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| Titel |
Age models for peat deposits on the basis of coupled lead-210 and radiocarbon data. |
| VerfasserIn |
Natalia Piotrowska, François De Vleeschouwer, Jarosław Sikorski, Barbara Sensuła, Adam Michczyński, Barbara Fiałkiewicz-Kozieł, Bernard Palowski |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033447
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| Zusammenfassung |
| The study presents three examples of age-model construction based on the results of 210Pb
and 14C dating methods applied to peat deposits. The three sites are ombrotrophic peat bogs:
the Misten (Belgium), Slowinskie Bloto (N Poland) and Puscizna Mala (S Poland). All sites
have been subjected to multiproxy studies aimed at reconstructing paleoenvironment and
human activity, covering the last 1500, 1300 and 1800 years, respectively (De Vleeschouwer
et al. 2009A, 2009B, in prep., Fialkiewicz-Koziel, ongoing PhD). A detailed comparison
between 210Pb and post-bomb 14C results in the Misten bog has also been carried out by
Piotrowska et al. (2009).
In all cores, the 210Pb activity was calculated using 210Po and 208Po activities
after acid-extraction from bulk samples, subsequent deposition on silver discs and
measurements by alpha spectrometry. Unsupported 210Pb was detected until 35cm
in Slowinskie Bloto, 15cm in the Misten and 19cm in Puscizna Mala. Constant
Rate of Supply (CRS) model was then applied to compute ages of each 1-cm core
interval.
For the Misten and Slowinskie Bloto, radiocarbon measurements were performed on
selected aboveground plant macrofossils, mainly Sphagnum spp. or Calluna vulgaris, Erica
tetralix, and Andromeda polyfolia. Radiocarbon ages were determined using accelerator mass
spectrometry (AMS) after acid-alkali-acid wash, combustion, purification of carbon dioxide
and graphitisation. For Puscizna Mala bulk samples were dated after chemical preparation of
benzene for liquid scintillation counting (LSC) or CO2 for gas proportional counting
(GPC).
Radiocarbon calibration was undertaken using the Intcal04 calibration curve and OxCal 4
software. As a priori information the 210Pb-derived ages were used in a P_Sequence model
(Bronk Ramsey, 2008). A number of dates characterized by low agreement with
stratigraphical order had to be considered as outliers and rejected from the final age
model.
For building a continuous age models a non-linear approach called generalized additive
model (GAM) was used, as described by Heegaard et al. (2005). The calculations were
performed within each period on the middle-point of the 95.4 % range of calibrated
age, while an uncertainty equal to half of this range was assumed. The results of
210Pb dating are described by Gaussian distribution and in their case 1-sigma range
was used. The resulting age-depth relationships provide a mean age and an age
range for each 1-cm thick slice of peat, and allows for calculation of sedimentation
rates.
This study highlights some important issues connected with radiocarbon dating of peat
sequences. Commonly known problems are wide ranges of calibrated ages caused by wiggles
in calibration curve for the period just beyond the range of precise 210Pb dates, which can
undoubtedly be solved by 14C-dating more samples from thinner slices. It also emphasizes
the great potential of Bayesian analysis applied in radiocarbon calibration. The high
suitability of coupled 210Pb-14C dating applied to dating of peat deposits was also
confirmed.
Bronk Ramsey C., 2008. Deposition Models for Chronological Records. Quaternary
Science Reviews 27: 42-60.
De Vleeschouwer F., Piotrowska N., Sikorski J., Pawlyta J., Cheburkin A., Le Roux G.,
Lamentowicz M., Fagel N., Mauquoy D., 2009A. Multiproxy evidences of Little Ice Age
palaeoenvironmental changes in a peat bog from northern Poland. The Holocene 19,4:
625-637.
De Vleeschouwer F., Fagel N., Cheburkin A., Pazdur A., Sikorski J., Mattielli N., Renson
V., Fialkiewicz B., Piotrowska N., Le Roux G., 2009B. Anthropogenic impacts in North
Poland over the last 1300 years and possible influence of climate on human activities. Science
of the Total Environment 407: 5674-5684.
De Vleeschouwer F., Luthers C., Court-Picon M., Mauquoy D., Wastiaux C., Le Roux
G., Moschen R., Pawlyta J., Pazdur A., Sikorski J., Piotrowska N. In preparation.
Multiproxy paleoenvironmental study in the Misten bog (East Belgium) during the last
millenium. To be submitted in Quaternary International in June 2010.
De Vleeschouwer F., Fagel N. Sikorski J., 2010. Development of lead 210 measurement
in peat using polonium extraction. A comparison of techniques. Â Submitted to
Geochronometria.
Heegaard E., Birks H.J.B. and Telford R.J., 2005. Relationships between calibrated ages
and depth in stratigraphical sequences: an estimation procedure by mixed-effect regression.
The Holocene 15: 612-618.
Piotrowska N., De Vleeschouwer F., Sikorski J., Pawlyta J., Fagel N., Le Roux G., Pazdur
A., 2009. Intercomparison of radiocarbon bomb pulse and 210Pb age models. A study in a
peat bog core from North Poland. Nuclear Instruments and Methods in Physics Research
Section B, document DOI: 10.1016/j.nimb.2009.10.124 (in print, accepted proofs). |
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