Back into the Icehouse: The Last 55 Million Years

1. Back into the Icehouse:The Last 55 Million Years Global Climate Change Since 55 Myr Ago

2. Evidence From Ice and Vegetation • Southern Hemisphere • No evidence for ice on Antarctica until 25 Mya. • Ice rafted debris in continental margin coastal sediments • Size of ice has increased towards the present • Large growth • 13 Myr ago • 7 7 Myr ago • Lower Middle Latitudes • Earliest evidence in Andes • Between 7 and 4 Myr ago

3. Antarctica Today • An ice sheet as much as 4 km thick covers more than 97% of the continent • Around the margins mountains protrude through the thinner ice

4. Fossil Evidence • Nothofagus • Type of beech tree • Found in Antarctica prior to 40 Myr ago • Disappeared with polar climate A modern beech forest at the southern tip of South America

5. Lichens • The only vegetation found in Antarctic today • Summer meltwater ponds in coastal valleys

6. Ice in the Northern Hemisphere • First developed on Greenland • Between 7 and 3 Myr ago • 1st glacial evidence in Alaska • High coastal mountains • About 5 Myr ago

7. North American and Eurasian Ice Sheets • Appeared 2.7 Myr ago • Formed and melted in repeated cycles • Maximum size increased after 0.9 Myr ago • Even though ice sheets developed in N. America 30 million years later than in the southern hemisphere • Part of the same overall cooling trend

8. North Polar Regions • Canadian Arctic 60 Myr ago (at 80o N) • Palm-like vegetation • Ancestors of modern alligators

9. North Polar Regions • Cold conditions developed • Conifer forests of spruce and larch by 20 Myr ago

10. Modern Tundra Has developed in only the last few million years

11. Tundra • Scrubby, grass-like or shrub-like vegetation • Grows on permafrost • Thawed layers lying above deeper frozen ground

12. Permafrost Alaska Siberia

13. Solifluction • During the warm season the surface melts and slides downslope over the frozen layer • Creates a hummocky appears

14. Shapes of Tree Leaves • Can be used to reconstruct climate • Smooth-edged leaves are found in tropics • Jagged-edged leaves grow in colder climates • Cause is unknown • Leaf-margin evidence in western N. America shows ongoing cooling over 55 Myr

15. Cooling in Western N. America Inferred from Leaf-Margin Evidence • Cooling trend in middle latitudes during the last 55 Myr. • Punctuated by small short-lived warmings

16. Oxygen Isotope Data

17. Periodic Table of the Elements

18. Recall that . . . • Atomic Number • Number of protons in the nucleus of an atom • All isotopes of an element have the same atomic number

19. Atomic Weight • Also called atomic mass. • Given in atomic mass units (amu) • Standard Mass is determined using a neutral atom of carbon-12 • It is set at exactly 12 mass units (12 u) • 1 atomic mass unit (amu) = 1/12 of this mass which is 1.66 X 10-26 g. • Also considers the fractional abundance of each isotope. • Example using chlorine: • Of the two isotopes of Cl, approximately: • 75% (75.53%) are the lighter isotope chlorine-35 and • 25% (24.47%) are chlorine-37 • Finding the atomic mass (34.969 amu x .7553) + (36.966 amu x .2447) = 35.45 amu

20. Two Climatically Important Isotopes of Oxygen O O 8 O 16 Usually written as 16 8 O 18 18 Usually written as

21. Isotopes of Oxygen • Provide information with regard to temperature • Oxygen trapped in ice indicates temperature at the time the oxygen was trapped • Oxygen trapped in shells is an indicator of water temperature

22. Ocean Water and Foraminifera • Foraminifera are marine Protists that secret shells composed of calcite (CaCO3). • Both isotopes of oxygen exist in seawater although 18O accounts for only about 0.2%. • Both isotopes of oxygen are found in the shells of forams. • The 18O/16O ratio in the shells provides information as to the seawater temperature at the time the forams lived.

23. Two Kinds of Foraminifera Live in Climatically Important Parts of the World’s Oceans

24. Planktonic Foraminifera • Live in the upper 100 m of the ocean • Shells contain oxygen taken from waters • near the surface

25. Benthic Foraminifera • Live on the seafloor and within • upper layers of sediment • Shells contain oxygen from • deep water

26. Symbols Used . . . • Parts per thousand is abbreviated as: • Infinitesimally small change is abbreviated using the lower case Greek letter delta. o /oo δ

27. 18O/160 Measurements • Individual measurements of 18O/160 ratios • Reported as departures in parts per thousand from a laboratory standard. • Large amounts of 18O compared to 16O • 18O – enriched (positive δ 18O values) • Or 16O – depleted • Small amounts of 18O compared to 16O • More negative δ 18O values • Referred to as 18O – depleted • Or 16O – enriched

28. Water Temperature and δ 18O Values in Foraminifera Shells • As temperature increases the δ 18Odecreases • Each 4.2o increase, δ 18O decreases 1 /oo • Modern Sub-Tropical Oceans (21o C) • Planktonic Foraminifer have δ 18O values of -1 /oo • Benthic Foraminifer in cold water (2o C) have δ 18O value of + 3.5 /oo o o o

29. Oxygen Isotopes and the Hydrologic Cycle -δ18O -δ18O 18O 18O +δ18O • Lighter 16O evaporates more easily from the ocean. • Heavier 18O is more easily removed from the atmosphere by precipitation • Along coastlines it quickly flows (runoff) back into the ocean

30. Isotope Fractionation • Process by which water vapor in the atmosphere becomes progressively enriched in 16O toward the higher latitudes • Each cycle of evaporation and condensation: • - Decreases the δ18O value of the water vapor by 10o/oo in • relation to the ocean water left behind

31. On a Growing Glacier • Fractionation results in lighter 16O being locked into glacial ice. • Heavier 18O builds up in seawater. • The δ18O is more positive than if no ice were present.

32. Evidence from Mg/Ca • Ratio of magnesium to calcium in foraminifera shells • Process of Mg replacing Ca • Depends on water temperature • Ratio increases 8 to 10% for each 1o C increase in temperature • Linear relationship • Mg/Ca trend is similar to δ18O “signal”

33. What if the ice sheets on Antarctica and Greenland melted over the next 10,000 years? And, how would this be recorded in the shells of planktonic and benthic foraminifera?

34. Melting of all Modern-Day Ice • The ocean’s average δ18O value would shift from its present 0 o/oo value to about -1o/oo • The result of lighter 16O from melting ice • This change would be recorded in the shells of benthic foraminifera everywhere in the world ocean

35. Possible Misinterpretation • Recall that each 4.2o C increase, δ 18O decreases 1 o/oo. • If in 10,000 years a future climate scientist measured foraminifera shells deposited during this interval and didn’t know about the deglaciation . . . • The -1 o/oo shift would be interpreted as a 4.2o C warming of the entire world ocean and not as a change in ice volume

36. Changes in Ocean Temperature and in the Amount of Water in Ice Sheets Must be Combined

37. The Equation Used by Climate Scientists Considers . . . • δ 18O measured in foraminifera shells • δ 18O mean value of ocean water at the time the shells were formed • The equation tells us that measured changes in the mean δ18O foraminifera are the result of: • Changes in the temperature of the water in which the shell formed and • Changes in the mean δ18O of the oceans

38. So, what is the equation? Don’t worry about it. We’re not going to get into the math. But, if you must find it look on page 101 of your text!

39. δ 18O Trends Over the Last 55 Myr • Shows global cooling • δ18O values increase towards the present-day.

40. δ 18O Trends Over the Last 55 Myr • This trend could be a result of: • Cooling of the deep ocean • Growth of ice sheets on land • A combination of both factors

41. δ 18O Trends Over the Last 55 Myr Between 55 and 50 Myr δ18O values increased +1.5 o/oo resulting in cooling of over 6o C (1.5o/oo x 4.2oC/o/oo) The volume of ice that did exist was negligible. Cooling of deep water must have been the main cause.

42. δ 18O Trends Over the Last 55 Myr 35 Myr Some ice had appeared on Earth The volume of ice is unknown. The composition of the ice is also unknown.

43. δ 18O Trends Over the Last 55 Myr Present • Increase due to a combination of: • Ice sheet growth • Deep water cooling

44. Cooling of the Deep Ocean • Between 40 Myr ago and today • Deep ocean δ18O values increased +2.75 o/oo • About 1 o/oo was due to δ18O deficient ice sheets • Additional cooling (another 7o C) of the deep ocean accounts for the residual 1.74 o/oo.(1.74o/oo x 4.2o C/o/oo) • Total deep ocean cooling has been about 14o C • When changes in temperature or ice volume occurred over the last 35 Myr can’t be determined • Both probably affected δ18O values simultaneously

45. More Temperate Polar Climates 55 Myr Ago • Deep ocean temperatures today average 2o C. • The deep ocean has cooled by at least 14o C in the last 55 Myr • 55 Myr ago the deep ocean temperature must have been 16o C. • Assuming that deep ocean water originated at high latitudes as today • The water originating in polar climates 55 Myr ago must have been warmer than today • indicating that those climates were much more temperate than they are today.

46. Why Did Global Climate Cool Over the Last 55 Myr?

47. Was it BLAG? • There should be evidence of: • A slowing of global mean seafloor spreading and subduction rates • Resulting slower rates of CO2 input into the atmosphere to cause global cooling

48. Changes in Spreading Rates • The average rate slowed until 15 Myr ago • Consistent with cooling • Increased in the last 15 Myr • But, ice appeared in the northern hemisphere in the last 15 Mry • BLAG may have caused global cooling before 15 Myr ago, but it doesn’t explain the cooling since then.

49. Was it the Uplift Weathering Hypothesis? • Its three main predictions must be verified: • High elevation terrain must be more common today than in the past 55 Myr. • This high terrain must be causing unusual amounts of rock fragmentation • This creates more surface area for greater rates of weathering. • There must be unusually high rates of chemical weathering.

50. Prediction 1 Extensive High Terrain