// lecture 20 Flashcards
paleoclimate
climate of the past.
science indicates that the solar system is about
4.5 billion years old, including earth. early forms of life on earth almost fro the beginning and solid evidence in rocks for bacteria 3 billion years before present.
the sun has changed its
energy output over its lifespan. over last billion years, its increased 10%. initially 75% as strong as it is now.
the orbit of the earth around the sun has set the climate over earth’s history:
- if the solar system has just the earth and the sun, the orbit would be a perfect ellipse that never changed.
- however, there are other plants/moons in the solar system which causes orbits to change with time. the tilt of earth changes over a 41,000 year cycle.
- a higher tilt would mean more seasonality (colder winters, warmer summers).
the location of continents has also had a role:
- these have shifted with time.
- mountain ranges appear, sometimes high latitude ice sheets aren’t possible, etc.
volcanoes as well:
- on short timescales, cause cooling.
- over very long timescales, can add significant CO2 to the atmosphere.
proxy
the agency, function, or office of a deputy who acts as a substitute for another.
proxy data
tells us about temp., precipitation, etc. through other indicators.
- biological data: tree rings, pollen, coral, fossils.
- cryological data: ice at the bottom of greenland is over 100,000 years old, antaractica is 800,000 years old. for example, ice cores have tiny bubbles of air trapped inside them that reveal past atmospheric composition.
- geological data: rocks, sediments, shape of land, etc.
- isotopic data: many of the previous data sets can be dated using carbon dating or other radiometric dating techniques. also isotopes tell us about precipitation and temp.
stable isotopes were
created in former stars, e.g. O16 (99.76%), O17, O18, C12 (98.9%), C12, N14 (99.6%), N15 (0.36%), H1 (99.99%), H2 = D.
radioactive isotopes can be
created by high energy radiation, but they tend to be metastable and decay into other isotopes over time, e.g. C14, O13, O15, H3.
people though earth could not have been ice covered because
life survived.
faint young sun paradox
raised by carl sagan in 1972. earth was warm most of this time when the sun was weak (geological evidence; e.g. rounded pebbles, mud cracks, ripple marks, microfossil algae). high GHG concentrations are likely key to keeping it warm.
snowball earth
occurred several times in the last billion when ice albedo feedback spiraled out of control. relatively recent discovery. happened several times around 700 million years ago.
dynamic snowball
two long live glaciations.
we know earth was covered in ice mostly because
- many independent, interlocking and consistent pieces of evidence.
- evidence that glaciers were present near the equator at low altitudes. found a deposit that can only have been produced by a glacier. used magnetic evidence to show these glacial deposits were within 10 degrees of equator.
- carbon-13 spikes in ocean sediments, means photosynthetic life was suppressed, which one would expect if the surface of the ocean was ice covered.
- banded iron deposits in ocean sediments, requires ocean to be anoxic so iron can build up, also to be expected if photosynthetic life is suppressed by ice cover.
- the glacial sediments and banded iron deposits are capped by thick layers of carbonate, indicating a large amount of CO2 came out of the atmosphere then, after the ice ball period. these cap carbonates are also low in carbon-13, indicating an enhanced weathering process and not plant burial.
extremely high GHG concentrations would be required to
deglaciate an ice covered earth.
release by volcanoes is relatively effective way of getting
CO2 into the atmosphere, this is small compared to current human emissions. volcanoes are important over hundred thousand year timescales.
land masses are key to
removing CO2 from the atmosphere over long timescales in a process called chemical weathering. when rain/snow falls on silicate rocks, it reacts and takes CO2 out of the atmosphere.
chemical weathering is a negative feedback:
- when climate is hotter, it’s easier for weathering to take CO2 out of the atmosphere.
- likely key for stabilization of climate over million of years.
- when earth is cold and ice-covered, weathering is suppressed and CO2 can build up.
during snowball earth, volcanic activity
injected CO2 into the atmosphere. with the land and oceans covered by ice, there was no chemical weathering to remove CO2 from the atmosphere, so it accumulated. eventually the GH effect became so strong that the ice began to melt, despite its high albedo. once initiated, melting would proceed very rapidly as the albedo dropped.
right after snowball earth thaws
CO2 concentrations would have been tremendously high. likely the hottest period in earth’s history. temps. jumped from -50 C to 50 C in just 1,000 years. massive weathering would gradually bring down CO2 and temps.
weathering of rock:
CO2 (carbon dioxide) + H2O (water) = H2CO3 (carbonic acid) then H2CO3 (carbonic acid) + CaCO3 (calcium carbonate) = Ca(HCO3)2 (calcium bicarbonate). in solution, calcium bicarbonate flows to ocean in streams.
reformation of limestone in ocean
- CO2 + H2O = H2CO3 (carbonic acid)
- H2CO2 + H2O + silicate minerals = HCO3- + cations (Ca++, Fe++, Na+, etc.) + clays. we made a bicarbonate ion, two of which are attracted to a calcium ion.
- Ca++ + 2HCO3- = CaCO3 + CO2 + H2O. to form limestone, burying one CO2 in the process.
- Net effect, buried one CO2 and made limestone. 2CO2 + ? + Ca++m = CaCO3 + CO2 + ?.
- this process does not require life - purely geochemical.
getting CO2 back through a volcano
shove the limestone down a subsudction zone with sand, make metamorphic rock and CO2 again. CaCo3 + SiO2 = CO2 + CaSiO3.
