Issues with radiocarbon age models

A record <40,000 old can be dated with radiocarbon method

Commonly dated materials are organic matter, or CaCO3. About 10 mg of CaCO3 is needed for an Accelerator Mass Spectrometry (AMS) 14C dating

In palaeoceanography, most records use marine CaCO3 shells for dating - notably foraminiferal shells

In terrestrial studies, the most commonly used material consists of plant remains

The 14C isotope forms from stable nitrogen isotopes (14N) due to the action of cosmic rays in the atmosphere.

The unstable 14C atom is incorporated into all processes where carbon is being used, including the formation of organic matter and carbonate.

Eventually, 14C atoms eject an electron and revert to 14N.

Changes in cosmic ray penetration to the atmosphere caused changes in the rate of 14C production.

Solar wind (stream of ionised particles from the sun) and the earth’s magnetic field, shield earth from cosmic rays.

High 14C production is expected at times of low solar output and/or a weak earth magnetic field.

Comparison between tree-ring ages and 14C ages on the same material showed that radiocarbon convention years are not equal to real years.

Why?

All figures in this slide by: University of southampton

In datings of marine materials, there is an additional complication, due to the vast carbon reservoir in the oceans.

Marine datings typically are several hundreds of years “older” than terrestrial counterparts because of the mean age of this carbon reservoir

Reservoir age needs a correction, to make it locally applicable - the reservoir age correction (R).

Put simply, this term R is the local deviation from the global mean reservoir age (~400 years).

The dating as supplied by dating labs is termed the “radiocarbon convention age”. It is reported with the analytical standard deviation.

In the case of marine datings, these first need correction for the (local) reservoir age, and then can be calibrated to so-called “calibrated ages (cal.

BP)”, using the marine calibration curve.

Note: Most software only needs to be told that a marine dating is being calibrated, and then what the appropriate value (with errors) for R is (e.g, in eastern Mediterranean, R = 53±43 years).

Definitions:

14C age BP = conventional radiocarbon age (half-live = 5568 yr; corrected for isotope fractionation; Stuiver and Polach, 1977)

Reservoir age = measured marine 14C - atmospheric 14C age at time t (Stuiver, Pearson, and Braziunas, 1986).

Atmospheric age is taken from a 5 point moving average of INTCAL98 (Stuiver et al., 1998a) for the Northern Hemisphere and New Zealand cedar radiocarbon ages (McCormac et al., 1998) for the Southern Hemisphere.

No fossil fuel correction is needed when the reservoir age is calculated according to the above definition.

R = difference between the regional and global marine 14C age = measured marine 14C - marine model 14C age at time t

For use when calibrating marine samples with the marine calibration curve MARINE98 (Stuiver et al., 1998a)

The uncertainty in R includes the standard error in the calibration dataset.

Note: R may be time-dependent in some regions

For ages in BP ==> Present = 1950 ! (So, age in BP is age in BC+1950 !!!)Consequently, these are calibrated ages, not calendar ages !

Marine Reservoir Correction Map

Showing R in years

Reimer, P. J., and Reimer, R. W. (2001). Marine Reservoir Correction Database [WWW database]. Belfast: 14CHRONO Centre, Queen's University Belfast. URL:http://www.calib.org

From:University of southampton

From:University of southampton

Now let’s have a look at what we really want to do with radiocarbon datings: Dating and correlating climate variability - an oceanic perspective

From:University of southampton

From:University of southampton

Dating of marine records:

Main limit to quality of the result is the quality of the material dated

In marine records: mostly foraminiferal tests (pick for 10 mg carbonate typically several hundreds of specimens).

Main Problems for the dating analysis.

1. Different species have different depth habitats, and thus live in waters with different “ages”.

2. Synsedimentary reworking. For example: current focussing, shelf-wash, etc. All means of transporting older specimens from previous depositional settings and depositing these along with actual shell flux at the study site.

3. Post-depositional reworking. For example: bioturbation, physical sedimentary processes. Create a mixture of specimens of different ages.

4. Post-depositional diagenesis. Coatings and carbonate alterations involving pore-water chemistry (could be interaction with very old or much younger DIC).

Solutions to Main Problems for the dating analysis.

(1). Different species have different depth habitats, and thus live in waters with different “ages”.

Solution: Date monospecific selections of specimens

Often difficult or impossible to pick the same species for all dated samples, since species abundances/availabilities change through time

Some basins well-ventilated, and no age differentiations between most species. However, this situation may change through time!

Solutions to Main Problems for the dating analysis.

(2). Synsedimentary reworking. For example: current focussing, shelf-wash, etc. (NB. turbidite deposits can normally be recognised and avoided).

There’s no way to avoid potential problems from the influences of synsedimentary reworking. If the reworked specimens are morphologically, and in terms of preservation, identical to the current test flux, then one cannot distinguish them.

In low accumulation rate sites, this problem is minimised. However, research increasingly focusses on high accummulation rate sites, which exist by virtue of the very focussing mechanisms that one would like to avoid!

Solutions to Main Problems for the dating analysis.

(3). Post-depositional reworking. For example: bioturbation, physical sedimentary processes. Create a mixture of specimens of different ages.

Common strategy for limiting the impact is by dating monospecific selections from temporally restricted abundance peaks.

Solutions to Main Problems for the dating analysis.

(4). Post-depositional diagenesis. Coatings, fillings and alterations involving pore-water chemistry (could be interaction with very old or much younger DIC).

Common strategy is to pick as pristine specimens as possible, but there always remains a degree of uncertainty with this...

When working with calibrated ages, uncertainties about reservoir age corrections enter the picture.

Have been discussed: massive problem in marine studies, especially since they can change (unconstrainable?) through time

Now consider the spatial context and “reproducibility” of ages for events from chronostratigraphic frameworks for individual - even closely spaced - cores.

Correlation uncertainties then further “muddle” the picture, even if a multi-proxy correlation framework is set up using very strong events (necessarily synchronous between adjacent sites) in continuously analysed series.

Example: Joint framework development for Aegean cores

Closely spaced suite of cores

Reproduced by permission of American Geophysical Union: Casford, J. S. L., Abu-Zied, R., Rohling, E.J., Cooke, S.,Fontanier, C.,Leng, M., Millard, A., Thomson, J., A stratigraphically controlled multiproxy chronostratigraphy for the eastern Mediterranean, Paleoceanography, v. 22, PA4215, 19 December 2007. Copyright [2007] American Geophysical Union.

Using correlation-based transformations of depth in other cores to depth in LC21, all AMS datings can be translated into the LC21 depth framework. (Fits only here to show that they do not appreciably change as good core data is added, relative to LC21’s own datings).

Comparison to data from 4 low-res cores

All figures on this slide are Reproduced by permission of American Geophysical Union: Casford, J. S. L., Abu-Zied, R., Rohling, E.J., Cooke, S.,Fontanier, C.,Leng, M., Millard, A., Thomson, J., A stratigraphically controlled multiproxy chronostratigraphy for the eastern Mediterranean, Paleoceanography, v. 22, PA4215, 19 December 2007. Copyright [2007] American Geophysical Union.

Use the LC21 age-depth relationship to predict ages at various depths, and compare these with values “correlated” into the LC21 framework.

Suggests that where correlations are involved, SE goes up to +/- 120 y or more while SD for individual values approaches +/- 600 y.

Reproduced by permission of American Geophysical Union: Casford, J. S. L., Abu-Zied, R., Rohling, E.J., Cooke, S.,Fontanier, C.,Leng, M., Millard, A., Thomson, J., A stratigraphically controlled multiproxy chronostratigraphy for the eastern Mediterranean, Paleoceanography, v. 22, PA4215, 19 December 2007. Copyright [2007] American Geophysical Union.

Following construction of initial chronology, “curve matching” may give further insight, but only if:

(a) there is a demonstrably direct process linking the two regions, and/or

(b) inferred phasings can be verified using synchronous markers (e.g., volcanic events)

Note. Dog-leg develops when bioturbation returns!

Rohling, E.J., Mayewski, P.A., Hayes, A., Abu-Zied, R.H., andCasford, J.S.L., Holocene atmosphere-ocean interactions: recordsfrom Greenland and the Aegean Sea. Climate Dynamics, 18, 587-593, 2002, Springer

Fig. 1. a Time-stratigraphic framework for Aegean core LC21. Depth scale adjusted forthe 10 cm thickness of the ash layer from theMinoan eruption of Santorini.  Follow linkfor full article and figure

Mechanism: changes in frequency/intensity of cold polar/continental northerly outbreaks.

A f r i c a

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T m

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A lboran Sea

Map: (Berenger 1955)

We have established that the centennial-scale cold spells were periods of very frequent outbreaks (at least within every 50 years (sample resolution)).

Hence, there is a well-established mechanism to “link” Greenland and the Aegean Sea, allowing signal synchronisation with a good degree of confidence.

Plot shows Outbreak example: Northerly winds over Aegean with air T down to -2ºC (N Aegean), RV Meteor 51-3, Dec 2001.Figure: Air temperature normally lies between 10 and 20oC however an outbreak with air temperatures dipping to -2ºC occurs between 0 and 90oC (Northerly). See link below for figure and article.

Casford, J.S.L., Rohling, E.J., Abu-Zied, R.H., Fontanier, C., Jorissen, F.J., Leng, M.J., Schmiedl, J. Thomson A dynamic concept for eastern Mediterranean circulation and oxygenation during sapropel formation Palaeogeography, Palaeoclimatology, Palaeoecology, v. 190, 15 January 2003, p. 103-119. Elsevier.

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