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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

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greenhouse earth 100 mya
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?
cretaceous tectonics
Cretaceous Tectonics
  • Pangaean continent broken into several smaller continents
    • High sea level flooded continental interiors
paleobotanical evidence for warm climate
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
paleobiological evidence for warm climate
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
cretaceous paleoclimate
Cretaceous Paleoclimate
  • Faunal and floral remains provide estimates of Cretaceous equator-to-pole temperatures
    • Zonal averaged temperature captures general temperature trend
cretaceous paleotemperatures
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
gcm models
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
cretaceous climate
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
data model mismatch
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
data model mismatch10
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
data model mismatch11
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
paleotemperature data
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
problems with models
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
ocean transfer of heat
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
ocean transfer of heat15
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
continental configuration favorable
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
model simulation
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
conclusions
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
sea level and climate
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
asteroid impacts and climate
Asteroid Impacts and Climate
  • Asteroid impacts can have apocalyptic consequences
    • Long-term climate change is not one of them
cool tropics paradox22
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
possible answers
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
data model mismatch24
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
diagenetic alteration of shells
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
evaluation of diagenetic effects
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?

diagenesis
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
prevailing view
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”
caveats
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
mollusks kobashi et al 2001
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
eocene mollusk data
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)
comparison of mollusk data
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
mollusk temperature trends
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
implications of mollusk study
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