Lecture 7: Paleozoic Era

1. Lecture 7: Paleozoic Era

2. Paleozoic Cooling & Plants Temperature Early Paleozoic Spread of land plants Late Paleozoic

3. Hypothesis • Spread of land plants caused cooling in Paleozoic

4. Details • Spread of land plants caused increased silicate-to-carbonate conversion

5. Recall: Silicate-to-Carbonate Conversion 1a. CO2 dissolves in rainwater, forming carbonic acid Rain 1b. Acid dissolves silicate rocks 1b: chemical weathering Land

6. Land Plants and Weathering: The Modern World • Modern plants increase the weatheringrate of the underlying rocks.

7. Effect of Plants on Chemical Weathering silicates silicates Vegetated region No vegetation Silicates dissolve at a certain rate Silicates dissolve 3 to 10 times faster

8. Modeled CO2 Values -- 1 RCO2 is the ratio of CO2 at the given time to current CO2 levels 450 mya. CO2 levels ~20 times current Origin of land plants ~ 450 million years ago (mya.)

9. Modeled CO2 Values -- 2 RCO2 dropped from ~20 to ~1 A 95% decrease! Spread of plants

10. Main Result of Model Calculations • 95% decrease in CO2 in mid-Paleozoic • CO2 levels dropped to near-current values 300 million years ago • Combined with weaker sun … • Would explain cold conditions

11. Very Warm

12. Evidence for late Mesozoic Warmth • Fossils of warm-weather plants/animals in high latitudes • Article from Science

13. Cenozoic Cooling

14. Cooling

15. Sediment Cores Sea Surface Ocean Sediment Core Layer of Sediment Bedrock

16. Late Cenozoic Cooling • Cooling ~10 million years ago may have been triggered by the collision of India with Asia • The collision caused the uplift of Tibet Plateau • Animation: http://www.scotese.com/pangeanim.htm

17. Collision  uplift of Tibet Plateau  development of Indian monsoon  increased regional rainfall  increased chemical weathering  increased removal rate of CO2

18. Indian Monsoon • Cherrapunji, India

19. Late Cenozoic Cooling and Human Evolution • Global Cooling (around 10 million years ago) • Reduced global evaporation • Reduced global rainfall • Altered vegetation patterns: • In Africa, tropical rainforest retreated and was replaced by grassland (savanna) • Earliest human-like primates appeared in the altered environment (http://www.mnh.si.edu/anthro/humanorigins/ha/primate.html)

20. Summary: Primary Causes of Long-Term Temperature Change • Changes in CO2 content of atmosphere • Caused by imbalances in long-term carbon cycle • The slow increase in solar energy output • (25% in 4 billion years)

21. Periods of cooling and warming i.e. glacial and interglacial ages, rapid changes

22. Late Cenozoic • Pleistocene Epoch • 1.8 million years ago to 10, 000 years ago • Holocene Epoch • 10, 000 years ago to today

23. Main Evidence For Pleistocene Climate Change • Sediment Cores • Ice cores

24. Temperature Fluctuations (from ocean sediments) • Here is a typical oxygen-isotope-ratio graph: Warmer

25. Data From an Antarctic Ice Core

26. Pleistocene Climate Changes • Large amplitude temperature oscillations • Around 5C • Principal period of oscillations: 100, 000 years

27. Pleistocene Glacial Cycles – Simplfied Glacial Maximum 100, 000 yrs Glacial Minimum Amount of Ice Time

28. The Most Recent Glacial Cycle Last Glacial Maximum ? Amount of Ice 18 0 Time (Thousands of Years before present)

29. Extent of ice at Last Glacial Maximum

30. The Most Recent Deglaciation • ~18, 000 years ago, almost all Canada and much of northern Eurasia were covered by ice sheets • Most of this ice was gone by ~10, 000 years ago

31. More Detail • Evidence exists for different frequencies of temperature change • Slowest: 100, 000 years • Intermediate: 40, 000 years • Fastest: 20, 000 years.

32. Cause of Temperature Fluctuations • Leading theory: Temperature oscillations were caused by small irregularities in Earth’s motions • Theory was developed by Milankovitch

33. Variations in Earth’s Motions • Shape of orbit • Period: 100, 000 years • Tilt of axis • Period: 40, 000 years • “Precession” (circular motion of axis) • Period: 20, 000 years • Same periods seen in ocean sediments!

34. Milankovitch cycles

35. Changes in Tilt of Axis • Higher tilt  more sunlight in high latitudes • Lower tilt  less sunlight in high latitudes

36. One Scenario • () tilt of axis  () high-latitude sun • Result: reduction (expansion) of high-latitude ice sheets

37. Hypothesis • Pleistocene temperature oscillations were driven by variations in Earth’s motions • Amplitude of oscillations was increased by positive feedbacks

38. Effect of positive feedbacks Temp Response with positive feedbacks time Response without feedbacks

39. Positive feedbacks • Ice-albedo, water vapor (already discussed) • Feedbacks involving CO2 and CH4 (methane)

40. Ice-Core Data: Temp and CO2 CO2 Temp

41. Ice-Core Data: Temp and CH4

42. T, CO2, CH4

43. Correlation • Ice-core data shows positive correlation between temperature and CO2 and between temperature and CH4 • i.e., when temperature was high (low), CO2 and CH4 were high (low)

45. Pluvial Lakes in Western U. S.

46. Pluvial Lakes in Western U. S. • These lakes existed in the Pleistocene Epoch • “Pluvial” means formed by excess rainfall (not glacier melt) • Lake Bonneville • Lake Lahontoan

47. Searles Lake • In Mojave Desert • Tufa Formations • (Trona Pinnacles)

49. One Theory • Canadian Ice Sheet Caused a Split in the Jet Stream • increased frequency of storms in southwestern U. S.

50. Migration of Vegetation Zones • Vegetation zones moved hundreds of miles northward when climate warmed • See http://stommel.tamu.edu/~baum/paleoveg.html