Category:Chronologies Working Group

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An imaginary chronology based on U/Th dates, generated via Bchron
Credit: Julien Emile-Geay (Own work)

Overview

In the Linked Earth context, a working group (WG) is a self-organized coalition of knowledgeable experts, whose activities are governed herewith. This page is dedicated to the discussion of data and metadata standards for chronologies, and aims to formulate a set of recommendations for such a standard.

This WG needs to work closely with other WGs, as constraints will vary by archive. Nevertheless, some aspects are common enough that it is worth pursuing some general recommendations, and devise ad hoc adjustments for individual archives if need be. It is recommended that every WG coordinator join this WG to keep track of discussions.

Members

The chronologies Working Group has one coordinator, Christof Pearce (Stockholm University)

Specific tasks

We recommend that discussions focus on the following techniques, and explore potential commonalities.

For each chronology type, we recommend:

  • structuring discussions around what scientific questions one would want to ask of the data
  • listing essential, recommended, and optional information for:
    • the age models themselves
    • the chronological measurements (ChronData tables, in LiDP/LinkedEarth parlance)
    • their uncertainties, and what those numbers correspond to (e.g. 1-sigma or 2-sigma?)
  • provide an ideal chronology table, so the community knows what to report and how to report it.
  • provide separate recommendations for new and legacy datasets

While it is recognized that most real-word chronologies are of mixed types (e.g. a Holocene lake sediment chronology may blend radiocarbon dates, 210Pb dates, and volcanic ash markers), it is critical to first define guidelines for how to report pure chronologies. Once the foundations are sound, they will be easier to compose together.

Nomenclature

In paleoclimatology, scientists tend to reference the time axis either starting from the present (present often being taken as 1950) and counting positively back in time. In the current LiPD model, this concept is referred to as 'age'. On the other hand, some scientists, especially focusing on the last 2,000 years, use the Gregorian Calendar. In LiPD, this concept is referred to as 'year'.

Should we keep these two separate concepts?
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poll-id D2C0068BD4358B564E5E598E295FF719

If yes, is the current nomenclature adopted by LiPD correct?

Age

How should we report age?
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poll-id 84A488E5E21C698D19615F535B80CA41

Year

How should we report year?
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poll-id EEF59B83600956C4360A6663F9661701

Recommendation from the literature

The recommendations below are from Govin et al. 2015 [1].

"Future climate-stratigaphic alignments should provide (1) a clear statement of climate hypotheses involved, (2) a detailed understanding of environmental parameters controlling selected tracers and (3) a careful evaluation of the synchronicity of aligned paleoclimatic records. We underscore the need to (1) systematically report quantitative estimates of relative and absolute age uncertainties, (2) assess the coherence of chronologies when comparing different records, and (3) integrate these uncertainties in paleoclimatic interpretations and comparisons with climate simulations."

Absolute dating of aragonitic corals

Dating fossil corals often requires the use of U-series age.

Age calculation and its intrinsic uncertainties

"In recent years, most ages have been obtained through high precision TIMS or MC-ICP-MS measurements are are generally given with a ±2σ. Earlier, they were mostly obtained through alpha particle counting and generally cited with a ±1σ uncertainty. [1]"
For U-series measurements in coral archive, should the uncertainty be reported as
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poll-id 40BD1B117B1FBFC6D8EB3EBACA7F577D

"Some vagueness persists with respect to the way quoted uncertainties include or not all potential sources of analytical errors (internal vs external precision), which makes direct comparison of databases somewhat hazardous. [1]"
"Moreover, radiometric age equations include decay constants of all concerned isotopes, which are known with some uncertainty. The comparison of such U-series ages to any other time scale, such as glaciological ages, astronomically-tunes ages and other radiometric ages, requires the U-series decay constant uncertainties to be carried into the final age error calculation [1]"
"
For U-series uncertainty in coral archive, should the uncertainty in the U-series decay constant for new datasets be:
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There were 2 votes since the poll was created on 20:57, 20 March 2017.
poll-id 9F38149046E9E5931EB22346B63BE3C8

For U-series uncertainty in coral archive, should the uncertainty in the U-series decay constant for legacy datasets be:
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poll-id 383DEB45E82A99D8069455AB247EFC4F

Dating of speleothems

Absolute dating and stratigraphic markers

Speleothems can be absolutely dated over the last 600ka with the U-Th methodology [2], with a dating uncertainty better than 0.5% [3] [4] [5].

Recommendation from Govin et al. (2015) [1]

"Ideally, new [spleothem] records should present growth rates as high as possible and numerous U-Th dating points in order to (1) reduce dating uncertainty, (2) enhance the robustness of radiometric chronologies on centennial to millennial time scales, (3) maximize constraints on the 'accordion effect' induced by growth rate changes and (4) optimize the assessment of the synchronicity between records. [1]"

Chronologies in marine sediments

Summary of methods used to derive chronologies in marine sediments. Adapted from Govin et al. (2015) [1]
Method Hypotheses Age Uncertainties Strengths Limitations/difficulties Recommendation
Radiocarbon Reservoir age correction is known and not changing over time See #Radiocarbon Absolute chronologies Reservoir Age correction
Synchronization of tephra horizons Correct identification of the same tephra in different archives
  • Absolute dating error (if available)
  • Synchronization error
  • Accurate synchronisation
  • Several tephra horizons identified during the same interval
  • Absolute dating control sometimes possible
  • Taphonomic issues (reworking, dissemination, dispersion of shards)
  • Unequivocal identification of source volcano and eruptive event via diagnostic fingerprinting

High potential of synchronization

Synchronization of geomagnetic excursions and of relative paleointensity records Global signal Potential to synchronize paleointensity to cosmogenic isotopic records from ice cores
  • Large dating uncertainty of geomagnetic excursions
  • Uncertainties associated with the magnetization acquisition process
 Method suitable for local to global synchronization when sediments pass criteria of paleomagnetic reliability
Benthic foraminifera δ18O alignment In-phase benthic δ18O variations
  • Alignment error (e.g. resolution, matching)
  • Uncertainty related to the hypothesis (e.g. possibility of diachronous high-latitude temperature changes)
  • Dating uncertainty of the reference
Common tracer: many available records Diachronous deep-water temperature and δ18O changes across water-masses Method suitable between sites located within the same water mass
SST alignment to ice cores In-phase high-latitude sea surface and air temperature changes High-resolution ice core records for the last 800 ka
  • Restricted to high latitudes
  • Precise geographical extension of synchronous temperature changes
  • Sensitive to uncertainties in SST reconstruction methods (e.g., seasonality, living depth, transport)
 Method suitable for North Atlantic and Southern Ocean sites
Marine alignment to speleothems In-phase regional changes in marine (e.g., SST, salinity, pollen) and continental (e.g., rainfall amount) tracers
  • High dating accuracy
  • Very high-resolution speleothem records
  • Mostly applicable in tropical and mid-latitude regions (where speleothem records are available)
  • Speleothem records sometimes discontinuous
  • Climatic drivers of speleothem tracers (moisture availability?)
 Method suitable for sites (mostly from tropical and mid-latitude regions) located close to well-dated speleothems with well-understood tracers

Tie-point chronologies

Radiocarbon

The following came from P. Reimer by way of T. Guilderson. 

Key references: Stuiver & Polach [1977] [6] and  Reimer et al., [2004] [7]

metadata

Table 1: Essential/Recommended/Desired metadata for Radiocarbon measurements
Metadata Essential (E)/Recommended(R)/Desired(D) Reason Added by (optional)
Sample ID E Allows to provide a unique identifier to the sample Deborah Khider (talk) 15:17, 15 March 2017 (PDT) on behalf of T. Guilderson
Type/Matrix E Deborah Khider (talk) 15:17, 15 March 2017 (PDT) on behalf of T. Guilderson
Location (lat/lon/alt or sample depth) E Deborah Khider (talk) 15:17, 15 March 2017 (PDT) on behalf of T. Guilderson
For radiocarbon measurements, should the sample ID be:
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poll-id E6013E78D3258F7514048F352CC002B6

For radiocarbon measurements, the type/matrix should be:
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There were 4 votes since the poll was created on 22:23, 15 March 2017.
poll-id 3A6E5ACD2E51DC714A46792D5F2145B1

Polls on whether location information should be essential, recommended or desired metadata for new dataset can be found here for new datasets and here for legacy datasets.

Reporting the Radiocarbon measurements

Table 2. Essential/Recommended/Desired metadata for reporting radiocarbon measurements
Metadata Essential (E)/Recommended(R)/Desired (D) Reason/Notes Added by (optional)
Chemical pretreatment/preparation E oxidation, chemical leach (%), a/b/a, soxhlet, ultrafiltration, none, etc Deborah Khider (talk) 15:37, 15 March 2017 (PDT) on behalf of T Guilderson
Method E GPC, LSC, AMS, etc Deborah Khider (talk) 15:37, 15 March 2017 (PDT) on behalf of T Guilderson
Laboratory ID# E eg. OS####; CAMS####; QUB#### Deborah Khider (talk) 15:37, 15 March 2017 (PDT) on behalf of T Guilderson
δ13C ratio actual/estimated (PDB/V-PDB) R Note, the AMS measured 13C has low precision (a few per mil, compared to IRMS). Most AMS labs do NOT report the machine measured 13C value because it is frequently misused. This should be estimated as per SP77 [6] or an IRMS-based value/estimate. Deborah Khider (talk) 15:37, 15 March 2017 (PDT) on behalf of T Guilderson
Conventional radiocarbon age as years BP E Note, that by definition, conventional radiocarbon age is the Libby based age. Deborah Khider (talk) 15:37, 15 March 2017 (PDT) on behalf of T Guilderson
one-sigma standard deviation E one-sigma standard deviation Deborah Khider (talk) 15:37, 15 March 2017 (PDT) on behalf of T Guilderson
F14C (13C and background correction applied) E This is slightly different than Stuiver and Polach [6] who have "D14C" as the optional parameter. 
We suggest the inclusion of the background and d13C corrected Fraction modern (F14C) as it is the primary reported value.  In the past there has been some inconsistencies in the literature/laboratories with regards to fraction modern.  By using/requesting F14C you will be explicitly requesting 13C and background corrected Fraction Modern (aka F14C, as per Reimer et al.[2004]) as intended by SP77.
Deborah Khider (talk) 16:13, 15 March 2017 (PDT) on behalf of T Guilderson
Correction applied to radiocarbon date prior to, or during conversion to calibrated age
Marine Reservoir effect
Marine reservoir age, , ± one-sigma sd (or square root of the variance) uncertainty, reference/source E Deborah Khider (talk) 16:13, 15 March 2017 (PDT) on behalf of T Guilderson
\Delta R value, ± one-sigma sd (or square root of the variance) uncertainty, reference/source E Deborah Khider (talk) 16:13, 15 March 2017 (PDT) on behalf of T Guilderson
hard/soft water effect: value, ± one-sigma sd (or square root of var) uncertainty, reference/source E Deborah Khider (talk) 16:13, 15 March 2017 (PDT) on behalf of T Guilderson
other corrections: value (14C years), ± one-sigma sd (or square root of var), reference/source E Deborah Khider (talk) 16:13, 15 March 2017 (PDT) on behalf of T Guilderson
Calibration or conversion to calibrated ages 
As of present, the international radiocarbon community recommends the use of the INTCAL13 (aka Marine 13[8]) and SHCAL13 [9] data products. Calibration of post-bomb (post 1954/1957) samples should also include the calibration data-set utilized,  description/reference of any corrections applied, and the software/algorithm utilized to convert the F14C data to years AD.
Deborah Khider (talk) 16:13, 15 March 2017 (PDT) on behalf of T Guilderson
For Radiocarbon measurements, the chemical pretreatment/preparation of the sample should be:
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poll-id 78202BE8846AE9F82E3C8D0FF8873D02

For radiocarbon measurements, the method should be:
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poll-id 6DB651F90B192B8EE2EFDDAAFF6C2B4F

For radiocarbon measurements, the laboratory ID# should be:
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poll-id 5054B8790D14CB3C1B3D267F3E22DE4C

For radiocarbon measurements, the d13C ratio actual/estimated should be:
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There were 4 votes since the poll was created on 23:18, 15 March 2017.
poll-id 8CF78D6D9F4CA9393100A7667DBE8A3F

For radiocarbon measurements, conventional radiocarbon age as years BP should be:
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There were 4 votes since the poll was created on 23:19, 15 March 2017.
poll-id 6654AC7134BAA4FBBA27A25B263B69B1

For radiocarbon measurements, the 1-sigma standard deviation on the conventional radiocarbon age should be:
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There were 4 votes since the poll was created on 23:20, 15 March 2017.
poll-id C7413E58CD0E740BE1015BF471706D18

For radiocarbon measurements, the F14C should be:
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There were 3 votes since the poll was created on 23:21, 15 March 2017.
poll-id 1F95A055C058CCD63002CDE71C31592F

For radiocarbon measurements converted to calendar ages, the marine reservoir age, one-sigma sd (or square root of the variance) uncertainty, reference/source should be:
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There were 5 votes since the poll was created on 23:22, 15 March 2017.
poll-id ECDA562DFA3D428A56997248E0D36175

For radiocarbon measurements converted to calendar ages, the DELTA_R value, ± one-sigma sd (or square root of the variance) uncertainty, reference/source should be:
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There were 5 votes since the poll was created on 23:38, 15 March 2017.
poll-id BABD6BFB79107997D8EDABC556177F82

For radiocarbon measurements, the hard/soft water effect: value, ± one-sigma sd (or square root of var) uncertainty, reference/source should be:
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poll-id 873E0B9012B11CAB074C44F049C17B87

Lead

no known existing standard/template

U-series

Layer-counted chronologies

Comboul et al[10] argue that it is critical to report uncertainties in layer-counted chronologies, and that these can be expressed in terms of an undercounting and overcounting rate. However, there needs to be agreement about how to measure and report this rate for various archives.

Varves

Growth rings

Trees, corals, speleothems

Tree-rings typically provide an absolute chronology but there needs to be the facility to also work with 'floating' chronologies anchored typically by radiocarbon. A hybrid dendro-radiocarbon chronological framework should be supported.

Tree-rings data can also be stored at subannual level e.g. with early and latewood parameters. These are typically stored as two different data series as they are normally used to reconstruct different climatic parameters. In chronological terms these datasets represent portions of the year but precisely which portions depends on the species and location. In the southern hemisphere the earlywood will typically grow in the final months of the year and the latewood the first months of the following year. For paleoclimate reconstructions it's therefore essential to include metadata regarding the months each data point covers.

A convention that continues to cause confusion when representing dendrochronological data is the use of the astronomical calendar. The astronomical calendar includes the year 0 so matches the AD calendar, but is one year different in the BC period. Astronomical dates are easier to handle in statistical analyses but have been erroneously quoted as BC years in even some very prestigious articles. Whatever method is used, it should be clear and consistent.

Ice layers

Role of flow models.


Age-modeling software

Content:

  • method (e.g. Bacon, OxCal, BChron, BAM)
  • version
  • parameters
Should the method by which the chronology is obtained be fully documented
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poll-id 995EC7654FD3A7F32B1C6029C31FB20A

If documented with the data, should the publication information about the software used in the chronology be:
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There were 4 votes since the poll was created on 23:38, 14 March 2017.
poll-id 857C1D02FA05E58B01A0BEB496D1E564

If documented with the data, should the code (i.e. parameters used) used for the chronology be:
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There were 3 votes since the poll was created on 23:39, 14 March 2017.
poll-id AFB996C4DF36B3A5D1E9EC6DF2938178

If documented with the data and using a technique creating ensembles of possible age models, should the ensemble age models be:
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There were 2 votes since the poll was created on 23:41, 14 March 2017.
poll-id 44A4F1955E6EC42862A54AA52689BCA9

If documented with the data and using a technique creating ensembles of possible age models, should the ensemble statistics (e.g., mean, median, standard deviation, quantiles) be:
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There were 2 votes since the poll was created on 23:42, 14 March 2017.
poll-id 7D3C2523FB122801F0BC56D8D9926882

Polls

Here are polls that the group might want to consider:

For LEGACY DATASETS:

Should legacy chronologies be updated to the current field standard (e.g. chronologies obtained from IntCal09 should be redone with IntCal13)?
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There were 4 votes since the poll was created on 23:35, 14 March 2017.
poll-id EE9B5AC53196826693167AAFFAD2B738

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Govin. A., E. Capron, P.C. Tzedakis, S. Verheyden, B. Ghaleb, C. Hillaire-Marcel, G. St-Onge, J.S. Stoner, F. Bassinot, L. Bazin, T. Blunier, N. Combourieu-Nebout, A. El Ouahabi, D. Genty, R. Gersonde, P. Jimenez-Amat, A. Landais, B. Martrat, V. Masson-Delmotte, F. Parrain, M.-S. Seidenkrantz, D. Veres, C. Waelbroeck, R. Zahn, 2015. Sequence of events from the onset to the demise of the Last Interglacial: Evaluating strengths and limitations of chronologies used in climatic archives. Quaternary Science Reviews, 129, 1-36, doi:10.1016/j.quadcirev.2015.09.018.
  2. ↑ Helmstrom, J., 2006. U-Th dating of speleothems with high initial 230Th using stratigraphical constraint Quaternary Geochronology, 1, 289-295.
  3. ↑ Dorale, J., Edwards, R.L., Alexander Jr., E.C., Shen, C.-C., Richards, D., Cheng, H., 2004. Uranium-series dating of speleothems: current techniques, limits, & applications. In: Sasowsky, I., Mylroie, J. (Eds), Studies of Cave Sediments, Springer, US, pp. 1777-197. http://dx.doi.org/10.1007/978-1-4419-9118-8_10.
  4. ↑ Li, H.-C., Ku, T.-L., You, C.-F., Cheng, H., Edwards, R.L., Ma, Z.-B., Tsai, W.-s, Li, M.-D., 2005. 87Sr/86Sr and Sr/Ca in speleothems for paleoclimate reconstruction in Central China between 70 and 280kyr ago. Geochimica and Cosmochimica Acta, 69, 3933-3947. http://dx.doi.org/10.1016/j.gca.2005.01.009.
  5. ↑ Cheng, H., Edwards, R.L., Broecker, W.S., Denton, G.H., Kong, X., Wang, Y., Zhang, R., Wang, X., 2009. Ice age terminations. Science 326, 248-252. http://dx.doi.org/ 10.1126/science.1177840.
  6. ↑ 6.0 6.1 6.2 Minze Stuiver and H. A. Polach, 1977. Discussion: Reporting of 14C Data. Radiocarbon 19, 3, 355-363.
  7. ↑ Paula J. Reimer, T. A. Brown, and R. W. Reimer, 2004. Discussion: Reporting and calibration of post-bomb 14C Data. Radiocarbon, 46, 3, 1299-1304.
  8. ↑ P. J. Reimer et al., 2013. INTCAL13 and MARINE13 Radiocarbon age calibration curves, 0-50,000 years CAL BP. Radiocarbon, 55, 1869-1887.
  9. ↑ A. G. Hogg, et al., 2013. SHCal13 Southern Hemisphere calibration, 0-50,000 years cal BP. Radiocarbon 55, 1889-1903.
  10. ↑ Comboul, M., J. Emile-Geay, M. N. Evans, N. Mirnateghi, K. M. Cobb, and D. M. Thompson (2014), A probabilistic model of chronological errors in layer-counted climate proxies: applications to annually banded coral archives, Climate of the Past, 10(2), 825–841, doi:10.5194/cp-10-825-2014

Pages in category "Chronologies Working Group"

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