Radiocarbon age-modelling I – introduction to age-models Dr. Maarten Blaauw School of Geography, Archaeology and Palaeoecology Queen’s University Belfast Northern Ireland 1

My background Born 1974 in Hardenberg (nl) ‘92-’98 Msc Biology, Groningen (nl) ‘98-‘03 PhD Palaeoecology, Amsterdam (nl) ‘03-’04 Postdoc Trinity College Dublin (ie) ‘04-’06 Postdoc CIMAT (mx) ‘07-... Postdoc Queen’s Univ Belfast (uk) 2

Today Lectures Introduction to age-depth modelling Bayesian age-depth modelling Practicals Calibrating Basic age-depth modelling 3

Pollen-diagram 4 Blaauw et al. in press, The Holocene 4 4

Poisson random walk 5 Blaauw et al. in press, The Holocene 5 5

Blaauw et al. in press, The Holocene 6 Blaauw et al. in press, The Holocene 6 6

Blaauw et al. in press, The Holocene 7 Blaauw et al. in press, The Holocene 7 7

Types of chronologies No uncertainties Yearly resolution Decadal / centennial – (multi-) millennial resolution 8

Annually layered ice cores 10

History peat dating Pre 14C dating (relative dating) Peat stratigraphy (Sernander 1866-1944) & pollen Link pollen with archaeology (bronze age etc.) Link with Swedish varve chronology (de Geer) (Sub) millennial precision Von Post 1946 11

History peat dating Carbon dating → 'absolute', independent dates Smith & Pilcher 1973: 14C dating vs. pollen zones 'Wishful thinking obscures reality' (von Post 1946) 14C date depths along peat core At levels with major proxy changes At regular intervals Assume linear accumulation between dated levels e.g.: Aaby 1976, van Geel 1978 12

History peat dating High-resolution 14C dating wiggle-match dating (van Geel&Mook 1989) Bayesian (e.g., Blaauw&Christen 2005) post-bomb dating (e.g., van der Linden et al. 2008) Tephra (e.g., Pilcher et al. 1995, Davies et al. 2003) 210Pb dating (e.g., Turetsky et al. 2004) All form age estimates for age-depth models The estimates and models are uncertain 13

Carbon dating 14

14C dating 14C unstable, half-life 5568 yr Ratio 14C/C gives age fossil 15

Belfast Seattle Tree-ring coverage for IntCal04: until 12.4 kcal BP Irish Oak Waikato German Oak Groningen German Oak Pretoria German Oak German Pine Swiss Pine Heidelberg Irish Oak German Oak Belfast PNW/CA German Oak German Pine Seattle 16

Tree-ring coverage for IntCal04 17

Tree-ring coverage for IntCal04 18

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http:///www.chrono.qub.ac.uk/blaauw/ 20

Age-depth modelling So now we have dates... what's next? Estimate ages of non-dated levels Use available information all dates together environmental settings site other comparable archives 21

Age-depth modelling 14C and other dates Basic age-modelling techniques interpolation, regression, spline, ... Bayesian approaches chron. ordering, wiggle-match dating Compare multiple archives tuning, eye-balling, Bayesian 22

Age-depth modelling 23

Age-depth modelling 24

Uncertainty accumulation 25

Many date$ 26

Few dates 27

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Which age-depth model? 800 cm core 9 14C dates surface = recent 29

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. Too much weight on individual dates? 30

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. date 6 = outlier 31

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. 470 cm = hiatus 32

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. Linear regression 470 cm = hiatus Too narrow error ranges? 33

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. Linear regression Polyn. regression 470 cm = hiatus second degree wide uncertainties when extrapolating! 34

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. Linear regression Polyn. regression Smooth spline 35

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. Linear regression Polyn. regression Smooth spline date 6 = outlier 36

Which age-depth model? 800 cm core 9 14C dates surface = recent Linear interpol. Linear regression Polyn. regression Smooth spline 470 cm = hiatus 37

Age-depth modelling How did our sediment accumulate over time? Constant? Varying? Pulses? Hiatuses? Site settings Should we try to fit a curve through all dates? Balance belief in dates and belief in model Use stratigraphic information How many dates do I need? The more, the better? The more problems? Depends on your questions “is my sediment of glacial age?” “early 8.2 k event?” 38

Dating uncertainties 39

14C dating 40

14C dating 41

Uncertainties dates 5 calibrated dates surface = recent simulate yr every date draw age-model linear interpolation repeat 42

Uncertainties dates 5 calibrated dates surface = recent simulate yr every date draw age-model polynomial regression does not cross all dots weighted by error sizes repeat 43

Uncertainties dates 44

Basic age-modelling Choose which one looks nicest... How treat point estimates? (mid/max, multimodal)‏ Why just one line? Not much literature Bennett 1994, Bennett and Fuller 2002, The Holocene, Telford et al. 2004, QSR Software Calib + Excel clam, Blaauw in press (Quaternary Geochronology) 45

Today Lectures Introduction to age-depth modelling Bayesian age-depth modelling Practicals Calibrating Basic age-depth modelling 46