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.

Definition of essential, recommended, and desired in regards to paleoclimate data standards

For each chronology type, we recommend:

  • structuring discussions around what scientific questions one would want to ask of the data
  • listing essential, recommended, and desired 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'.

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Results of the poll placed on Twitter between July 12th and 19th 2017
Should we keep these two separate concepts?
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There were 5 votes since the poll was created on 10:59, 15 March 2017.
poll-id D2C0068BD4358B564E5E598E295FF719

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

Age

Results of the poll placed on Twitter between July 15th and 22nd 2017
How should we report age?
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There were 6 votes since the poll was created on 11:02, 15 March 2017.
poll-id 84A488E5E21C698D19615F535B80CA41

Year

Results of the poll placed on Twitter between July 15th and 22nd 2017
How should we report year?
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There were 6 votes since the poll was created on 11:03, 15 March 2017.
poll-id EEF59B83600956C4360A6663F9661701


Unifying representations

The aforementioned concepts are in fact the same: time. One way to unify its representation is to recognize that all known applications use something of the form:

prefix _ yearlyIncrement _ suffix


  • Possible instances of yearlyIncrement are : year, years, annum, annums, etc
  • Possible instances of prefix are: none, kilo, mega, giga; this conveys information about the multiplicative factor that needs to be applied to the increments.
  • Possible instances of suffix are: none, BP, CE, AD. the suffix conveys information about the direction of time flow, and the reference point (with some ambiguity that needs resolving).

- For years CE, the prefix is none, the yearlyIncrement is years, the suffix is CE. Thus time flows forward from the origin of the Common Era, which is sometimes placed at year 0 (astronomical calendar, or ISO8601), sometimes at year 1. A system for disambiguation is needed (perhaps by attaching an ISO standard).

- For kyr BP the suffix conveys that time flows in reverse from the present day, again defined with some ambiguity as 1950 CE or 2000 CE (see Age poll). The prefix is kilo, indicating that a factor of 1000 needs to be applied.

- For "Ga", the prefix is giga, indicating a billion factor. The increment is "annum", and the suffix is none, indicating implicitly something like BP (where the 1950 or 2000 reference point is immaterial given the time spans involved.

Thus, the direction of time flow (forward or backward) can always been inferred from context; however, the diversity of spelling and abbreviations makes it difficult for machines to guess this, and thus a more disciplined representation would be helpful. For all representations, one might consider an internal representation like `kilo_years_BP`, and an external representation for display purposes like `kyr BP`.

Lastly, we note that the excellent ontology of geologic time by Cox & Richard[1], which deals with >1Ma time scales, does not encompass such distinctions, and thus a new representation is needed.

Recommendations from the literature

The recommendations below are from Govin et al. 2015 [2].

"Future climate-stratigraphic 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

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"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. [2]"
Results of the poll placed on Twitter between July 5th and 12th 2017
For U-series measurements in coral archive, should the uncertainty be reported as
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There were 4 votes since the poll was created on 12:39, 29 March 2017.
poll-id 8F6390D1C46244A8CB20D9F3BFE83F22

"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. [2]"
"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 [2]"
Results of the poll placed on Twitter between July 5th and 12th 2017
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 13:57, 20 March 2017.
poll-id 9F38149046E9E5931EB22346B63BE3C8

Results of the poll placed on Twitter between July 5th and 12th 2017
For U-series uncertainty in coral archive, should the uncertainty in the U-series decay constant for legacy datasets be:
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There were 2 votes since the poll was created on 13:57, 20 March 2017.
poll-id 383DEB45E82A99D8069455AB247EFC4F

Comment from User:Kcobb made on Twitter on July 5th 2017:
"Should be propagated through full uncertainty analysis in published dating table. But if not there, it's easy to figure out by pub date."

Dating of speleothems

Absolute dating and stratigraphic markers

Speleothems can be absolutely dated over the last 600ka with the U-Th methodology [3], with a dating uncertainty better than 0.5% [4] [5] [6].

Recommendation from Govin et al. (2015) [2]

"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. [2]"

Chronologies in marine sediments

Summary of methods used to derive chronologies in marine sediments. Adapted from Govin et al. (2015) [2]
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] [7] and Reimer et al., [2004] [8]

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

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Results of the poll ran on Twitter from March 23rd 2017 to March 30th 2017
For radiocarbon measurements, should the sample ID be:
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There were 4 votes since the poll was created on 15:21, 15 March 2017.
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 15:23, 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 [7] 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 [7] 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[9]) and SHCAL13 [10] 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

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Results of the poll conducted on Twitter between March 27th 2017 and April 3rd 2017
For Radiocarbon measurements, the chemical pretreatment/preparation of the sample should be:
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There were 5 votes since the poll was created on 16:17, 15 March 2017.
poll-id 78202BE8846AE9F82E3C8D0FF8873D02

Results of the poll placed on Twitter between July 12th and 19th 2017
For radiocarbon measurements, the method should be:
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There were 4 votes since the poll was created on 16:17, 15 March 2017.
poll-id 6DB651F90B192B8EE2EFDDAAFF6C2B4F

Results of the poll placed on Twitter between July 12th and 19th 2017
For radiocarbon measurements, the laboratory ID# should be:
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There were 5 votes since the poll was created on 16:17, 15 March 2017.
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 16:18, 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 16:19, 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 16:20, 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 16:21, 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 16:22, 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 4 votes since the poll was created on 16:38, 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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There was one vote since the poll was created on 16:38, 15 March 2017.
poll-id 873E0B9012B11CAB074C44F049C17B87

Lead

Proposed standards from Doug Hammond (University of Southern California).

Recommendation to report 210Pb measurements
Metadata Description Essential (E)/ Recommended (R)/ Desired (D)
calculated excess 210Pb values Total - supported E for both New and Legacy datasets
226Ra The supported 210Pb D for both new and legacy datasets
Total 210Pb D for both new and legacy datasets
Whether a salt correction is made D for both new and legacy datasets
Porosity necessary to do a salt correction D for both new and legacy datasets
Measurement methodology alpha spectrometry, gamma spectrometry... R for both new and legacy datasets
Units Recommend to report units in Bq/kg
Depth A way to locate where on the archive the measurements was made E
mid-depth E for both new and legacy datasets
interval top/bottom R for both new and legacy datasets
core collectionDate Necessary to convert the excess 210Pb values to calendar age

Recommend to correct the reported activities to the collectionDate

E for both new and legacy datasets
Methodology used to calculate age E for both new and legacy datasets

The same standards (excluding supporting measurements, which do not apply) are proposed for 137Cs measurements which are often made alongside 210Pb.

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Results of the poll placed on Twitter between July 17th-24th 2017
Should the calculated excess 210Pb values be considered:
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There were 3 votes since the poll was created on 21:31, 22 March 2017.
poll-id F2B50A2D9E7A28C350C863BC5344EFAC

Results of the poll placed on Twitter from March 30th to April 6th 2017
Should the supported activities (226Ra) be considered:
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There were 3 votes since the poll was created on 21:31, 22 March 2017.
poll-id 6B9986C74DE1D835AD2134D0FBC42F88

Results of the poll placed on Twitter from July 17th to 24th
Should the measured total 210Pb be considered:
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There were 3 votes since the poll was created on 21:32, 22 March 2017.
poll-id 340D1FB46BC1E0E0A44950EE33A97108

Results of the poll placed on Twitter between July 17th and 24th 2017
For 210Pb chronologies, should the reporting of a salt correction be considered:
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There were 2 votes since the poll was created on 21:40, 22 March 2017.
poll-id 05501AFC8D35560467941FCD24576B8A

For 210Pb chronologies, should the archive porosity be considered:
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There were 2 votes since the poll was created on 21:40, 22 March 2017.
poll-id 181DCFBC24604291F434FB9CD7F51A20

For 210Pb chronologies, should the measurement methodology be considered:
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There were 2 votes since the poll was created on 21:40, 22 March 2017.
poll-id 891B68A7B5BA1B1C466ACAA587B9F439

For 210Pb chronologies, should the activities be reported in:
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There were 2 votes since the poll was created on 21:40, 22 March 2017.
poll-id AE0852F3B711AAAB64C353C8FA5E4940

For 210Pb chronologies, should depth be considered:
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There were 2 votes since the poll was created on 21:40, 22 March 2017.
poll-id 02CFD3844C4DAF8F0EA94040B9D17EE3

For 210Pb chronologies, should the archive collectionDate be considered:
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There were 2 votes since the poll was created on 21:40, 22 March 2017.
poll-id CB743DD19273EF77DC03BD5640161C9B

For 210Pb chronologies, should the method used to convert to calendar age be considered:
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There was one vote since the poll was created on 21:40, 22 March 2017.
poll-id 0885F6472066B693111BCD8598DAA74E

U-series

A standard was recently proposed by Dutton et al [2017][11]. We suggest to re-use it wholesale.

Layer-counted chronologies

Comboul et al[12] 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

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Results of the poll placed on Twitter between April 25th and May 2nd 2017
Should the method by which the chronology is obtained be fully documented
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There were 3 votes since the poll was created on 16:38, 14 March 2017.
poll-id 995EC7654FD3A7F32B1C6029C31FB20A

Results of the poll placed on Twitter between April 25th and May 2nd 2017
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 16:38, 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 2 votes since the poll was created on 16:39, 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 was one vote since the poll was created on 16:41, 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 was one vote since the poll was created on 16:42, 14 March 2017.
poll-id 7D3C2523FB122801F0BC56D8D9926882

Polls

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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 16:35, 14 March 2017.
poll-id EE9B5AC53196826693167AAFFAD2B738

References

  1. Cox, S. J. D., and S. M. Richard (2015), A geologic timescale ontology and service, Earth Science Informatics, 8(1), 5–19, doi:10.1007/s12145-014-0170-6.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 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.
  3. Helmstrom, J., 2006. U-Th dating of speleothems with high initial 230Th using stratigraphical constraint Quaternary Geochronology, 1, 289-295.
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Pages in category "Chronologies Working Group"

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