TephrochronologyTephrochronology(tephra Gk ‘ashes’)

David J. Lowe

Photo: M. Gehrels

1. What is1. What is tephrochronologytephrochronology??

2. How are2. How are tephrastephras dated?dated?–– Bayesian improvementsBayesian improvements

3. Identifying uncertainties3. Identifying uncertainties–– Six steps to miscorrelationSix steps to miscorrelation

1. What is1. What is tephrochronologytephrochronology??

A stratigraphic linking, dating & synchronizing tool– for palaeoenvironmental, geological or archaeological sequences or events

Linking/correlating tephras relies on:– Statigraphic position (law of superposition relative age)

– Characterisation using physical, mineralogical and geochemical properties= ‘fingerprinting’ (fingerprints must be diagnostic, ideally persistent)

– Numerical age

Hence unique power (advantages) of tephrochronology:– Deposits and palaeoarchival information positioned precisely on common time

scale using tephra isochron as stratigraphically fixed tie-point even if ageunknown or imprecise

– Numerical age acquired for tephra, or relative age, can be transferred fromone site to next because tephra eruption age geologically ‘instantaneous’

– Provides independent test of sequence chronology

Hence F tephra:

- Correlated ~3000 km

- NGRIP age & environ. proxies- Marine cores environ. proxies- Ice-core age vs marine 14C ages correct marine reservoir effect LGM

Fugloyarbanki tephra

Ice-core age:26,740 ± 390 yr before 2000 AD(1)

(from Siwan Davies et al. 2008 JQS)

Example 1:Example 1: FugloyarbankiFugloyarbanki tephratephra (Iceland)(Iceland)

Example 2: KawakawaExample 2: Kawakawa tephratephra (New Zealand)(New Zealand)

Kohuora crater, Auckland

Galway TarnPhoto: M. Vandergoes

Kk

Correlated ~1000 km

KawakawaKawakawa tephratephraLinks palaeoarchival records to common tie-point in

Auckland (A) Taranaki (B) West Coast (C)

(After Rewi Newnham et al. 2007 JQS)

Grass peaks= coldest

Qtz peaks= coldest

Mild

Mild

Beetle studyshows warminterstadial

- Even though age imprecise 27,097 ± 957 cal yr BP (2)

2. How are2. How are tephrastephras dated?dated?

HistoricalHistorical RadiometricRadiometric

–– 1414CC–– Fission track esp. isothermal plateau FTDFission track esp. isothermal plateau FTD

on glass (also FTD on zircon)on glass (also FTD on zircon)–– 4040Ar/Ar/3939ArAr–– Luminescence (TL, OSL)Luminescence (TL, OSL)

IncrementalIncremental–– DendrochronologyDendrochronology–– VarvedVarved sedimentssediments–– IceIce--core layeringcore layering

Age equivalentAge equivalent–– PalaeomagnetismPalaeomagnetism–– Correlation with marine oxygen isotopeCorrelation with marine oxygen isotope

stages orstages or biostratigraphybiostratigraphy (e.g.(e.g. palynostratigraphypalynostratigraphy))

NZNZ--INTIMATE records linked withINTIMATE records linked with tephratephra isochronsisochrons

HOL

LGIT

eLGM

Improve ages on tephras at Kaipo bogsequence using OxCal and Bpeat

Kaipo bog

Section

Upper section

Grey mud

Kaharoa

Taupo

Waimihia

WhakataneTuhua

Mamaku

Rotoma

Opepe

Poronui Karapiti

Okupata Konini

Poronui Karapiti Okupata Konini

Grey mudPeaty mud B

Peaty mud AWaiohau

Rotorua

Mud Rerewhak.

Lower section

20 radiometr. 14Cages from Waikatolab in red- via Dr Alan Hogg

Chronology of Kaipo- 3 sets of 14C dates

14 dates on tephras(via tephrochronology)in black (Froggatt & Lowe 1990,Lowe et al. 1999)

Total of 51 independentlydated points

20 AMS 14C ages from ETHlab (Switzeland) in blue- via Dr Irka Hajdas

(Lowe et al. 1999 NZJGG,Irka Hajdas et al. 2006 QR)

Kaipo bog sequencefitted to INTCAL04using Bpeat &stratigraphic ordering(Dr Maarten Blaauw)

Age model

Interpolated dates viaBpeat - maximumposterior densitiesof chronologicalordering-constrainedcalibration ranges(red = 100% outlier)

Depth chronologygrey scalelikelihoods(darker = morelikely calendar ages)

TephraTephra age models derived fromage models derived from KaipoKaipo BpeatBpeat analysisanalysis

69106832-70136850-7160

17,76017,200-18,05017,426-17,999

15,45015,192-15,52415,100-15,750

13,67013,502-13,74413,470-13,800

54905474-55655470-5590

80007979-80397960-8050

94909488-95189480-9530

11,32011,269-11,45011,220-11,600

11,18011,118-11,23911,110-11,270

33903362-34223370-3450

52205021-52334970-5270

99809699-10,1829920-10,230

Red = OkatainaYellow = TaupoGreen = Tong./EgmontPurple = Tuhua (Mayor Is)

Point age estimates fromsingle-best Bpeat iteration

Bpeat 95.4% HPD (2 range)OxCal 95.4% HPD (2 range)

94909488-95189480-9530

Volcanic centres

(After Lowe et al. QSR 2008 with help of Dr Maarten Blaauw)

11,56011,480-11,75211,500-11,940

11,51011,420-11,66111,430-11,810

Konini

Okupata

3. Identifying uncertainty3. Identifying uncertainty

Miscorrelation ofMiscorrelation of tephratephra layer maylayer may –– IsochronIsochron drawn at incorrect positiondrawn at incorrect position–– MisassociationMisassociation ofof palaeoclimaticpalaeoclimatic oror archaeologicarchaeologic

events = incorrect linkingevents = incorrect linking//synchronizationsynchronization–– Incorrect age transferredIncorrect age transferred

Six steps to uncertaintySix steps to uncertainty……

Uncertainty inUncertainty in tephrochronologytephrochronology

1. Faulty characterisation1. Faulty characterisation–– Analytical data compromised,Analytical data compromised, e.g.e.g. Unsuitable anal. method usedUnsuitable anal. method used

(bulk sample(bulk sample vsvs. single grain). single grain) Glass or crystals not polishedGlass or crystals not polished Appropriate standards not usedAppropriate standards not used Incorrect probe conditionsIncorrect probe conditions

(e.g. glass needs de(e.g. glass needs de--focussedfocussed~20~20--mm beam)beam) Grains weatheredGrains weathered Electron microprobe,

Victoria Univ. of Wellington

Block for EMPA

2. Non2. Non--unique fingerprintsunique fingerprints–– Major element composition ofMajor element composition of

glass can be identical forglass can be identical fordifferentdifferent tephrastephras e.g. Holocenee.g. Holocene TaupoTaupo tephrastephras

–– Need additional dataNeed additional data Trace element/rare earth dataTrace element/rare earth data

from single grain LAfrom single grain LA--ICPMSICPMS StratigraphicStratigraphic or chronological dataor chronological data

Uncertainty inUncertainty in tephrochronologytephrochronology

(Stokes et al.1992 QI)

3. Multiple fingerprints3. Multiple fingerprints–– Compositional heterogeneityCompositional heterogeneity

Some eruptions tapped 2Some eruptions tapped 23 magmas3 magmassimultaneously or sequentiallysimultaneously or sequentially

compositions vary in different falloutcompositions vary in different falloutdirectionsdirections

iinadequate to characterise few samplesnadequate to characterise few samplesfrom limited dispersal directionsfrom limited dispersal directions May have miscorrelatedMay have miscorrelated tephrastephras in pastin past How many grains need we analyse?How many grains need we analyse?

–– Wide variability andesitic glassWide variability andesitic glasscomposition from crystal inclusionscomposition from crystal inclusions((microlitesmicrolites)) Can now correct using petrologic methodCan now correct using petrologic method

Uncertainty inUncertainty in tephrochronologytephrochronology

Andesitic glass + microlites

Photo:M. Gehrels

North lobe

Biotite analyses - Kaharoa tephra(Phil Shane et al. 2008 QI)

Eruptionduration

Southlobe

4. Reworking to different4. Reworking to differentstratigraphicstratigraphic positionposition–– Results in wrong positioning ofResults in wrong positioning of isochronisochron–– Dissemination of grains through sedimentDissemination of grains through sediment

e.g. Shard concentration zone in peat:e.g. Shard concentration zone in peat:where iswhere is isochronisochron ‘‘lineline’’??

–– TephraTephra sinking in v. soft lake sedimentssinking in v. soft lake sediments

How to recognise reworking?How to recognise reworking?–– Grain characterGrain character–– StratigraphicStratigraphic/sedimentary context/sedimentary context–– Multiple populations shown byMultiple populations shown by

singlesingle--grain analysesgrain analyses–– ReproducibilityReproducibility

Uncertainty inUncertainty in tephrochronologytephrochronology

Photos: M. Gehrels

Tp

5. Dating5. Dating ‘‘errorserrors’’ Wrong age transferredWrong age transferred

–– MisassociationMisassociation–– ContaminationContamination

e.g. Old carbon ine.g. Old carbon in--wash in lakes; young carbon migration;wash in lakes; young carbon migration;inin--built age; hardbuilt age; hard--water, marine reservoir effectswater, marine reservoir effects

–– Large counting errors (low precision)Large counting errors (low precision)–– Annealing problem glassAnnealing problem glass

fissionfission--track datingtrack dating ageage underestimated,underestimated,

now corrected ITPFTnow corrected ITPFT

Uncertainty inUncertainty in tephrochronologytephrochronology

ZFTD

6. Statistical misadventure6. Statistical misadventure–– Miscorrelation because databaseMiscorrelation because database

not comprehensive or poornot comprehensive or poorquality data (etc)quality data (etc) e.g. DFA generally OK (providese.g. DFA generally OK (provides

probability of miscorrelation,probability of miscorrelation,MahalanobisMahalanobis distance stat.) butdistance stat.) butdependent on quality &dependent on quality &comprehensiveness of training setscomprehensiveness of training sets

–– SomeSome tephrastephras very similarvery similarcompositionally, otherscompositionally, othersseparable, e.g.separable, e.g. KkKk vsvs ReRe vsvs OO

Uncertainty inUncertainty in tephrochronologytephrochronology

DFA from Shane Croninet al. 1997 NZJGG

KawakawaKk

Rotoehu Re

Okaia O

NeedNeed–– Multiple criteria = safer correlationMultiple criteria = safer correlation–– Rigorous analyses, standards must be documented,Rigorous analyses, standards must be documented,

uncertainty reporteduncertainty reported–– Ideally use comprehensive sequences esp. withIdeally use comprehensive sequences esp. with

interfingeringinterfingering tephrastephras from different sourcesfrom different sources Distal can be best, e.g. lake sediments, marine cores, ice coresDistal can be best, e.g. lake sediments, marine cores, ice cores

Even if age onEven if age on tephratephra is imprecise, correctis imprecise, correctcharacterisation meanscharacterisation means isochronisochron still transferrablestill transferrable–– EffectivelyEffectively tephratephra tietie--point fixed (point fixed (stratigraphicstratigraphic position)position)

onon ‘‘elasticatedelasticated’’ chronologychronology

Reducing uncertaintyReducing uncertainty