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Palaeo- Atmospheric composition (Glaciation of Antarctic (Theories that…
Palaeo- Atmospheric composition
Palaeoclimates
Why study?
Uncertainties in climate prediction involve climate changes in the past
Palaeoclimatology assess whether changes are consistent with current theories of climate system
Provides a unique test-bed for models that project large changes in future to be evaluated
Palaeo=
Ancient, relating to geological past
Cenozoic era
Began with mass extinction-
loss of 3/4 of plant and animal species (extinction of dinosaurs)
Boundary between Eocene & Oligocene marks first extensive glaciation of Antarctic
Climate
Deep sea temps track estimates of atm CO2
CO2 higher during "greenhouse" periods of past: CO2 around 2000ppm (5x greater than today)
CO2 reduced over time due to silicate weathering: upthrusting mountains exposed minerals to weathering -> conversion to carbonates (CO2 removed) -> cooling of Earth
CO2 most frequently estimated atmospheric gas from proxy records
CO2 emission & consumption kept in balance by negative feedback- operates on million year time scale
Measuring Palaeo CO2
Oldest record based on age of oldest ice < 1 million years
Clearly defined glacial-interglacial cycles of the Pleistocene
Ice core drilling- high resolution records of CO2 & methane
Ice core data from EPICA Dome C ice core: deuterium is a proxy for local temps, CO2 measured from ice core air
Proxies for CO2
Stomatal density
Balance must be struck between CO2 gain and H2O loss
High CO2 concs in air: plant produces fewer stomata to conserve water- provides basics for CO2 proxy based on leaf stomatal density
Stomata are specialised epidermal cells on leaves of plants- allow for CO2 to enter leaf and water diffuses out
(evapotranspiration)
Boron
Boron has two stable isotopes 11-B & 10B, 80% and 20% respectively.
formation of boron in seawater is pH- dependent
CO2 of ocean can then be calculated from pH
Ice core records
Glaciation of Antarctic
Benthic oxygen isotopes are combined measure of bottom water temps and global ice volume
Around 34 million years ago there was a drop in temp- associated w/ abrupt growth of Antarctic ice sheet
Transition from Eocene to Oligocene
Theories that caused glaciation
Thermal isolation of Antarctica due to Southern Gateway opening
Declining levels of atmospheric co2
Thresholds: height-mass balance between 2x & 3x preindustrial levels of atm co2
CO2 decline:
boron isotope analysis of carbonate shells to get ocean pH and dissolved CO2 conc.
O2 isoptoes display positive shift suggesting drop in temp and increase in ice volume of O-E transition
Analogues for future
Palaeoclimate modelling
Simulations of air temp difference from PE for 8 models show large differences in degree of polar amplification
In leeds: project compares climate model predictions of Pliocene period
All models are wrong- some are useful
Late Pliocene
Chosen to study as proxy evidence shows this period as time of warmer than modern climate and continents in near same position
Late Pliocene showed CO2 levels up to 400ppm
Characteristics
Global mean temps of 2-3 degree c higher than pre-industrial
Reduced extent of ice sheets- 25m higher than today
Sea ice evidence only in Arctic