Greenhouse Earth 100 mya

1. Greenhouse Earth 100 mya • Important for understanding potential anthropogenic changes in climate • Cretaceous • Most recent example of Greenhouse world • Geologic record reasonably preserved • Indicates warm intervals • Continental configuration known • Can estimate rates of seafloor spreading • Do climate models simulate the warmth of this greenhouse climate? • If so, are high levels of atmospheric CO2 required?

2. Cretaceous Tectonics • Pangaean continent broken into several smaller continents • High sea level flooded continental interiors

3. Paleobotanical Evidence for Warm Climate • Warm-adapted evergreen vegetation found above Arctic circle • Leaves of breadfruit tree found north of Arctic Circle • Today breadfruit found in tropical to subtropical environments • Equator-to-pole temperature gradient different in Cretaceous

4. Paleobiological Evidence for Warm Climate • Warm-adapted animals found at high latitudes • Dinosaurs, turtles and crocodiles found pole-wards of the Arctic and Antarctic circles • Coral reefs indicative of warm tropical waters found within 40° of equator

5. Cretaceous Paleoclimate • Faunal and floral remains provide estimates of Cretaceous equator-to-pole temperatures • Zonal averaged temperature captures general temperature trend

6. Cretaceous Paleotemperatures • Equatorial temperatures a few degree-C warmer than today • Polar temperatures 20°-30°C warmer • Cretaceous an ice-free world • Modern Antarctic ice at high latitude are also at high altitude • Temperature very cold • Understanding Cretaceous climate requires understanding unusual equator-to-pole temperature gradient

7. GCM Models • Changes in geography without ice sheets • Tropical T okay • T above 40° well below range of paleotemperatures • Change in geography and CO2 required • CO24-10 X PAL • Improved match but tropical T too high • T above 40° still too low

8. Cretaceous Climate • CO2 at least 4x PAL • Conclude from lack of ice sheets • Geography and high CO2 do not replicate global temperature gradient • Higher CO2 levels increase global average temperature • Questions remain on how to handle • Albedo-temperature feedback • Water vapor–temperature feedback • Role of clouds

9. Data-Model Mismatch • Problems with the data or interpretation • Could temperature tolerance of organisms changed over time? • Pervasive and gradual shift towards a lower tolerance for temperature • Interpret climate as being too warm • No reason why such a trend would exist for diverse groups of organisms • Evolutionary change in ecology of fauna and flora unlikely

10. Data-Model Mismatch • Faunal and floral evidence for warm climate • Coastal environments • Coastal environments may be maritime • Not indicative of cold continental interiors with harsh winters • Fossil record from continental interior scarce • Fossil preservation in coastal maritime environments could bias the geologic record

11. Data-Model Mismatch • Diagenetic alteration of geochemical records • Particularly isotopic records • Colder isotopic temperatures requires alteration on the seafloor • Sea floor alteration of foraminifera shells has been documented • Alteration of Cretaceous shells have not been studied systematically

12. Paleotemperature Data • If isotopic records are biased by alteration on the cold seafloor • Current records underestimate equatorial paleotemperatures • Actual tropical temperature could be 5°C higher • Model simulations with high CO2 • Warm the tropics sufficiently • Polar temperatures would not be underestimates

13. Problems with Models • Ocean general circulation crude • Coastal and equatorial upwelling not in global model • Deep water formation not easily modeled • If Cretaceous ocean transported 2x the heat as modern ocean • Poles warmed by greater heat influx • Tropics would be cooled by greater export of heat

14. Ocean Transfer of Heat • Heat transfer through deep ocean today • Formation of cold dense water in polar regions with some warm saline water from Mediterranean

15. Ocean Transfer of Heat • Deep ocean 100 mya may have been filled with warm saline bottom water • Formed in tropics or subtropics and flowed pole-ward transferring heat

16. Continental Configuration Favorable • Large seaway covered N tropical and subtropical latitudes • Seaways should have been under sinking arm of Hadley cell • Dry air would have caused evaporation to exceed precipitation • Increased salinity of surface water • Explanation consistent with several large oceanic anoxic events • AOE may have been caused by warm saline bottom waters

17. Model Simulation • Warm saline water could have formed in N hemisphere when salinity exceeded 37 • Would have been curtailed by freshwater runoff from continents into coastal regions in epicontinental seaways

18. Conclusions • Attempts to model Cretaceous partly successful • Simplest explanation tropical temperatures were higher • Need more detailed studies of diagenetic alteration of tropical fossils • Need to be able to estimate Cretaceous atmospheric CO2 levels

19. Sea Level and Climate • Change in sea level can affect climate • Changes the heat capacity • Flood land with low heat capacity with seawater that has high heat capacity • Formation of epicontinental seas will create moderate maritime climate • During Cretaceous, large epicontinental seas formed • Replaced arid interior with coastal environment • Created widespread moderate maritime climate conditions

20. Asteroid Impacts and Climate • Asteroid impacts can have apocalyptic consequences • Long-term climate change is not one of them

21. Cool Tropics Paradox

22. Cool Tropics Paradox • Distribution of nearshore marine and terrestrial fauna and flora • Low-latitude temperature higher than today • However, models of Cretaceous-Eocene warm climate require greenhouse • Equator-to-pole temperature gradients cannot be modeled • Tropical and low-latitude SST determined by oxygen isotopic analyses too low

23. Possible Answers • Increased ocean heat transfer • Fundamentally different mode of deep water formation and circulation • Diagenetic alteration of foraminiferal tests • Pervasive sea floor alteration in deep sea oozes and chalks • Regional upwelling • Delivery of cool deep water to surface • Upwelling not easily modeled

24. Data-Model Mismatch • Mismatch particularly evident during the Eocene • Similar patterns emerged for Cretaceous and Paleocene • Generally evident record during last 500 my • Authors have questioned the primary role of atmospheric CO2 in determining global temperature • Over the next 200 years, CO2 levels may reach 4-6 x PAL

25. Diagenetic Alteration of Shells • Colder isotopic temperatures requires alteration on the seafloor • Diagenetic modeling suggests overgrowth and infilling of shell microstructure • Probably results in 1-2°C decrease from SST • Far short of that required to explain mismatch

26. Evaluation of Diagenetic Effects • Expect the d13C of foraminiferal calcite to approach bulk carbonate values (~3‰) • Significant isotopic differentials are observed in most fossil assemblages • Fit well the expected depth habitat of various organisms Question: are the fossils represented by these data diagenetically altered so that they are giving low SST?

27. Diagenesis? • Significant species-specific isotopic differentials observed • Differentials consistent between different sites • Species-specific relationships between d13C and size observed in surface-dwelling taxa • Shells with secondary euhedral calcite crystals on surface easily recognized and avoided • Data and observations has led most authors to conclude that substantial diagenetic overprinting of shell chemistry is unlikely • Even when microstructural preservation imperfect

28. Prevailing View • Tom Crowley and Jim Zachos (2000) • “There is little robust geological evidence indicating that tropical sea surface temperatures increased as atmospheric CO2 increased”

29. Caveats • Oxygen in calcareous oozes mostly in porewater whereas carbon is in minerals • Oxygen isotopic alteration is water dominated • Carbon isotopic alteration is rock dominated • Studies of exceptionally well preserved mollusks, inorganic cements and phosphates • Indicate considerably warmer temperatures during Cretaceous-Eocene

30. Mollusks (Kobashi et al., 2001) • Diagenesis easily recognized • Metastable aragonite converts to calcite • Nearshore organisms record seasonality • If seasonality preserved, d18O accurate • Could be influenced by freshwater runoff • Paleobathymetry can be estimated • Mollusks generally do not exhibit vital oxygen isotope effects

31. Eocene Mollusk data • Excellent preservation • All shells > 99% aragonite • Comparison of oxygen isotopic data from modern and ancient mollusk shells • Seasonality preserved in shells • d18O of oldest shells considerably more negative (warmer SST)

32. Comparison of Mollusk Data • Oxygen isotope trend parallels benthic record • Mollusk record in agreement with results from fish otoliths • Records show same amplitude of cooling in surface and deep water

33. Mollusk Temperature Trends • Climate at 30°N changed from tropical (26-27°C) to paratropical (22-23°C) from Eocene  Oligocene • Agrees with terrestrial fauna and floral data • Increased seasonality during same interval • Summer T decreased ~3°C • Winter T decreased ~5°C • Winter mollusk SST agree with foraminiferal SST • Suggests winter growth

34. Implications of Mollusk Study • If results from Mississippi Embayment are representative of open ocean • SST in general and winter SST in particular higher at low latitudes in Eocene • Results are consistent with prediction of GCM models with high atmospheric CO2 • Decrease in atmospheric CO2 and more significant winter cooling • Consistent with oxygen isotopic record from mollusks