Age-depth modelling in the EPD

By far most of the sites in the EPD have some some of age control. Age models are vitally important for palaeoecological/palaeoclimatological studies, especially when multiple proxy archives need to be compared (e.g., an EPD search for mid-Holocene sites). Basically we can distinguish two types of age-controls in EPD sites: pollen-stratigraphic and radiocarbon dates. Whereas pollen-stratigraphy can give us relative and often rather imprecise information (e.g., a site with a mid-Holocene pollen spectrum), radiocarbon (14C) can potentially provide more precise and absolute ages to pollen cores. On the other hand, cores often contain only a handful of 14C dates owing to constraints on budget and abundance of datable organic material. From this limited amount of information, and through assumptions about the accumulation history (sedimentation rate, hiatuses), the ages of all depths of a core need to be estimated somehow (age-depth modelling).

Radiocarbon

Atmospheric CO2 contains a tiny proportion (only 1 in every 1012 atoms) of the 14C isotope. As the CO2 is incorporated into the carbon cycle, living plants in equilibrium with the atmosphere (and their consumers) will obtain the same concentration of 14C atoms (the concentration will be lower in systems with old carbon present, such as oceans and hard-water lakes). Because radiocarbon is radioactive, as time goes on the 14C atoms will fall apart (click http://www.chrono.qub.ac.uk/Members/MBlaauw/C14decay.html for an animation). When exactly an individual 14C atom will disappear cannot be predicted; it could happen within a few seconds, but it could also survive several tens of thousands of years. In organic material, after waiting 5568 years (the half-life of 14C) only half of the original concentration of 14C atoms will remain. By measuring the remaining concentration of radiocarbon, the 14C age of fossil material can thus be determined. Currently the limit for 14C dating lies around c. 10 14C half-lifes; thus 14C dates older than c. 40-60,000 years obtain infinite ages when measured (Is Infinite column in EPD Dating table).

To-be-dated fossil material (Material Dated in the EPD Dating table) can either be bulk sediment/lake mud, or a selection of material from the sediment (e.g. macrofossils such as seeds, twigs and leaves). The 14C/C ratio of the fossil material can be measured in two ways: through counting the number of 14C disintegrations over a time-interval (conventional radiometric dating, either using liquid scintillation or proportional gas counting, see http://www.c14dating.com), or through counting the 14C/C mass ratio directly (accelerator mass spectrometry or AMS). For the former method, large samples are needed, often spanning several cm of sediment (Thickness in the EPD Dating tables), and consisting of bulk sediments. The AMS technique can handle much smaller samples, and can thus do with thinner sediment slices (more precise/accurate age estimates).

Owing to laboratory treatments, machine drift, and statistical scatter, 14C ages are always reported with an uncertainty or error estimate, e.g., 2450 ± 50 years (Age, Limit Younger and Limit Older in the EPD Dating tables). The dates always have a unique label given by the 14C laboratory (Lab Number in the EPD Dating tables) so that its details can be retrieved if needed.

Most laboratories measure the d13C value of the dates (delta13C in EPD Dating tables). This is done to correct for fractionation; during assimilation of CO2, organisms will slightly favour the lighter isotopes 12C and 13C over the heavier 14C. This discrimination against 14C is corrected for by measuring the ratio of the stable isotopes 13C/12C, and by assuming that the 14C/12C fractionation in the sample is twice that of the 13C/12C.

From measuring the 14C ages of material of exactly known calendar age (dendrochronologically dated tree-rings), it has become clear that the concentration of atmospheric 14C has fluctuated in the past. Thus there is no one-to-one relationship between real age and 14C age; 14C ages need to be calibrated. For most northern hemisphere terrestrial samples with 14C ages up to c. 21,000 years, the IntCal04 calibration curve (Reimer et al. 2004) is recommended; no accepted calibration curve exists yet for samples older than 21,000 years, and for the most recent, “post-bomb” 14C dates, dedicated calibration curves should be used (Hua and Barbetti 2004).

Calibration of 14C ages (e.g. on CALIB, http://calib.qub.ac.uk) and subsequent interpretation of the calibrated ages is not a straightforward task. Owing to the wiggly nature of the calibration curve, calibrated ages often possess multiple and asymmetric local maxima in their probability range (see Fig 1). It is not recommended to obtain point calendar age estimates using the intercepts with the calibration curve, because they do not provide a robust indicator of sample calendar age. Instead the weighted average or the median probability of the probability distribution are preferred (Telford et al 2004).


Fig. 1. Example of a calibrated 14C date

Age-depth modelling

As mentioned above, constraints in budget and datable material means that not every depth of a pollen profile can be dated individually. Most cores only have a handful of dates spanning several millennia and meters of sediment. Thus, some sort of interpolation and/or extrapolation is needed to estimate the ages of non-dated levels (Bennett, 1994). Most age-depth models in the EPD were constructed using interpolation between the dated levels (often including the core top and its year of sampling as extra dating point), while some EPD age-depth models use linear, polynomial or power regression to estimate the ages of the non-dated depths. Sometimes the age-depth model is extrapolated beyond the dated levels; extrapolated ages should be treated with much caution, as should core sections with low dating resolution.

The choice of age-depth model (see Default Chronology in EPD site information) is somewhat subjective. Often a range of models is tested on the pollen core, and then the model which seems the most “realistic” is used. The settings of the site could help in deciding about which model to use; sites which can be assumed to have accumulated in a relatively stable manner over the studied period, could for example be modelled using linear regression, while sites with many likely changes in sedimentation rate (e.g. sites which changed from a lake to a mire) could perhaps be better modelled using linear interpolation between the dated levels. Indeed, it is recommended to check how different modelling assumptions would impact the age-depth models of individual sites.

Sometimes some dates appear to disagree with the other dates and the model used. If this is the case, such outlying dates are removed from the analysis, and a remark about this is made in the EPD Default Chronology or Dating Samples table.

Most of the age-depth models currently provided in the EPD are based on uncalibrated 14C ages. As 14C dates need to be calibrated for reliable age-depth models, we are currently working on updating our age-depth modelling routines. We are also working on ways to estimate the age uncertainties for the cores (e.g. Heegaard et al. 2005, Blaauw et al. 2007).

References

Bennett KD, 1994. Confidence intervals for age estimates and deposition times in late-Quaternary sediment sequences. The Holocene 14: 337-348

Bennett KD, 2007. Psimpoll and pscomb programs for plotting and analysis. http://www.chrono.qub.ac.uk/psimpoll/psimpoll.html

Blaauw M, JA Christen, 2005. Radiocarbon peat chronologies and environmental change. Applied Statistics 54: 805-816

Blaauw M, Christen JA, Mauquoy D, van der Plicht J, Bennett KD, 2007. Testing the timing of radiocarbon-dated events between proxy archives. The Holocene 17: 283-288

Heegaard E, Birks HJB, Telford RJ, 2005. Relationships between calibrated ages and depth in stratigraphical sequences: an estimation procedure by mixed-effect regression. The Holocene 15: 612-618

Hua Q, Barbetti M, 2004. Review of tropospheric bomb C-14 data for carbon cycle modeling and age calibration purposes. Radiocarbon 46: 1273-1298

Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW, Bertrand CJH, Blackwell PG, Buck CE, Burr GS, Cutler KB, Damon PE, Edwards RL, Fairbanks RG, Friedrich M, Guilderson TP, Hogg AG, Hughen KA, Kromer B, McCormac FG, Manning SW, Ramsey CB, Reimer RW, Remmele S, Southon JR, Stuiver M, Talamo S, Taylor FW, van der Plicht J, Weyhenmeyer CE, 2004. IntCal04 terrestrial radiocarbon age calibration, 26 - 0 ka BP. Radiocarbon 46: 1029-1058.

Telford RJ, Heegaard E, Birks HJB, 2004. The intercept is a poor estimate of a calibrated radiocarbon age. The Holocene 14: 296-298.

age-depth_chronologies.txt · Last modified: 2015/06/23 17:22 (external edit)
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