TephrochronologyTephrochronology (tephra Gk ‘ashes’) David J. Lowe 1. What is tephrochronology? 2. How are tephras dated? – Bayesian improvements 3. Identifying uncertainties – Six steps to miscorrelation Photo: M. Gehrels 1.1. What What is 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 age unknown or imprecise – Numerical age acquired for tephra, or relative age, can be transferred from one site to next because tephra eruption age geologically ‘instantaneous’ – Provides independent test of sequence chronology ExampleExample 1: 1: FugloyarbankiFugloyarbanki tephratephra (Iceland)(Iceland) Fugloyarbanki tephra Ice-core age: 26,740 ± 390 yr before 2000 AD Hence F tephra: (1) - Correlated ~3000 km (from Siwan Davies et al. 2008 JQS) - NGRIP age & environ. proxies - Marine cores environ. proxies - Ice-core age vs marine 14C ages correct marine reservoir effect LGM ExampleExample 2: 2: Kawakawa Kawakawa tephratephra (New(New Zealand) Zealand) Kk Galway Tarn Photo: M. Vandergoes Correlated ~1000 km Kohuora crater, Auckland KawakawaKawakawa tephratephra Links palaeoarchival records to common tie-point in Auckland (A) Taranaki (B) West Coast (C) - Even though age imprecise 27,097 ± 957 cal yr BP (2) Grass peaks Qtz peaks = coldest = coldest Mild Beetle study shows warm interstadial Mild (After Rewi Newnham et al. 2007 JQS) 2.2. How How are are tephrastephras dated?dated? Historical Radiometric – 14C – Fission track esp. isothermal plateau FTD on glass (also FTD on zircon) – 40Ar/39Ar – Luminescence (TL, OSL) Incremental – Dendrochronology – Varved sediments – Ice-core layering Age equivalent – Palaeomagnetism – Correlation with marine oxygen isotope stages or biostratigraphy (e.g. palynostratigraphy) NZNZ--INTIMATEINTIMATE records records linked linked with with tephratephra isochronsisochrons HOL LGIT eLGM Improve ages on tephras at Kaipo bog sequence using OxCal and Bpeat Upper section Kaharoa Kaipo bog Taupo Section Waimihia Whakatane Tuhua Mamaku Rotoma Opepe Poronui Karapiti Okupata Konini Grey mud Poronui Karapiti Okupata Konini Peaty mud B Grey mud Peaty mud A Waiohau Rotorua Mud Rerewhak. Lower section Chronology of Kaipo - 3 sets of 14C dates 20 radiometr. 14C 14 dates on tephras ages from Waikato (via tephrochronology) lab in red in black (Froggatt & Lowe 1990, - via Dr Alan Hogg Lowe et al. 1999) 20 AMS 14C ages from ETH lab (Switzeland) in blue - via Dr Irka Hajdas Total of 51 independently dated points (Lowe et al. 1999 NZJGG, Irka Hajdas et al. 2006 QR) Age model Depth chronology grey scale likelihoods (darker = more likely calendar ages) Interpolated dates via Bpeat - maximum Kaipo bog sequence posterior densities of chronological fitted to INTCAL04 ordering-constrained using Bpeat & calibration ranges stratigraphic ordering (red = 100% outlier) (Dr Maarten Blaauw) TephraTephra ageage models models derived derived from from KaipoKaipo BpeatBpeat analysisanalysis Volcanic centres Point age estimates from 9490 Red = Okataina single-best Bpeat iteration Yellow = Taupo 9488-9518 Bpeat 95.4% HPD (2 range) Green = Tong./Egmont 9480-9530 Purple = Tuhua (Mayor Is) OxCal 95.4% HPD (2 range) 9490 11,510 9488-9518 11,420-11,661 Okupata 9480-9530 11,430-11,810 11,180 8000 11,118-11,239 7979-8039 3390 Konini 11,560 11,110-11,270 7960-8050 3362-3422 11,480-11,752 3370-3450 11,500-11,940 9980 5490 9699-10,182 5474-5565 9920-10,230 5470-5590 13,670 13,502-13,744 6910 13,470-13,800 6832-7013 15,450 6850-7160 15,192-15,524 11,320 17,760 15,100-15,750 5220 11,269-11,450 17,200-18,050 5021-5233 11,220-11,600 17,426-17,999 4970-5270 (After Lowe et al. QSR 2008 with help of Dr Maarten Blaauw) 3.3. Identifying Identifying uncertainty uncertainty MiscorrelationMiscorrelation of of tephratephra layerlayer may may –– IsochronIsochron drawndrawn at at incorrect incorrect position position –– MisassociationMisassociation ofof palaeoclimaticpalaeoclimatic oror archaeologicarchaeologic eventsevents = = incorrect incorrect linking linking//synchronizationsynchronization –– IncorrectIncorrect age age transferred transferred SixSix steps steps to to uncertainty uncertainty…… UncertaintyUncertainty in in tephrochronologytephrochronology 1.1. Faulty Faulty characterisation characterisation –– AnalyticalAnalytical data data compromised, compromised, e.g. Unsuitable anal. method used . Block for EMPA (bulk sample vs. single grain) . Glass or crystals not polished . Appropriate standards not used . Incorrect probe conditions (e.g. glass needs de-focussed ~20-m beam) . Grains weathered Electron microprobe, Victoria Univ. of Wellington UncertaintyUncertainty in in tephrochronologytephrochronology 2.2. Non Non--uniqueunique fingerprints fingerprints –– MajorMajor element element composition composition of of glassglass can can be be identical identical for for differentdifferent tephrastephras . e.g. Holocene Taupo tephras (Stokes et al. 1992 QI) –– NeedNeed additional additional data data . Trace element/rare earth data from single grain LA-ICPMS . Stratigraphic or chronological data UncertaintyUncertainty in in tephrochronologytephrochronology Eruption North lobe 3.3. Multiple Multiple fingerprints fingerprints duration – Compositional heterogeneity South . Some eruptions tapped 23 magmas lobe simultaneously or sequentially compositions vary in different fallout directions Biotite analyses - Kaharoa tephra inadequate to characterise few samples (Phil Shane et al. 2008 QI) from limited dispersal directions Photo: . May have miscorrelated tephras in past M. Gehrels . How many grains need we analyse? – Wide variability andesitic glass composition from crystal inclusions (microlites) . Can now correct using petrologic method Andesitic glass + microlites UncertaintyUncertainty in in tephrochronologytephrochronology 4.4. Reworking Reworking to to different different stratigraphicstratigraphic positionposition – Results in wrong positioning of isochron – Dissemination of grains through sediment . e.g. Shard concentration zone in peat: where is isochron ‘line’? – Tephra sinking in v. soft lake sediments How to recognise reworking? – Grain character – Stratigraphic/sedimentary context – Multiple populations shown by single-grain analyses – Reproducibility Tp Photos: M. Gehrels UncertaintyUncertainty in in tephrochronologytephrochronology 5.5. Dating Dating ‘‘errorserrors’’ WrongWrong age age transferred transferred – Misassociation – Contamination . e.g. Old carbon in-wash in lakes; young carbon migration; in-built age; hard-water, marine reservoir effects – Large counting errors (low precision) – Annealing problem glass fission-track dating age underestimated, now corrected ITPFT ZFTD UncertaintyUncertainty in in tephrochronologytephrochronology Kawakawa 6.6. Statistical Statistical misadventure misadventure Kk – Miscorrelation because database – Miscorrelation because database Okaia O not comprehensive or poor quality data (etc) . e.g. DFA generally OK (provides Rotoehu Re probability of miscorrelation, Mahalanobis distance stat.) but dependent on quality & comprehensiveness of training sets – Some tephras very similar compositionally, others separable, e.g. Kk vs Re vs O DFA from Shane Cronin et al. 1997 NZJGG ReducingReducing uncertainty uncertainty NeedNeed – Multiple criteria = safer correlation – Rigorous analyses, standards must be documented, uncertainty reported – Ideally use comprehensive sequences esp. with interfingering tephras from different sources . Distal can be best, e.g. lake sediments, marine cores, ice cores EvenEven if if age age on on tephratephra isis imprecise, imprecise, correct correct characterisationcharacterisation means means isochronisochron stillstill transferrable transferrable – Effectively tephra tie-point fixed (stratigraphic position) on ‘elasticated’ chronology.