// lecture 22 Flashcards
bubbles in ice cores
contain ancient air with the CO2 and CH4 of the time when the ice was formed.
antarctic ice cores
800kyr record from antarctica
land ice
- last glacial age - 20kbp
- disappearing ice - 12kbp
- present conditions - 0kbp
the previous ice age (-20kbp)
sea level was 120 meters lower than now. possible to walk from siberia to alaska.
- ice volume in NH 15 times today’s.
- ice 2-3 km thick over much of north america. much more sea ice.
- global temp. about 6 C colder than today.
- CO2 about 180 ppm compared to 275 ppm in preindustrial era and 400 ppm now.
- windier and dustier than now.
most of ice gone by 8000 bp
- laurentide ice sheet entirely covered canada and some northern states.
- lower sea level allowed for migrations across the bering strait.
- meltwater was trapped inland as the ice sheet retreated.
- the great lakes are the last vestige of this ice sheet.
ice ages are
simultaneous in both hemispheres
ice cores and ice ages:
- temperature, dust, CO2, CH4.
- cold, dusty, low on GHGs.
- warm, wet, more GHG.
heinrich events
armadas of ice bergs in the north atlantic during the ice ages. find continental rock in mud.
the ice ages lasted
2.7 million ybp to about 10,000 years ago. large ice sheets covered northwestern europe and norther north america.
antarctic ice core record of past 800,000 years
- last glacial max: 20 kbp
- lack glacial min: 130 kbp
- CO2 and methane are low during ice ages
- ice ages are dusty
the last glacial maxium (LGM) occured
around 20,000 years ago. sea level was lower by ~120 m at the time of the LGM because of the storage of water in the continental ice sheets.
lake missoula
~18,000 ybp ice core at clark fork river.
evidence of lake missoula flood
channeled scablands with lava plugs, potholes drilled by giant whirlpools, giant current ripples formed by deep, fast flowing water, and deep gorges with steep walls formed by flood: Multnomah Falls
drumlin
an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine. first recorded in 1833.
glalcial erractic
piece of rock that differs from the size and type of rock native to the area in which it rests. carried by glacial ice, often over distances of hundreds of kilometers.
ice core have been taken from
greenland and antarctica. up to 800,000 years old and up to 3,700 m deep.
evaporation favors
light isotopes, rain favors heavy isotopes. cloud above ice is enriched in light isotopes.
isotopes in ocean sediments record
glacial ice volume.
isotopes in ice cores indicate
local temp.
isotopic evidence: ice cores
- the water vapor in the atmosphere starts out with a certain fraction of heavy isotopes of O18 and Deuterium usually in the subtropics where its warm.
- as it moves along toward the ice sheet, it cools and the heavy isotopes condense, just like they are less likely to evaporate than heavier isotopes.
- the more it cools, the less heavy isotopes remain as the vapor condenses.
- so when it forms ice, the colder it has gotten, the less heavy isotopes.
- so more heavy isotope in ice means the air was warmer when the snow formed.
huge temp. changes in ice ages:
up to 10 C in antarctica, 5 C globally.
slow onset of
glacial periods and rapid decay. suggests that ice dynamics may play a role in the decay process (one reason we’re concerned about dynamic loss in greenland/ west antarctica).
isotopic evidence: ocean sediments
- during an ice age water evaporates from the ocean and is deposited as ice on land. the lighter isotopes, O16 and H, evaporate first.
- the heavier isotopes get left behind, so ocean sediments get enriched in the heavy isotopes O18 and Deueterium (heavy hydrogen, has a neuron).
- so you can estimate the ice volume from the O18 ratio in ocean sediments. more O18 in ocean sediment, more land ice.
can extend records back (to 3 million years) with
ocean sediment data.
from oxygen isotopes in ocean sediments:
more frequent switching between glacial/interglacial before 600,000 years ago.
carbon dioxide does not
drive ice ages, it is a feedback. CO2 changed due to temp. changes and amplified the changes (positive feedback).
the driver of ice ages is
changes in solar radiation due to changes in earth’s orbit around the sun.
solar radiation in the NH summer is key for
growth/melt of ice sheets.
- it’s always cold enough for snow in the winter at high latitudes, so winter temp. doesn’t matter.
- a colder summer means snow doesn’t melt and can accumulate.
- less sunlight in the summer means colder summers and expansion of high latitude ice sheets.
- and vice versa.
theory of the ice ages:
orbital induced solar radiation changes and global ice volume.
- deglaciation events happen when there’s more summer sunlight.
- onset of ice ages occur when there’s less summer sun.
tilt angle of earth is currently
23.44 degrees. tilt is the reason why we have seasons.
tilt of axis of rotation varies from
22.5 to 24.5 degrees. dominant period of 41 kyr.
higher tilt makes
summers hotter and winters colder, especially in high altitudes.
currently at the middle of the road for tilt, but
tilt is decreasing and will reach its minimum in 8,000 years
