
foraminifera (red symbols),
• thermocline-living planktonic
foraminifera (blue symbols), and
• benthic foraminifera (black
symbols).
All samples are from the North
Atlantic.
Norris, R.D., and P.A. Wilson, 1998, Low-latitude
sea-surface temperatures for the mid-Cretaceous
and the evolution of planktic foraminifera, Geol.,
26, 823-826.
Cooling down from the Cretaceous
Greenhouse World.
Prior to the boloid impact at 65 My,
the Cretaceous world was already
cooling.
Mantle plumes were less-frequent,
global temperatures were
decreasing,
Sea level was falling,
Atmospheric CO2 levels were
falling.
Figure from textbook,
showing global temperatures
decreasing;
from the end-Cretaceous estimate
of about 28°C.
(remember that present global
temperature average is +15°C)
This temperature decrease was not
smooth, and there were still a few
LIPS (mantle plumes) erupting.
But in general, the Eocene (58 My
to about 38 My) was warm
compared to today.
An example you have seen before.
Remember: when d18O goes negative, that means that the seawater
temperature is getting WARMER.
The PETM – a massive release of
Greenhouse Gases into the
atmosphere – and the associated
climate change
Low-resolution marine stableisotope records of the PETM and
the carbon isotope excursion,
together with the seafloor sediment
CaCO3 record.
The carbon isotope (a) and oxygen
isotope (b) records are based on
benthic foraminiferal records and
the from drill holes in the South
Atlantic.
The decrease in sedimentary
CaCO3 reflects increased
dissolution and indicates a severe
decrease in seawater pH (that is,
ocean acidification).
From Zachos et al. Nature, 2008
There are two common stable isotopes of carbon: 12C and 13C.
The ratio of these isotopes is expressed in relation to a standard
(PeeDeeBelemnite) as
d13C = [(Rsample/Rstandard) -1] x 1000
where R = (13C/12C).
As d13C values increase, the abundance of the heavier isotope (13C) increases.
Biological activity fractionates in favor of 12C
High 12C input
12C
enriched
This enrichment of 12C within the biological
reservoir, depletes the 12C in the exterior
seawater, and the d13C ratio of the SEAWATER
becomes HIGHER.
PETM
PETM
Sea water
High 12C input to biology:
12C
enriched
Seawater becomes
depleted in 12C,
Sea water d13C ratio
becomes HIGH and
POSITIVE
High 12C output to
seawater as methane
(d13C = -60):
Sediment
12C
enriched
X X
Seawater becomes
richer in 12C and
depleted in 13C,
d13C ratio is LOW and
NEGATIVE.
And this is the broad
Eocene thermal maximum
Low bioproductivity
This is the PETM ‘spike’
Methane
spike
High bioproductivity
Lots of 12C stored
as ‘biology’ leaving
13C behind in the
seawater
The Paleocene-Eocene thermal maximum (PETM)
(1) sea surface temperature rose by 5°C in the tropics;
(2) by more than 7°C in the Antarctic and Arctic.
(3) ocean acidification was strong (CCD was shallow).
(4) with the extinction of 30 to 50% of deep-sea benthic
formaminiferal species.
A good ‘Rule of Thumb’ is that temperature changes in the polar regions are
about TWICE those of the Global Average Temperature Change.
That is => 7°C temperature increase in the Antarctic means about a 3.5°C
increase in global temperatures.
Or about the temperature increase expected over the next 100 years due to
anthropogenic greenhouse gas emissions.
The trigger for the initiation of the PETM was (probably) a period of intense flood basalt
magmatism (surface and sub-surface volcanism) associated with the opening of the
North Atlantic, by generating additional methane from sill intrusion into basin-filling
carbon-rich sedimentary rocks.
The theory.
North Atlantic VP erupted in a warm climate with
abundant methane hydrates stored in continental
margin sediments.
This increase in global temperatures destabilized
the hydrates, releasing massive amounts of
methane into the atmosphere.
The additional warming due to the released
methane/converted CO2 had a strong positive
feedback, causing more hydrates to be destabilized over a 20,000 year period.
The methane from the hydrates had a very
negative d13C signature (-60 0/00), giving the
observed correlation between global temperature
spike and carbon isotope anomaly.
Possible source of the Eocene
warmth; the eruption of another LIPS;
The North Atlantic Volcanic Province.
The initiation of the PETM is marked by
an abrupt decrease in the d13C proportion of marine and
terrestrial sedimentary carbon,
which is consistent with the rapid addition of >1200
gigatons of 13C depleted carbon, most likely in the form
of methane, into the hydrosphere and atmosphere.
The broad Eocene thermal maximum lasted only 210,000
to 220,000 years,
with most of the decrease in d13C occurring over a 20,000year period (the PETM) at the beginning of the event.
The PETM – a massive release of
Greenhouse Gases into the
atmosphere – and the associated
climate change
Low-resolution marine stableisotope records of the PETM and
the carbon isotope excursion,
together with the seafloor sediment
CaCO3 record.
The carbon isotope (a) and oxygen
isotope (b) records are based on
benthic foraminiferal records and
the from drill holes in the South
Atlantic.
The decrease in sedimentary
CaCO3 reflects increased
dissolution and indicates a severe
decrease in seawater pH (that is,
ocean acidification).
From Zachos et al. Nature, 2008
The initiation of the PETM is marked by
an abrupt decrease in the d13C proportion of marine and
terrestrial sedimentary carbon,
which is consistent with the rapid addition of >1500
gigatons of 13C depleted carbon, most likely in the form of
methane, into the hydrosphere and atmosphere.
The Eocene Climate Optimum (warm period) lasted only
210,000 to 220,000 years,
with most of the decrease in d13C occurring over a 20,000year period (the PETM).
Methane is a powerful Greenhouse Gas:
24 times more effective at global warming than CO2.
The Fate of methane gas in the atmosphere.
It mixes quickly in the atmosphere,
rises to the stratosphere where it is rapidly oxidized to CO2 –
with an 8 to 10 years residence time
CH4+O2= CO2+H20
And becomes another powerful greenhouse gas, CO2, with a
much longer residence time.
This is the window
of long-wave
radiation that
keeps the earth
habitable.
Acid Oceans?
During this massive methane release, the oxidation and ocean
absorption of this carbon would have lowered deep-sea pH
(increased ocean acidity dramatically).
This low ocean pH would have led to rapid shoaling of the
calcite compensation depth (CCD), followed by a gradual
recovery.
Evidence of a rapid acidification of the deep oceans would be
evident in the abrupt transition from carbonate-rich sediment
to clay, followed by a gradual recovery to carbonate.
Samples of the ocean sediment from five South Atlantic deepsea sites, all within the geologic time frame of the PETM.
During the Eocene, the CCD is inferred to have shoaled more than 2 km within a
few thousand years.
Graphs of the core samples show an abrupt transition from carbonate-rich
sediment to clay, followed by a gradual recovery (100K years) to carbonate.
Carbon dioxide levels and Cenozoic climate change. [note these are low resolution records]
The pCO2 record is converted to radiative forcing (a measure of global warming).
The benthic foraminifer d18O time series is a smoothed record of many observations.
The trend towards more positive d18O results from a combination of deep-sea cooling and global
ice volume increases.
from Pearson and Palmer, Science, v406, 2000.
Estimates of Cenozoic atmospheric pCO2 based on two independent proxies as
measured in sub-tropical deep-sea sediment cores from the Pacific.
The first curve is estimated from surface ocean pH as derived from the boron isotope
ratios of planktonic foraminifers. I.e. 11B/10B isotope ratio depends on seawater pH.
The second pCO2 curve is based on the d13C values of phytoplankton organic
compounds known as alkenones.
Both approaches assume chemical equilibrium between the ocean and atmosphere.
Sea surface pH for the past 60 Myr. These results are inferred from calculated
pCO2 data – obtained from boron isotope ratios (from foram shells).
Vertical error bars result from analytical error in determining d11B ratio.
During the Eocene, the oceans were TOO ACIDIC for corals with carbonate shells.
Pearson and Palmer, Nature 406, 2000
SEQUENCE OF EVENTS AT PETM (Eocene climate maximum).
1. Hypothesis – initial cause? The eruption of the North Atlantic Igneous Province (the
head of the Iceland mantle plume). 60 to 55 My.
2. Climate gets (generally) warm.
3. Ocean circulation changes – conveys surface warmth to deep ocean. Bottom
water starts warming up.
4. Methane hydrates stored in the sediments now become unstable. Some continental
margin slopes are de-stabilized, and slump, exposing more hydrates – which
decompose to methane. Strong positive feed-back!
5. Large (1,500 gigatons) amounts of organic carbon are vented into the ocean and
atmosphere.
6. The methane in the atmosphere and the oxidation of CH4 to CO2 (both greenhouse
gases), cause a strong temperature spike (the PETM).
7. This increased global temperature causes more water evaporation, more coastal
run-off, more nutrients into the ocean.
8. This increases biological productivity which removes the CO2 from the atmosphere
(the biological pump), and temperatures ‘cool off’.
9. Heat spike (few 1000 years). Cooling off period – 70K to 100K years.
Can it happen again? Is the PETM a good analog for future climate change?
There are presently large quantities
of methane stored as hydrates in the
arctic permafrost and in the shallow
continental margins.
In the continental margin sediments,
the low thermal conductivity of the
sediments slows the transfer of a
warming ocean to the hydrates
(PETM took 20K years).
But in the arctic, the melting
permafrost exposes the hydrates to
the atmosphere (or in shallow ponds
and lakes) and the hydrates can be
dissociated quickly.
And they are.
Using Eocene data to simulate future
climate (Zachos et al, Nature, 2008)
(a), ocean surface pH
(b), ocean surface calcite saturation
(c) and deep-ocean temperature
changes
(d) in response to the input of 5,000
Gt C of anthropogenic CO2 into the
atmosphere, starting from preindustrial CO2 levels.
Blue and green are w/wo a silicateweathering feedback. Projected
changes in deep-ocean temperature
in d assume a homogeneous
warming following temperature
sensitivities to a doubling of CO2
concentration: short-dashed line, 4.5
°C; solid line, 3.0 °C; long-dashed
line, 1.5 °C.
Present day carbon reservoirs
More than half the present day (non-carbonate) carbon is locked up in Gas hydrates.
What is methane hydrate – and how does it form?
Methane hydrate is methane gas (CH4)
‘locked’ into a water-ice lattice.
Places where you find
large methane hydrate
deposits.
Shallow continental
margins. (why?)
Methane forms from the
organic matter that rains down
from the surface of the ocean
– and is preserved in the
sediments at the seafloor.
Once this organic matter is
buried, it does not oxidize.
When the sediments are
heated (by being buried) to
100°C, the organic matter is
converted to methane gas, oil,
and other hydrocarbons.
During subduction on
continental margins, the
sediments are compressed
and the methane escapes.
Methane Hydrate
CH4 + H2O = (at low temperature and high pressure) methane hydrate
Looks like ice, but is unstable at atmospheric pressure and room
temperature.
Also called ‘clathrate’.
Phase diagram showing the water
depths (and pressures) and
temperatures for gas hydrate (grey
area) stability.
Many sediments lie within the range
denoted by the box.
The line shows the temperature in the
Earth as a function of depth
(geotherm).
At greater depths in the sediments, the
geotherm crosses from the hydrate
zone (purple region) to the gas zone.
This means that gas hydrate in
sediments usually overlies free gas.
Because hydrate is a solid, it
forms at the cold surface of the
sediments, and traps the
methane gas below (in the
warmer sediments).
But as seawater warms, the
solid lid decomposes, and the
methane escapes as bubbles.
Which can reach the surface
as methane gas.
An acoustic profile over a methane vent, using a 200 kHz sonar.
Taken from the Washington continental shelf, off Grays Harbor,
last September.
Rising methane bubble plume
Probably fish
Layers of high krill concentrations
RED layer is seafloor
Courtesy Julie Keister, UW/Oceanography
Release of Methane hydrate
Under stable ocean temperatures, methane hydrate can be released through
slumping and permafrost warming due to rising arctic waters.
Another way to release methane hydrate is through warming ocean temperatures.
NOTE: While this figure is required by law to be shown in all paleoclimate
courses, do not try this at home.
It is not dangerous, just extremely disappointing…
Notice that there are major climate shifts in the Cenozoic climate record that are not reflected
solely in the atmospheric CO2 concentration.
Other global disasters during the (troubled) Eocene.
The Chesapeake Bay impact crater was formed by a bolide that impacted the eastern
shore of North America about 35.5 million years ago, in the late Eocene.
It is the second largest impact crater in the U.S.
Continued slumping of sediments over the rubble of the crater have helped shape
Chesapeake Bay.
During the Eocene climate
Maximum, there were no ice
sheets, and the value of d18O
for seawater was probably near
zero (surface value today).
The changes in d18O in deep
seawater are shown on the
curve on the right.
Clearly it has been getting
cooler (more positive d18O ratio)
since the Eocene.
But since 30 My ago, the d18O
from the carbonate shells
contains both ice volume and
temperature information.
Because substantial amounts of
16O is stored in ice sheets, we
need to estimate HOW MUCH
ice is present to use this for
paleotemperatures.
Benthic, deep-water forams
Another possible mechanism for ‘cooling the earth’ - by removing CO2 from the
atmosphere. The raising of continental plateaus.
How would this decrease CO2 levels?
Increased weathering of the rocks/mountains that are uplifted.
Other geological processes related to seafloor
spreading that can cause major climate change.
The formation of high plateaus – and the increase
in weathering (and the draw-down of atmospheric
CO2 that results).
Classic example – the Tibetan Plateau.
The flux of sediments from the Tibetan plateau has increased dramatically over
the last 40 My (by factor of 10).
This both pulls CO2 directly from the atmosphere (as carbonic acid) but also
fertilizes the upper ocean and increases the ‘biological pump’.
This ‘fertilization’ is largely iron (which
is a limiting nutrient for both forams
and diatoms).
But it can also provide dissolved silica,
which promotes diatom blooms.
And diatoms are more effective at
pulling CO2 out of the atmosphere
than forams.
Some climate models have predicted
that this difference (diatom blooms) is
the amplifier of the orbital cycles that
produces the glacial/interglacial
cycles.
Even the types of organisms that dominate the surface ocean can have a major
impact on climate – i.e., foram vs diatom blooms.
i.e., organisms building shells
Ca++ + 2HCO3- --> CaCO3 + H2O + CO2
H4SiO4 --> SiO2·2H2O
(forams) and
(diatoms)
Diatoms are silica-limited. Forams are (mostly) iron limited in growth.
If environmental conditions favor diatom blooms over forams, more CO2 is withdrawn
from the atmosphere, and the global temperature can cool.
The uplifted Tibetan plateau also creates it’s ‘own weather’ – the Monsoons.
The summer sun on the plateau causes an upwelling zone over the plateau,
which draws (warm, very moist) air from the continental margins.
This causes very heavy seasonal rains, which are effective in weathering of the
rocks and flushing nutrients into the surrounding ocean.
Possible problem with this hypothesis: negative feed-backs on weathering
as a control on global climate. Increasing weathering in high plateaus can
cause a DECREASE in weathering in areas that are NOT high plateaus.
Other processes that control climate, on a shorter timescale – ‘Gateways’
(restrictive passages between continents) can open and close, and force changes
in ocean circulation.
Ocean circulation is the ‘pump’ that moves heat from the equator to the higher
latitudes, and distributes heat over the earth.
Opening of Drake passage at
20 My coincided with a new ice
sheet forming at Antarctica
Closing of the Isthmus of Panama
between 10 and 4 My coincided with
the formation of polar (and then midlatitude) ice sheets.
Weathering can produce both positive
and negative feedbacks.
By producing glaciers that grind-up
the rocks, and exposing more rock
surface to weathering, the resulting
drawdown in CO2 can make things
cooler. A positive feedback.
An example you have seen before.
Remember: when d18O goes negative, that means that the seawater
temperature is getting WARMER.