18O records Ice Volume Every 10-m change in sea level produces an Every 10-m change in sea level produces an

~0.1~0.1‰‰ change in change in 1818O of benthic foraminiferO of benthic foraminifer The age of most prominent The age of most prominent 1818O minimaO minima Correspond with ages of most prominent Correspond with ages of most prominent

reef recording sea level high standsreef recording sea level high standsAbsolute sea levels estimates from reefsAbsolute sea levels estimates from reefs

•Correspond to shifts in Correspond to shifts in 1818OO Reef sea level record agreement with Reef sea level record agreement with

assumption of orbital forcingassumption of orbital forcing125K, 104K and 82K events forced by 125K, 104K and 82K events forced by precessionprecession

Astronomical 18O as a Chronometer

Relationship between orbital forcing and Relationship between orbital forcing and 1818O so strongO so strong 1818O values can orbitally tune sediment O values can orbitally tune sediment

ageageConstant relationship in time between Constant relationship in time between insolation and ice volumeinsolation and ice volume

Constant lag between insolation Constant lag between insolation change and ice volume changechange and ice volume change

Date climate records in ocean sedimentsDate climate records in ocean sedimentsIn relation to the known timing of In relation to the known timing of orbital changesorbital changes

Orbital Tuning 41,000 and 23,000 41,000 and 23,000

year cycles from year cycles from astronomically dated astronomically dated insolation curvesinsolation curves Provide tuning Provide tuning

targetstargets Similar cycles Similar cycles

embedded in the embedded in the 1818O ice volume O ice volume curves are curves are matched and datedmatched and dated

Now most accurate Now most accurate way to date marine way to date marine sedimentssediments

Orbital-Scale Change in CH4 & CO2 Important climate records from last 400 kyaImportant climate records from last 400 kya

Direct sampling of greenhouse gases in iceDirect sampling of greenhouse gases in ice Critical questions must be addressedCritical questions must be addressed

Before scale of variability in records Before scale of variability in records determineddeterminedReliability of age dating of ice core?Reliability of age dating of ice core?Mechanisms and timing of gas trapping?Mechanisms and timing of gas trapping?Accuracy of the record?Accuracy of the record?

•How well gases can be measured?How well gases can be measured?•How well do they represent How well do they represent

atmospheric compositions and atmospheric compositions and concentrations?concentrations?

Vostok Climate Records Illustrates strong correlation Illustrates strong correlation

between paleotemperature between paleotemperature and the concentration of and the concentration of atmospheric greenhouse atmospheric greenhouse gasesgases

Concentrations of COConcentrations of CO22 and CH and CH44

moved in tandem with moved in tandem with paleotemperatures derived paleotemperatures derived from stable isotope recordsfrom stable isotope records

Mechanisms of relationships Mechanisms of relationships poorly understoodpoorly understood

To what extent did higher To what extent did higher greenhouse gases cause greenhouse gases cause greater radiative warming of greater radiative warming of the Earth's atmosphere?the Earth's atmosphere?

Dating Ice Core Records Ice sheets thickest in centerIce sheets thickest in center

Ice flow slowly downwardIce flow slowly downward Then flows laterally outwardThen flows laterally outward

Annual layers may be preserved and countedAnnual layers may be preserved and counted Deposition of dust during winterDeposition of dust during winter

Blurred at depth due to ice deformationBlurred at depth due to ice deformation

Reliability of Dating Dust layer countingDust layer counting

Best when ice deposition rapidBest when ice deposition rapidGreenland ice accumulates at 0.5 m yGreenland ice accumulates at 0.5 m y-1-1

•Layer counting good to 10,000 yearsLayer counting good to 10,000 yearsAntarctica ice accumulates at 0.05 m yAntarctica ice accumulates at 0.05 m y-1-1

•Layering unreliable due to slow Layering unreliable due to slow depositiondeposition

Where unreliable, ice flow models usedWhere unreliable, ice flow models usedPhysical properties of icePhysical properties of iceAssumes smooth steady flowAssumes smooth steady flow

•Produces “fairly good estimates” of Produces “fairly good estimates” of ageage

Dust Layers Greenland has two primary sources for dustGreenland has two primary sources for dust

Particulates from Arctic Canada and coastal Particulates from Arctic Canada and coastal GreenlandGreenland

Large volcanic eruptions anywhere on the globeLarge volcanic eruptions anywhere on the globe

Gas Trapping in Ice Gases trapped during ice Gases trapped during ice

sinteringsintering When gas flow to When gas flow to

surface shut downsurface shut down Crystallization of iceCrystallization of ice Depths of about 50 to Depths of about 50 to

100 m below surface100 m below surface Gases younger than Gases younger than

host icehost ice Fast accumulation Fast accumulation

minimizes age difference minimizes age difference (100 years)(100 years)

Slow deposition Slow deposition maximizes age difference maximizes age difference (1000-2000 years)(1000-2000 years)

Implication of Age Difference If change in greenhouse gas If change in greenhouse gas

concentrationsconcentrations Force changes in ice volumeForce changes in ice volume

Gas concentration should lead ice Gas concentration should lead ice volumevolume

Gas age is younger than ice ageGas age is younger than ice ageTherefore offset between changes in Therefore offset between changes in atmospheric gas concentrationsatmospheric gas concentrations•Which should be relatively rapidWhich should be relatively rapid

Closer to change in ice volumeCloser to change in ice volume•Which should be relatively slowWhich should be relatively slow

Reliability and Accuracy of Records

Can be evaluated by comparing instrumental recordCan be evaluated by comparing instrumental record With records from rapidly accumulating ice sheetsWith records from rapidly accumulating ice sheets

Instrumental records date to 1958 for COInstrumental records date to 1958 for CO22 and and 1983 for CH1983 for CH44

• Mauna Loa Observatory (David Keeling)Mauna Loa Observatory (David Keeling)

NOAA/CMDL Air Sampling Network

35 Sampling stations or about half world-wide stations35 Sampling stations or about half world-wide stations

CSIRO CH4 Sampling Network

Carbon Dioxide Measurements of COMeasurements of CO22 concentration concentration

Core from rapidly accumulating iceCore from rapidly accumulating ice Merge well with instrumental dataMerge well with instrumental data

Methane Measurements of CHMeasurements of CH44 concentration concentration

Core from rapidly accumulating iceCore from rapidly accumulating ice Merge well with instrumental dataMerge well with instrumental data

CH4 and CO2 in Ice Cores Given agreement between records from Given agreement between records from

rapidly accumulating icerapidly accumulating ice Instrumental dataInstrumental data

Accuracy and variability about the Accuracy and variability about the trendstrends

Assume that longer-term records Assume that longer-term records collected from ice corescollected from ice coresReliable for determining the scale of Reliable for determining the scale of variabilityvariability

Orbital-Scale Changes in CH4 CHCH44 variability variability

Interglacial maxima Interglacial maxima 550-700 ppb550-700 ppb

Glacial minima 350-450 Glacial minima 350-450 ppbppb

Five cycles apparent in Five cycles apparent in recordrecord 23,000 precession 23,000 precession

periodperiod Dominates low-Dominates low-

latitude insolationlatitude insolation Resemble monsoon Resemble monsoon

signalsignal•Magnitude of Magnitude of

signals matchsignals match

Monsoon forcing of CH4

Match of high CHMatch of high CH44 with strong monsoon with strong monsoon Strongly suggests connectionStrongly suggests connection

Monsoon fluctuations in SE AsiaMonsoon fluctuations in SE Asia Produce heavy rainfall, saturate groundProduce heavy rainfall, saturate ground

Builds up bogsBuilds up bogs•Organic matter deposition and Organic matter deposition and anaerobic respiration likelyanaerobic respiration likely–Bogs expand during strong Bogs expand during strong summer monsoonsummer monsoon

–Shrink during weak summer Shrink during weak summer monsoonmonsoon

Alternative Explanation High-latitude soils and continental margins High-latitude soils and continental margins

source of atmospheric methanesource of atmospheric methane CHCH44 stored in frozen soils (permafrost) stored in frozen soils (permafrost) Continental margin sediments (hydrates)Continental margin sediments (hydrates)

Released during exceptionally warm Released during exceptionally warm summerssummers Precessional changes in summer Precessional changes in summer

insolation affects high latitudesinsolation affects high latitudes Cycles of summer warming should also Cycles of summer warming should also

occur on 41,000 year cyclesoccur on 41,000 year cycles Lack of 41,000 cycle in record argues Lack of 41,000 cycle in record argues

against high latitude sourceagainst high latitude source

Orbital-Scale Changes in CO2

COCO22 record from Vostok record from Vostok Interglacial maxima 280-Interglacial maxima 280-

300 ppm300 ppm Glacial minima 180-190 Glacial minima 180-190

ppmppm 100,000 year cycle dominant100,000 year cycle dominant Match ice volume recordMatch ice volume record

Timing Timing AsymmetryAsymmetry

Abrupt increases in COAbrupt increases in CO22 match rapid ice meltingmatch rapid ice melting

Slow decreases in COSlow decreases in CO22 match slow build-up of match slow build-up of iceice

Orbital-Scale Changes in CO2 Vostok 150,000 recordVostok 150,000 record

23,000 and 41,000 23,000 and 41,000 cyclescycles

Match similar cycles Match similar cycles in ice volumein ice volume

Agreement suggests Agreement suggests cause and effect cause and effect relationshiprelationship Relationship Relationship

unknownunknowne.g., does COe.g., does CO22 lead lead

ice volume?ice volume? Correlations not Correlations not

sufficient to provide sufficient to provide definite evaluationdefinite evaluation

Problems with Records Ice cores poorly datedIce cores poorly dated

COCO22 older than ice by variable amount older than ice by variable amount Greenland ice core well-dated (dust layers)Greenland ice core well-dated (dust layers)

Dust is CaCODust is CaCO33-rich-richDissolution of CaCODissolution of CaCO33 releases CO releases CO22

Precise timing between changes in COPrecise timing between changes in CO22 and and ice volume uncertainice volume uncertain New data provide better correlationNew data provide better correlation

Data do show that signals correlateData do show that signals correlate Some causal link must existSome causal link must exist

Big question – how did COBig question – how did CO22 vary by 30%? vary by 30%?

Covariation Between CO2 and D

Substantial mismatch in Vostok records (Substantial mismatch in Vostok records (rr22 = = 0.64 over the last 150 kya)0.64 over the last 150 kya)

Values shown Values shown normalizednormalizedto their mean valuesto their mean valuesduring the mid-Holoceneduring the mid-Holocene(5–7 kya BP) and the(5–7 kya BP) and thelast glacial (18–last glacial (18–60 kya BP)60 kya BP)

Clearly visible are theClearly visible are thedisproportionately lowdisproportionately lowdeuterium values duringdeuterium values duringthe mid-glacial (60–80the mid-glacial (60–80Kya BP), the glacialKya BP), the glacialinception (95–125 Kyainception (95–125 KyaBP), and the BP), and the penultimatepenultimateglacial maximum (140–glacial maximum (140–150 Kya BP)150 Kya BP)

If the If the D change reflects a D change reflects a proportional T drop, then proportional T drop, then more than ½ of the more than ½ of the interglacial-to-glacial change interglacial-to-glacial change occurred before significant occurred before significant removal of atmospheric COremoval of atmospheric CO22

Temperature from Ice Cores Snow falling on ice sheets under colder Snow falling on ice sheets under colder

temperatures is more negativetemperatures is more negative A plot of the A plot of the 1818O of snow versus temperature O of snow versus temperature

shows an excellent correlationshows an excellent correlation Thus Thus 1818O serves as a paleothermometerO serves as a paleothermometer

18O in Ice Cores Several factors in addition to temperature of Several factors in addition to temperature of

precipitationprecipitation Affect the Affect the 1818O of snow and ice on glaciersO of snow and ice on glaciers

Meteoric Water Line D and D and 1818O in precipitation correlatedO in precipitation correlated

Determined by evaporation/precipitation and rainoutDetermined by evaporation/precipitation and rainout Mixture of equilibrium and non-equilibrium processesMixture of equilibrium and non-equilibrium processes Deuterium excess Deuterium excess ((dd = = D – 8D – 81818O)O) quantifies intercept quantifies intercept

and disequilibriaand disequilibria

Deuterium Excess in Marine Rain

Deuterium-excess value in marine environmentsDeuterium-excess value in marine environments Established at the site of the air-sea interactionEstablished at the site of the air-sea interaction

The offset from equilibrium conditionsThe offset from equilibrium conditions•Determined by the humidity deficit above Determined by the humidity deficit above

the sea surfacethe sea surfaceThis deuterium-excess value is conserved This deuterium-excess value is conserved

during the rainout over the continentsduring the rainout over the continents If humidity deficit is known or can be modeledIf humidity deficit is known or can be modeled

Can be used to correct Can be used to correct D/D/1818O of precipitationO of precipitationDetermine more precisely ambient Determine more precisely ambient

temperature during precipitationtemperature during precipitation

D on Antarctica Determine by the temperature, humidity Determine by the temperature, humidity

and and D of the vapor source regionD of the vapor source region Cuffy and Vimeux (2001, Cuffy and Vimeux (2001, NatureNature, ,

415:523-527) showed using deuterium 415:523-527) showed using deuterium excessexcessMismatch is an artifact caused by Mismatch is an artifact caused by variations in climate of the vapor variations in climate of the vapor source regionsource region

Used a climate model and measured Used a climate model and measured deuterium excessdeuterium excess•Calculate Southern Hemisphere Calculate Southern Hemisphere temperature variationstemperature variations

Vostok Temperature and CO2 Deuterium excess corrected Southern Deuterium excess corrected Southern

Hemisphere temperature correlate remarkably Hemisphere temperature correlate remarkably well with COwell with CO22 variations variations

Covariation of COCovariation of CO22 and andtemperature havetemperature haverr22 = 0.89 for last 150 = 0.89 for last 150kya and kya and rr22 = 0.84 for = 0.84 forlast 350-400 kyalast 350-400 kya

Implications of Results COCO22 is an important climate forcing on the Modern is an important climate forcing on the Modern

EarthEarth Long-term synchrony of glacial-interglacial cyclingLong-term synchrony of glacial-interglacial cycling

Between Northern and Southern HemispheresBetween Northern and Southern HemispheresDue to greenhouse gas variations and Due to greenhouse gas variations and

feedbacks associated with variationsfeedbacks associated with variations Southern Hemisphere Southern Hemisphere T explained byT explained by

COCO22 variations variationsWithout considering changes in N. Hemisphere Without considering changes in N. Hemisphere

insolationinsolation Delay between CODelay between CO22 decrease and decrease and TT

During last glacial inception only ~5,000 yearsDuring last glacial inception only ~5,000 years

Unresolved Issues Cuffy and Vimeux (2001) show thatCuffy and Vimeux (2001) show that

90% of 90% of T can be explained by T can be explained by variations in COvariations in CO22 and CH and CH44

Reasonably firm grasp on causes of CHReasonably firm grasp on causes of CH44 variations (Monsoon forcing)variations (Monsoon forcing) What produced COWhat produced CO22 variations? variations?

Variations are large – 30%Variations are large – 30%Show rapid changes – drop of 90 ppm Show rapid changes – drop of 90 ppm from interglacial to glacialfrom interglacial to glacial

Physical Oceanographic Changes in CO2

During glaciations physical properties changeDuring glaciations physical properties change Temperature and salinityTemperature and salinity Affect solubility of COAffect solubility of CO22(aq) and thus pCO(aq) and thus pCO22

90% of the CO90% of the CO22

decrease unexplaineddecrease unexplainedby physical processesby physical processes

Exchange of Carbon Carbon in rock reservoir exchanges slowlyCarbon in rock reservoir exchanges slowly

Cannot account for 90 ppm change in 10Cannot account for 90 ppm change in 1033 y y Rapid exchange of carbon must involve near-Rapid exchange of carbon must involve near-

surface reservoirssurface reservoirs

Changes in Soil Carbon Expansion of ice sheets Expansion of ice sheets

Covered or displaced forestsCovered or displaced forestsConiferous and deciduous treesConiferous and deciduous trees

•Displaced forests replaced by steppes Displaced forests replaced by steppes and grasslandsand grasslands– Have lower carbon biomassHave lower carbon biomass

Pollen records in lakesPollen records in lakes Indicate glacial times were dryer and less Indicate glacial times were dryer and less

vegetated than interglacialvegetated than interglacialEstimates of total vegetation reduced by Estimates of total vegetation reduced by

25% (15-30%) during glacial maxima25% (15-30%) during glacial maxima•COCO22 removed from atmosphere did not removed from atmosphere did not

go into vegetation on land!go into vegetation on land!

Where is the Missing Carbon? Carbon from reduced COCarbon from reduced CO22 during glacial times during glacial times

Not explained by physical properties of Not explained by physical properties of surface oceansurface ocean

Did not go into biomass on landDid not go into biomass on land Must have gone into oceansMust have gone into oceans

Surface ocean not likelySurface ocean not likely•Exchanges carbon with atmosphere too Exchanges carbon with atmosphere too

rapidlyrapidly•Most areas of ocean within 30 ppm of Most areas of ocean within 30 ppm of

atmosphereatmosphere– Glacial surface ocean must also have Glacial surface ocean must also have

been lower, like atmospherebeen lower, like atmosphereDeep ocean only likely remaining reservoirDeep ocean only likely remaining reservoir