Difference between revisions of "Category:Chronologies Working Group"

Latest revision as of 19:04, 18 September 2017

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'.

Note: Do not edit the polls (even for typos) once voting has started as it will reset the vote counts to zero. If a change needs to be made, make an annotation above the old poll (i.e., above the poll tags) and place the new poll below the first one.

Results of the poll placed on Twitter between July 12th and 19th 2017

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

Age

Results of the poll placed on Twitter between July 15th and 22nd 2017

Year

Results of the poll placed on Twitter between July 15th and 22nd 2017

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

Note: Do not edit the polls (even for typos) once voting has started as it will reset the vote counts to zero. If a change needs to be made, make an annotation above the old poll (i.e., above the poll tags) and place the new poll below the first one.

"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

"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

Results of the poll placed on Twitter between July 5th and 12th 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

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

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Results of the poll ran on Twitter from March 23rd 2017 to March 30th 2017

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
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

Note: Do not edit the polls (even for typos) once voting has started as it will reset the vote counts to zero. If a change needs to be made, make an annotation above the old poll (i.e., above the poll tags) and place the new poll below the first one.

Results of the poll conducted on Twitter between March 27th 2017 and April 3rd 2017

Results of the poll placed on Twitter between July 12th and 19th 2017

Results of the poll placed on Twitter between July 12th and 19th 2017

Lead

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

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

Results of the poll placed on Twitter from March 30th to April 6th 2017

Results of the poll placed on Twitter from July 17th to 24th

Results of the poll placed on Twitter between July 17th and 24th 2017

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

Results of the poll placed on Twitter between April 25th and May 2nd 2017

Relative chronologies

Example of relative chronologies include benthic δ18O alignment, paleomagnetic data...

Polls

Note: Do not edit the polls (even for typos) once voting has started as it will reset the vote counts to zero. If a change needs to be made, make an annotation above the old poll (i.e., above the poll tags) and place the new poll below the first one.

Here are polls that the group might want to consider:

For LEGACY DATASETS:

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.
4. ↑ 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.
5. ↑ 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.
6. ↑ 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.
7. ↑ ^{7.0 7.1 7.2} Minze Stuiver and H. A. Polach, 1977. Discussion: Reporting of 14C Data. Radiocarbon 19, 3, 355-363.
8. ↑ 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.
9. ↑ P. J. Reimer et al., 2013. INTCAL13 and MARINE13 Radiocarbon age calibration curves, 0-50,000 years CAL BP. Radiocarbon, 55, 1869-1887.
10. ↑ A. G. Hogg, et al., 2013. SHCal13 Southern Hemisphere calibration, 0-50,000 years cal BP. Radiocarbon 55, 1889-1903.
11. ↑ A. Dutton, K. Rubin, N. McLean, J. Bowring, E. Bard, R.L. Edwards, G.M. Henderson, M.R. Reid, D.A. Richards, K.W.W. Sims, J.D. Walker, Y. Yokoyama, Data reporting standards for publication of U-series data for geochronology and timescale assessment in the earth sciences, Quaternary Geochronology, Volume 39, April 2017, Pages 142-149, DOI
12. ↑ 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