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 20-30C 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
• CO2 4-10 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 vaportemperature 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 5C 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 1-2C 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 gt 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 30N changed from tropical (26-27C)
  to paratropical (22-23C) from Eocene ? Oligocene
• Agrees with terrestrial fauna and floral data
• Increased seasonality during same interval
• Summer T decreased 3C
• Winter T decreased 5C
• 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