Greenhouse Earth 100 mya Important for understanding potential anthropogenic changes in climate Important for understanding potential anthropogenic changes in climate Cretaceous 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? Do climate models simulate the warmth of this greenhouse climate?  If so, are high levels of atmospheric CO 2 required?

Cretaceous Tectonics Pangaean continent broken into several smaller continents Pangaean continent broken into several smaller continents  High sea level flooded continental interiors

Paleobotanical Evidence for Warm Climate Warm-adapted evergreen vegetation found above Arctic circle 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 Warm-adapted animals found at high latitudes 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 Faunal and floral remains provide estimates of Cretaceous equator-to-pole temperatures Faunal and floral remains provide estimates of Cretaceous equator-to-pole temperatures  Zonal averaged temperature captures general temperature trend

Cretaceous Paleotemperatures Equatorial temperatures a few degree-C warmer than today Equatorial temperatures a few degree-C warmer than today Polar temperatures 20°-30°C warmer 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 Changes in geography without ice sheets Changes in geography without ice sheets  Tropical T okay  T above 40° well below range of paleotemperatures Change in geography and CO 2 required Change in geography and CO 2 required  CO X PAL  Improved match but tropical T too high  T above 40° still too low

Cretaceous Climate CO 2 at least 4x PAL CO 2 at least 4x PAL  Conclude from lack of ice sheets Geography and high CO 2 do not replicate global temperature gradient Geography and high CO 2 do not replicate global temperature gradient  Higher CO 2 levels increase global average temperature Questions remain on how to handle Questions remain on how to handle  Albedo-temperature feedback  Water vapor–temperature feedback  Role of clouds

Data-Model Mismatch Problems with the data or interpretation Problems with the data or interpretation Could temperature tolerance of organisms changed over time? 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 Mismatch Faunal and floral evidence for warm climate Faunal and floral evidence for warm climate  Coastal environments  Coastal environments may be maritime Not indicative of cold continental interiors with harsh wintersNot 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 Mismatch Diagenetic alteration of geochemical records Diagenetic alteration of geochemical records Particularly isotopic 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 If isotopic records are biased by alteration on the cold seafloor 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 CO 2 Model simulations with high CO 2  Warm the tropics sufficiently  Polar temperatures would not be underestimates

Problems with Models Ocean general circulation crude 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 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 Heat transfer through deep ocean today Heat transfer through deep ocean today  Formation of cold dense water in polar regions with some warm saline water from Mediterranean

Ocean Transfer of Heat Deep ocean 100 mya may have been filled with warm saline bottom water 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 Large seaway covered N tropical and subtropical latitudes 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 Explanation consistent with several large oceanic anoxic events  AOE may have been caused by warm saline bottom waters

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

Conclusions Attempts to model Cretaceous partly successful 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 CO 2 levels

Sea Level and Climate Change in sea level can affect 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 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 can have apocalyptic consequences Asteroid impacts can have apocalyptic consequences  Long-term climate change is not one of them

Cool Tropics Paradox

Distribution of nearshore marine and terrestrial fauna and flora Distribution of nearshore marine and terrestrial fauna and flora  Low-latitude temperature higher than today However, models of Cretaceous-Eocene warm climate require greenhouse 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 Increased ocean heat transfer Increased ocean heat transfer  Fundamentally different mode of deep water formation and circulation Diagenetic alteration of foraminiferal tests Diagenetic alteration of foraminiferal tests  Pervasive sea floor alteration in deep sea oozes and chalks Regional upwelling Regional upwelling  Delivery of cool deep water to surface  Upwelling not easily modeled

Data-Model Mismatch Mismatch particularly evident during the Eocene 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 CO 2 in determining global temperature Authors have questioned the primary role of atmospheric CO 2 in determining global temperature  Over the next 200 years, CO 2 levels may reach 4-6 x PAL

Diagenetic Alteration of Shells Colder isotopic temperatures requires alteration on the seafloor Colder isotopic temperatures requires alteration on the seafloor Diagenetic modeling suggests overgrowth and infilling of shell microstructure 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 Expect the  13 C of foraminiferal calcite to approach bulk carbonate values (~3‰) Expect the  13 C of foraminiferal calcite to approach bulk carbonate values (~3‰) Significant isotopic differentials are observed in most fossil assemblages 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? Significant species-specific isotopic differentials observed Significant species-specific isotopic differentials observed  Differentials consistent between different sites  Species-specific relationships between  13 C and size observed in surface-dwelling taxa Shells with secondary euhedral calcite crystals on surface easily recognized and avoided 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 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 Tom Crowley and Jim Zachos (2000) Tom Crowley and Jim Zachos (2000)  “There is little robust geological evidence indicating that tropical sea surface temperatures increased as atmospheric CO 2 increased”

Caveats Oxygen in calcareous oozes mostly in porewater whereas carbon is in minerals 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 Studies of exceptionally well preserved mollusks, inorganic cements and phosphates  Indicate considerably warmer temperatures during Cretaceous-Eocene

Mollusks (Kobashi et al., 2001) Diagenesis easily recognized Diagenesis easily recognized  Metastable aragonite converts to calcite Nearshore organisms record seasonality Nearshore organisms record seasonality  If seasonality preserved,  18 O accurate  Could be influenced by freshwater runoff  Paleobathymetry can be estimated Mollusks generally do not exhibit vital oxygen isotope effects Mollusks generally do not exhibit vital oxygen isotope effects

Eocene Mollusk data Excellent preservation Excellent preservation  All shells > 99% aragonite Comparison of oxygen isotopic data from modern and ancient mollusk shells Comparison of oxygen isotopic data from modern and ancient mollusk shells  Seasonality preserved in shells   18 O of oldest shells considerably more negative (warmer SST)

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

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

Implications of Mollusk Study If results from Mississippi Embayment are representative of open ocean 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 CO 2  Decrease in atmospheric CO 2 and more significant winter cooling  Consistent with oxygen isotopic record from mollusks