Greenhouse Earth 100 mya Important for understanding potential Important for understanding potential

anthropogenic changes in climateanthropogenic changes in climate CretaceousCretaceous

Most recent example of Greenhouse worldMost recent example of Greenhouse world Geologic record reasonably preservedGeologic record reasonably preserved

Indicates warm intervalsIndicates warm intervals Continental configuration knownContinental configuration known Can estimate rates of seafloor spreadingCan estimate rates of seafloor spreading

Do climate models simulate the warmth of this Do climate models simulate the warmth of this greenhouse climate?greenhouse climate? If so, are high levels of atmospheric COIf so, are high levels of atmospheric CO22

required?required?

Cretaceous Tectonics Pangaean continent broken into several Pangaean continent broken into several

smaller continentssmaller continents High sea level flooded continental interiorsHigh sea level flooded continental interiors

Paleobotanical Evidence for Warm Climate

Warm-adapted evergreen Warm-adapted evergreen vegetation found above vegetation found above Arctic circleArctic circle Leaves of breadfruit Leaves of breadfruit

tree found north of tree found north of Arctic CircleArctic Circle

Today breadfruit found Today breadfruit found in tropical to in tropical to subtropical subtropical environmentsenvironments

Equator-to-pole Equator-to-pole temperature gradient temperature gradient different in Cretaceousdifferent in Cretaceous

Paleobiological Evidence for Warm Climate

Warm-adapted Warm-adapted animals found at high animals found at high latitudeslatitudes Dinosaurs, turtles Dinosaurs, turtles

and crocodiles and crocodiles found pole-wards of found pole-wards of the Arctic and the Arctic and Antarctic circlesAntarctic circles

Coral reefs Coral reefs indicative of warm indicative of warm tropical waters tropical waters found within 40° of found within 40° of equator equator

Cretaceous Paleoclimate Faunal and floral remains provide estimates of Faunal and floral remains provide estimates of

Cretaceous equator-to-pole temperaturesCretaceous equator-to-pole temperatures Zonal averaged temperature captures general Zonal averaged temperature captures general

temperature trendtemperature trend

Cretaceous Paleotemperatures Equatorial temperatures a few degree-C Equatorial temperatures a few degree-C

warmer than todaywarmer than today Polar temperatures 20°-30°C warmerPolar temperatures 20°-30°C warmer

Cretaceous an ice-free worldCretaceous an ice-free world Modern Antarctic ice at high latitude Modern Antarctic ice at high latitude

are also at high altitudeare also at high altitudeTemperature very coldTemperature very cold

Understanding Cretaceous climate Understanding Cretaceous climate requires understanding unusual requires understanding unusual equator-to-pole temperature gradientequator-to-pole temperature gradient

GCM Models Changes in geography Changes in geography

without ice sheetswithout ice sheets Tropical T okayTropical T okay T above 40° well T above 40° well

below range of below range of paleotemperaturespaleotemperatures

Change in geography Change in geography and COand CO22 required required COCO22 44-10 X PAL-10 X PAL Improved match but Improved match but

tropical T too hightropical T too high T above 40° still too T above 40° still too

lowlow

Cretaceous Climate COCO22 at least 4x PAL at least 4x PAL

Conclude from lack of ice sheetsConclude from lack of ice sheets Geography and high COGeography and high CO22 do not replicate do not replicate

global temperature gradientglobal temperature gradient Higher COHigher CO22 levels increase global levels increase global

average temperatureaverage temperature Questions remain on how to handleQuestions remain on how to handle

Albedo-temperature feedbackAlbedo-temperature feedback Water vapor–temperature feedbackWater vapor–temperature feedback Role of cloudsRole of clouds

Data-Model Mismatch Problems with the data or interpretationProblems with the data or interpretation Could temperature tolerance of Could temperature tolerance of

organisms changed over time?organisms changed over time? Pervasive and gradual shift towards a Pervasive and gradual shift towards a

lower tolerance for temperaturelower tolerance for temperature Interpret climate as being too warmInterpret climate as being too warm

No reason why such a trend would No reason why such a trend would exist for diverse groups of organismsexist for diverse groups of organisms

Evolutionary change in ecology of Evolutionary change in ecology of fauna and flora unlikelyfauna and flora unlikely

Data-Model Mismatch Faunal and floral evidence for warm climateFaunal and floral evidence for warm climate

Coastal environmentsCoastal environmentsCoastal environments may be maritimeCoastal environments may be maritime

•Not indicative of cold continental Not indicative of cold continental interiors with harsh wintersinteriors with harsh winters

Fossil record from continental interior Fossil record from continental interior scarcescarce

Fossil preservation in coastal maritime Fossil preservation in coastal maritime environments could bias the geologic environments could bias the geologic recordrecord

Data-Model Mismatch Diagenetic alteration of Diagenetic alteration of

geochemical recordsgeochemical records Particularly isotopic recordsParticularly isotopic records

Colder isotopic Colder isotopic temperatures requires temperatures requires alteration on the seaflooralteration on the seafloor

Sea floor alteration of Sea floor alteration of foraminifera shells has foraminifera shells has been documentedbeen documented

Alteration of Cretaceous Alteration of Cretaceous shells have not been shells have not been studied systematicallystudied systematically

Paleotemperature Data If isotopic records are biased by alteration If isotopic records are biased by alteration

on the cold seaflooron the cold seafloor Current records underestimate Current records underestimate

equatorial paleotemperaturesequatorial paleotemperatures Actual tropical temperature could be Actual tropical temperature could be

5°C higher5°C higher Model simulations with high COModel simulations with high CO22

Warm the tropics sufficientlyWarm the tropics sufficiently Polar temperatures would not be Polar temperatures would not be

underestimatesunderestimates

Problems with Models Ocean general circulation crudeOcean general circulation crude

Coastal and equatorial upwelling not in Coastal and equatorial upwelling not in global modelglobal model

Deep water formation not easily Deep water formation not easily modeledmodeled

If Cretaceous ocean transported 2x the If Cretaceous ocean transported 2x the heat as modern oceanheat as modern ocean Poles warmed by greater heat influxPoles warmed by greater heat influx Tropics would be cooled by greater Tropics would be cooled by greater

export of heatexport of heat

Ocean Transfer of Heat Heat transfer through deep ocean todayHeat transfer through deep ocean today

Formation of cold dense water in polar regions Formation of cold dense water in polar regions with some warm saline water from Mediterraneanwith some warm saline water from Mediterranean

Ocean Transfer of Heat Deep ocean 100 mya may have been filled with Deep ocean 100 mya may have been filled with

warm saline bottom waterwarm saline bottom water Formed in tropics or subtropics and flowed Formed in tropics or subtropics and flowed

pole-ward transferring heatpole-ward transferring heat

Continental Configuration Favorable

Large seaway covered N tropical and Large seaway covered N tropical and subtropical latitudessubtropical latitudes Seaways should have been under Seaways should have been under

sinking arm of Hadley cellsinking arm of Hadley cellDry air would have caused Dry air would have caused evaporation to exceed precipitationevaporation to exceed precipitation

Increased salinity of surface waterIncreased salinity of surface water Explanation consistent with several large Explanation consistent with several large

oceanic anoxic eventsoceanic anoxic events AOE may have been caused by warm AOE may have been caused by warm

saline bottom waters saline bottom waters

Model Simulation Warm saline water could have formed in N hemisphere Warm saline water could have formed in N hemisphere

when salinity exceeded 37when salinity exceeded 37 Would have been curtailed by freshwater runoff from Would have been curtailed by freshwater runoff from

continents into coastal regions in epicontinental seawayscontinents into coastal regions in epicontinental seaways

Conclusions Attempts to model Cretaceous partly Attempts to model Cretaceous partly

successfulsuccessful Simplest explanation tropical Simplest explanation tropical

temperatures were highertemperatures were higherNeed more detailed studies of Need more detailed studies of diagenetic alteration of tropical fossilsdiagenetic alteration of tropical fossils

Need to be able to estimate Cretaceous Need to be able to estimate Cretaceous atmospheric COatmospheric CO22 levels levels

Sea Level and Climate Change in sea level can affect climateChange in sea level can affect climate

Changes the heat capacityChanges the heat capacityFlood land with low heat capacity with Flood land with low heat capacity with

seawater that has high heat capacityseawater that has high heat capacity Formation of epicontinental seas will create Formation of epicontinental seas will create

moderate maritime climatemoderate maritime climate During Cretaceous, large epicontinental seas During Cretaceous, large epicontinental seas

formedformed Replaced arid interior with coastal Replaced arid interior with coastal

environmentenvironmentCreated widespread moderate maritime Created widespread moderate maritime

climate conditionsclimate conditions

Asteroid Impacts and Climate Asteroid impacts can have apocalyptic Asteroid impacts can have apocalyptic

consequencesconsequences Long-term climate change is not one of themLong-term climate change is not one of them

Cool Tropics Paradox

Cool Tropics Paradox Distribution of nearshore marine and Distribution of nearshore marine and

terrestrial fauna and floraterrestrial fauna and flora Low-latitude temperature higher than Low-latitude temperature higher than

todaytoday However, models of Cretaceous-Eocene However, models of Cretaceous-Eocene

warm climate require greenhousewarm climate require greenhouse Equator-to-pole temperature gradients Equator-to-pole temperature gradients

cannot be modeledcannot be modeled Tropical and low-latitude SST Tropical and low-latitude SST

determined by oxygen isotopic determined by oxygen isotopic analyses too lowanalyses too low

Possible Answers Increased ocean heat transferIncreased ocean heat transfer

Fundamentally different mode of deep Fundamentally different mode of deep water formation and circulationwater formation and circulation

Diagenetic alteration of foraminiferal Diagenetic alteration of foraminiferal teststests Pervasive sea floor alteration in deep Pervasive sea floor alteration in deep

sea oozes and chalkssea oozes and chalks Regional upwellingRegional upwelling

Delivery of cool deep water to surfaceDelivery of cool deep water to surfaceUpwelling not easily modeledUpwelling not easily modeled

Data-Model Mismatch Mismatch particularly evident during the Mismatch particularly evident during the

EoceneEocene Similar patterns emerged for Similar patterns emerged for

Cretaceous and PaleoceneCretaceous and Paleocene Generally evident record during last Generally evident record during last

500 my500 my Authors have questioned the primary role Authors have questioned the primary role

of atmospheric COof atmospheric CO22 in determining global in determining global temperaturetemperature Over the next 200 years, COOver the next 200 years, CO22 levels levels

may reach 4-6 x PALmay reach 4-6 x PAL

Diagenetic Alteration of Shells Colder isotopic Colder isotopic

temperatures requires temperatures requires alteration on the seaflooralteration on the seafloor

Diagenetic modeling Diagenetic modeling suggests overgrowth and suggests overgrowth and infilling of shell infilling of shell microstructuremicrostructure Probably results in 1-Probably results in 1-

2°C decrease from SST2°C decrease from SST Far short of that Far short of that

required to explain required to explain mismatchmismatch

Evaluation of Diagenetic Effects Expect the Expect the 1313C of foraminiferal calcite to approach bulk C of foraminiferal calcite to approach bulk

carbonate values (~3carbonate values (~3‰‰)) Significant isotopic differentials are observed in most fossil Significant isotopic differentials are observed in most fossil

assemblagesassemblages Fit well the expected depth habitat of various organismsFit well the expected depth habitat of various organisms

Question: are the Question: are the fossils represented fossils represented by these data by these data diagenetically diagenetically altered so that they altered so that they are giving low SST?are giving low SST?

Diagenesis? Significant species-specific isotopic differentials Significant species-specific isotopic differentials

observedobserved Differentials consistent between different sitesDifferentials consistent between different sites Species-specific relationships between Species-specific relationships between 1313C C

and size observed in surface-dwelling taxaand size observed in surface-dwelling taxa Shells with secondary euhedral calcite crystals Shells with secondary euhedral calcite crystals

on surface easily recognized and avoidedon surface easily recognized and avoided Data and observations has led most authors to Data and observations has led most authors to

conclude that substantial diagenetic conclude that substantial diagenetic overprinting of shell chemistry is unlikelyoverprinting of shell chemistry is unlikely Even when microstructural preservation Even when microstructural preservation

imperfectimperfect

Prevailing View Tom Crowley and Jim Zachos (2000)Tom Crowley and Jim Zachos (2000)

““There is little robust geological There is little robust geological evidence indicating that tropical sea evidence indicating that tropical sea surface temperatures increased as surface temperatures increased as atmospheric COatmospheric CO22 increased” increased”

Caveats Oxygen in calcareous oozes mostly in Oxygen in calcareous oozes mostly in

porewater whereas carbon is in mineralsporewater whereas carbon is in minerals Oxygen isotopic alteration is water Oxygen isotopic alteration is water

dominateddominated Carbon isotopic alteration is rock Carbon isotopic alteration is rock

dominateddominated Studies of exceptionally well preserved Studies of exceptionally well preserved

mollusks, inorganic cements and mollusks, inorganic cements and phosphatesphosphates Indicate considerably warmer Indicate considerably warmer

temperatures during Cretaceous-Eocenetemperatures during Cretaceous-Eocene

Mollusks (Kobashi et al., 2001) Diagenesis easily recognizedDiagenesis easily recognized

Metastable aragonite converts to Metastable aragonite converts to calcitecalcite

Nearshore organisms record seasonalityNearshore organisms record seasonality If seasonality preserved, If seasonality preserved, 1818O accurateO accurate Could be influenced by freshwater Could be influenced by freshwater

runoffrunoff Paleobathymetry can be estimatedPaleobathymetry can be estimated

Mollusks generally do not exhibit vital Mollusks generally do not exhibit vital oxygen isotope effectsoxygen isotope effects

Eocene Mollusk data Excellent preservationExcellent preservation

All shells > 99% All shells > 99% aragonitearagonite

Comparison of oxygen Comparison of oxygen isotopic data from isotopic data from modern and ancient modern and ancient mollusk shellsmollusk shells Seasonality Seasonality

preserved in shellspreserved in shells 1818O of oldest shells O of oldest shells

considerably more considerably more negative (warmer negative (warmer SST)SST)

Comparison of Mollusk Data Oxygen isotope Oxygen isotope

trend parallels trend parallels benthic recordbenthic record

Mollusk record in Mollusk record in agreement with agreement with results from fish results from fish otolithsotoliths

Records show same Records show same amplitude of cooling amplitude of cooling in surface and deep in surface and deep waterwater

Mollusk Temperature Trends Climate at 30°N changed Climate at 30°N changed

from tropical (26-27°C) to from tropical (26-27°C) to paratropical (22-23°C) from paratropical (22-23°C) from Eocene Eocene Oligocene Oligocene Agrees with terrestrial Agrees with terrestrial

fauna and floral datafauna and floral data Increased seasonality during Increased seasonality during

same intervalsame interval Summer T decreased Summer T decreased

~3°C~3°C Winter T decreased ~5°CWinter T decreased ~5°C

Winter mollusk SST agree Winter mollusk SST agree with foraminiferal SSTwith foraminiferal SST Suggests winter growthSuggests winter growth

Implications of Mollusk Study If results from Mississippi Embayment are If results from Mississippi Embayment are

representative of open oceanrepresentative of open ocean SST in general and winter SST in SST in general and winter SST in

particular higher at low latitudes in particular higher at low latitudes in EoceneEocene

Results are consistent with prediction of Results are consistent with prediction of GCM models with high atmospheric COGCM models with high atmospheric CO22

Decrease in atmospheric CODecrease in atmospheric CO22 and more and more significant winter coolingsignificant winter coolingConsistent with oxygen isotopic record Consistent with oxygen isotopic record from mollusksfrom mollusks