06 - From greenhouse to icehouse (8) (1).pptgre

7/27/2019 06 - From greenhouse to icehouse (8) (1).pptgre

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From Greenhouse to Icehouse

• Climate has changed a lot over the last 50-100 million

years!• Large-scale cooling at the poles and lower latitudes.




• Two kinds of glaciations occur as climate cools:

- relatively small mountain glaciers

- large ice sheets




• No evidence for persistentice on Antarctica until ~35

Mya.• Greenland ice sheet beganto grow between 7 and 3Mya.

• Northern ice sheetsdeveloped almost 30 Myrlater than Southern ones,but still a response to samecooling trend.



Evidence from Fossils



Evidence from Oxygen Isotopes

• Perhaps the most important record of past climatefrom the ocean is the oxygen isotope signal.

• Most oxygen occurs as the very abundant 16O isotopeor as the much less abundant 18O isotope.

• Scientists refer to changes in the relative amounts of these isotopes as d18O.

8 P+

8 N

8 P+

10 N

Oxygen-1616

O Oxygen-1818

O




• Variations in d18O are preserved in the shells of foraminifera (CaC16O3 vs CaC18O3).

• Climate affects d18O through two processes.

• The first is by temperature. A change of 4.2 °Ccauses a change of 1‰ in the d18O value of foraminiferal calcite.




• Climate affects d18O through two processes.

• The second way is through ice sheet growth anddecay.

• Water (H2O) contains molecules with different oxygencombinations (H2

16O, H218O).

• When water evaporates, the lighter isotope moleculeis preferentially evaporated. When precipitation occurs,the heavier isotope preferentially condenses.




• As the watervapor movestowards higher

latitudes, theprecipitation hasless and less H2

18O.

• Snow and ice is

mostly H216O withvery little H2

18O(which is left behindin the ocean).




• A record of d18O for the last 70 Myr has been compiledfrom foraminifera that lived on the sea floor.



Evidence from Mg/Ca

Measurements• Another valuable measure of the ocean response toclimate forcing comes from analyzing the ratio of

magnesium (Mg) to calcium (Ca) in the shells of foraminifera.

• The ratio of these two elements in the foraminiferaskeleton is temperature-dependant.



Evidence from Mg/Ca

Measurements• The two records are similar, but also have somedifferences.



Does Geography Explain the

Cooling?• No!



Do Ocean Gateways Explain the

Cooling?• Maybe yes, maybe no. The computer models of theseevents don’t necessarily support the hypotheses.

• However, gateway changes definitely redistribute heatand salt at high latitude sites.



Do Changes in CO2 Explain the

Cooling?• Sea floor spreading slowed from 65 to 15 Mya, andcould explain the cooling over that interval, but the trendsince 15 Mya does not support the hypothesis.



What about Uplift Weathering?

• We would need persistent uplift over the last 50 Myrto explain the cooling. If this is so…

- we would expect large regions of young, highterrain.

- we would expect an increase in rock fragmentation to create the surface area

necessary for chemical weathering.

- we would expect an increase in chemicalweathering rates over the same period.




• Extensive high terrain…

• The Tibetan plateau, Andes Mountains, and the

European Alps all uplifted in the last 50 Myr.




• India crashes into Asia beginning some time around20-30 million years ago.




• Increased rock fragmentation…




• The highest amounts of suspended sediments in riverscome rivers draining the Himalayas and the Andes.




• Uplift of the Tibetan plateau created its own weather – the Southeast Asian Monsoon.

• Increased precipitation causes increased mechanicaland chemical weathering.




• Increased chemical weathering? Chemical weatheringrates are difficult to determine even on regional scales,much less the entire planet.

• Difficult to distinguish in rivers between ions producedby weathering of silicate rocks (hydrolysis) vs carbonaterocks (dissolution).

• Also impossible to study enough rivers to reach anaccurate measure of global weathering.



In Summary

• The cause of global cooling over the last 50 Myrremains uncertain.

• It is most likely driven by a decrease in atmosphericCO2.

• Not clear if the decrease was driven by decreasedinput tied to slowing sea floor spreading, or increased

removal tied to chemical weathering.

• Probably some combination of the two!

06 - From greenhouse to icehouse (8) (1).pptgre

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