Glaciation part 1 Flashcards
Glacier
A perennial mass of ice that moves or is capable of movement over land (movement due to gravity or compression force- needs at least 50 m)
Pleistocene glaciations
20 glacial advances in the past 2 million years
Causes of glaciation
Climatic cooling
Milankovitch cycles
Positive Feedback
Climatic cooling driven by
Continental coalescence (huge continental areas) (not enough time)
Continental drift into far souther latitudes (near south pole) (not enough time)
Variation in solar output (stable during this time)
Volcanic activity resulting in particulate matter in the atmosphere (not associated with pleistocene)
Milankovitch Cycles
Variation in the earth’s elipticallness in orbit around sun, axial tilt, and precession comprise three dominant cycles.
The three cycles impact the seasonality and location of solar energy around the earth.
Milankovitch Cycles: eccentricity
Shape of the Earth’s Orbit around the sun changes. When orbit is most ellipitcal the earth recieves 20-30% more solar radiation at the perihelion than at the aphelion. Today the orbit is at low eccentricity (i.e less ellipticallity)
Cyclic period: 100, 000 years
Perihelion- closest point
Aphelion- farthest point
Milankovitch Cycles: Axial tilt (obliquity)
Inclination of the Earth’s title in relation to its plane of orbit around the sun.
Minimum tilt solar radiation is more evenly distributed across seasons
Hypothesis- minimum tilt could promote growth of ice sheets (more snow with warmer winter, less opportunity to melt in cooler summer)
Max tilt- 24.5o
Min tilt- 22.1o
Current- 23.5
Cyclic period- 41 000 years
Milankovitch Cycles: Precession
When axis is tilted towards Vega, the position of the Northern Hemisphere winter solstice coincides with the aphelion and the summer solstice with the perihelion resulting in greater seasonal contrasts.
Currently, Earth experiences perihelion close to the winter solstice. (i.e. lesser seasonal contrasts)
Cyclic period: 22 000 years
Milankovitch Cycles in combination
When the three different cycles combine they are responsible for major changes in the Earth’s climate.
Growing Ice sheets
eliminate plant/animal life over large areas
Natural production of greenhouse gases decline
more white surfaces increase reflection of light energy
Retreating ice sheets
Increases plant/animal life
Increases production of greenhouse gases
albedo decreases
Characteristics of the Pleistocene glaciations
Most of the glaciations had the same range and extent
Glacial sheets consisted of massive ice formations up to 4 km thick and covered up to one third of Earth’s land mass
Much more extensive in the N hemisphere (more land mass and more continental climate)
Greater extent in NA than Eurasia (ex, last glaciation 80% of ice was in NA)
Changes Associated
1) Elimination of Terrestrial habitat (ice covered 33% of terrestrial area last glaciation, but some land opened up due to falling sea levels)
2) Shifts in Climate regimes
3) changes in sea level
Glacial Refugium
-An area that has remained relatively unchanged while surrounding areas are glaciated and which has thus served as a refuge for species displaced by the glacial advance. -Caused by glaciation
Some refugia: beringian (alaska) , columbia (oregon), atlantic , Mississippi
-are centers of diversity or centers of endemism (evidence that these areas provided safe retreat during glaciation)
Microrefugia
glacial refugia of smaller geographical area that may have been important to fewer species than larger main refugia for survival during glaciation
Three types: distal/remote, widespread/diffuse, proximal or ecotonal
Ex: Lemming range across eastern beringia and Canada.
Eastern refugia had several mini refugia everywhere
This is also evident because fossils occur in places where there were glaciers
Proglacial lakes (lakes that form just beyond frontal margin of advancing or retreating glacier)
glaciation caused more interconnected with proglacial lakes
Shifts in climate regimes
Large changes in distribution of biomes, especially due to changes in precipitation
Ex. as cool air descends down the face of glaciers the air warms as pressure increases. The warm dry air tends to make environments near glacial margins moderate temp and dry.
General aridification and more sand dunes during glaciation
ex. NA ice sheets tended to cause jet stream splitting -> dry easterly winds
Lahontan cutthraoat trout evolved in the resulting pluvial lakes and fed on endemic forage fish tui chub - trout got really big due to big sized lakes
Changes in sea level
Global changes: tremendous uptake of moisture (last glaciation 84 million cubic m)
Ocean decreased by 130 m
These drops in sea level caused formation of land bridges
ex. beringia and southeast asia and austrailia
Local changes: in high north latitude the mass of 2-3km high glaciers caused a depression of land up to 300 m in coastal areas
When icesheets retreat the rebound of land takes much longer. This results in massive inflow of marine waters that form wide expanses of shallow seas.
ex. In eastern NA influx of marine waters inundated the St. Lawrence River Valley such that large inland sea extended from the Atlantic coast to Lake ontario
Eustatic changes
global changes that occur in all oceans at the same time
Isostatic changes
local changes in sea level when global levels remain unchanged.
Biogeographic consequences
Latitudinal Shift in Biomes- ex in NA the biomes shift northward.
Elevational shift in biomes- more rapid that latitudinal shifts, biomes move up the mountain
Range shifts- where animals live/are found change
Biotic exchanges
- Great american interchange: exchange of mammals between N and SA following formation of Central american land bridge (3.5 Ma)
- Other examples btwn africa and asia, Asia to new guinea.. etc
Great American Interchange
The bridge was more of a filter than a highway
more things came from the north than the south (asymmetric exchange)
Reasons for asymmetric exchange
Advantage from northern forms to southern forms
1) better migrators (a lot of them came across beringia land bridge)
2) better survivors who diversified more readily
3) better competitors
