Earth Sciences 240A - Lecture 33 - Climate II
Three Icehouse
periods in last 500 My
~430 Mya: brief
325-240 Mya: very major
35-0 Mya: major
Plate Tectonics:
Importance
Watch: influences on
CO2 changes
The Polar Position Hypothesis
Ice sheet should appear only if
continents at polar positions.
Test (See Table):
430 Mya:
425-325 Mya:
325-240 Mya:
240-35 Mya:
35-0 Mya:
The Lesson of Pangaea
Examine from 200 Mya
200 – 180 Mya
Pangaea:
pole-to-pole (~symmetrical)
Sea levels ~ same
as now
Sun ~ 1% weaker
than now (so cooler)
Evidence: no ice
sheets!
So higher CO2
Assume 1600 ppm (pre-industrial: 280 ppm)
Dry interior
(winter snow readily melted later)
Conclusion: only geography not
enough
Tectonic Controls of
CO2
The BLAG Hypothesis
The Uplift Weathering Hypothesis
The Ocean Heat Transfer
Hypothesis
The BLAG Hypothesis
or ‘Spreading Rate Hypothesis’
Faster spreading = more volcanism
Thermostat: rate of weathering
Requires cycling of C
Limestone:
subducted → volcanism
Test (see Table)
Check spreading
rates
All floor over
~175 my subducted!
Last 100 my: Rates >50% higher
Climate warmer
The Uplift Weathering
Hypothesis
Weathering: NOT response but
forcing factor
Continent-continent collision
High elevation,
fresh rock
Test (see Table)
Looks good but
difficult to find thermostat
Continental
ice sheet growth?
~100 Mya Pangaea had fragmented
Sea levels high, temperatures warm,
no ice
Ocean Heat Transport
Hypothesis
Placement of continents to change
ocean flow paths
‘Superplume’ warming of water
Poor records
Sea Level Change Factor
Why do sea levels change? [because the ocean volumes change]
More abundant ocean ridges= sea
level rise
Continent collisions=sea level drop
Volcanic sea plateaus=sea level
rise
Continental ice sheets=sea level
drop
Thermal expansion=sea level rise
Correlation of sea levels with
glaciers:
100 Mya high seas
should have aided glacier growth [Not]
Low sea levels
today should oppose Icehouse [Not]
Current Icehouse
Cooling for past 55 my; Icehouse
for last 35 my
Best evidence from oxygen isotopes
The BLAG Hypothesis
50-15 mya: slow spreading (cooling
- √)
15-0 mya: increased spreading
(cooling – x)
Uplift Weathering Hypothesis
Massive Tibetan Plateau
But what’s the character of
weathered products?
Ocean Heat Transport Hypothesis
Drake’s Passage
So, Why Are We In The
Icehouse?
High rate of chemical weathering must
be a factor
Milankovitch cycles: Probably a
(minor) factor right now
Long-Term Forecast:
`MUCH COLDER!
Next is Last
Friday
Climate III
Next Monday
Review of previous exam (the one
posted)