// lecture 21 Flashcards
the amount of carbon in the atmosphere is always
tiny compared to that in ocean and rock.
the rate of outgassing from earth is probably
pretty steady, but could speed up or slow down with plate tectonics or volcanism.
the rate of weathering, and the speed of the biological pump (the rate at which life takes CO2 and makes sediment) are dependent on
the climate, hence a strong feedback between the atmospheric CO2 and the climate, which appears on time scales of 100’s of millions of years and also on the time scale of thousands of years during the Pelistocene (most recent) glacial ages.
warm climate means
more weathering > less CO2 > cooler climate
Cooler climate means
less weathering > more CO2 > warmer climate
climate is driven towards
the middle by CO2 weathering feedback. a stabilizing feedback. works on a time scale of hundreds of thousands of years and relatively big climate changes.
prokaryotes
single celled organism that lacks a membrane bound nucleus, e.g. bacteria, archaea.
eukaryotes
life with cells with nucleus.
life might have survived snowball earth because
of hydrothermal vents or cracks in the sea-ice, which could be a potential refigum for phototrophs.
steve warrren (UW ATM S)
studies ice types in antarctic as a way to understand snowball earth. also grows other types of ice in labs that don’t exist on earth.
blue ice in antarctica might be
the most like ice on snowball earth.
iceball earth/hothouse cycles occurred when they did because:
- continents were bunched together on the equator.
- very high rate of weathering? low CO2?
- ice albedo feedback runs away and covers earth.
- weathering stops, CO2 builds back slowly.
- CO2 reaches tipping point and ice melts.
- now have high CO2, very warm climate, very high weathering rate, CO2 drops, cycle repeats.
continental drift (wegener, 1920s)
last 250 million years; another major factor in the history of climate change
movement of antarctica over the south pole
allowed an ice sheet to form. higher planetary albedo.
decline in atmosphere CO2 starting 60 million years ago
coincides with the rise of the Himalays and Rockies (more weathering as fresh rock exposed). also a concurrent slowdown in continental drift (less volcanism, less CO2).
closing of the isthmus of panama was the
last major change to the land distribution (about 4 million years ago).
warm mesozoic (250-65 million years ago)
dinosaurs; 2-6 C warmer globally. poles were especially warm - mystery. evidence for polar warmth was lush ferns and alligators in siberia.
cretaceous sea levels were
200 m higher than today. the entire middle of N. america was a giant seaway.
hot climates in last 250 million years:
- CO2 levels were several times higher than present. know this from many lines of evidence: isotopes in rocks/fossils, examination of plant fossils, carbon cycle models. increased undersea volcano activity was likely important for releasing more CO2.
- methane hydrates (methane frozen in ice) may have been important for a couple of the warmest times in these periods. may have rapidly been released from the bottom of the ocean, increasing GHG concentrations quickly.
cretaceous warm
large shallow sea in tropics, seas to both poles.
last 35 million years (since end of early Cenozoic)
- earth slowly cooling.
- life retreats from poles.
- polar ice caps established.
- most recent ice ages begin ~ 3 million years ago.
- cause of decline in CO2? - Himalayas form when India collides with Asia and the fresh rock and high precipitation around mountains increased weathering (maybe?)
cores can be drilled out of ocean sediment, lake sediment, soil, or a glacier. they contain different types of proxy climate indicators:
- leftover biology - pollen, leaves, shells of small sea animals.
- physical evidence - ice rafted pebbles, chemistry of sledge, sand, etc.
- isotopic evidence - abundance of various stable and radioactive isotopes.
most elements have several
stable isotopes, differing only by the number of neutrons and hence the weight of the atom and the molecule of which it is a part of.
most of the stable isotopes are
heavier by one neutron than their more common versions.
their abundance varies due to mass fractionization
- water containing a heavy isotope will evaporate less readily and condense more readily than a lighter isotope.
- after a bunch of vapor comes off the ocean, and moves north and starts condensing, the heavier isotopes are condensed first, so the colder it gets and the more vapor is removed by condensation, the lighter the remaining sample will become. so isotopes in snow are a measure of how cold the air was when the water was condensed.
- plants prefer lighter carbon to heavier carbon, so plant material tends to be light in C13.
standard mean ocean water (SMOW)
samples that are depleted in the heavy isotope have negative deltas. SMOW is thus 0%.
ocean sediment cores
can get 5 million years out of them.
why does )18 in ocean indicate more land ice?
- when water evaporates from the ocean, the lighter O16 isotope evaporates more readily, leaving more of the heavier O18 behind in the ocean.
- if this evaporated water does not return to the ocean, but is stored in an ice sheet, then the fractional abundance of O18 in the ocean will increase because of the removed O16.
- High O18 in the ocean sediment must mean more stored ice on land, greater land ice volume.
ice cores
give variable time resolution and time depth.
14C is produced
by cosmic rays hitting 14N, or by nuclear explosions in air. decays with a half life of 5,730 years.
the 14C of coal or petroleum is
zero because it’s been in the ground away from the atmosphere for millions of years.
the 13C of coal or petroleum is low because
plants try to avoid it during photosynthesis.
14C and 13C are both
decreasing n atmospheric CO2
