Difference between revisions of "Category:Chronologies Working Group"

Revision as of 22:17, 15 March 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.

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'.

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

Age

Year

Tie-point chronologies

Radiocarbon

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

Key references: Stuiver & Polach [1977] [1] and  Reimer et al., [2004] [2]

metadata

Radiocarbon Measurements

• Chemical pretreatment/preparation (oxidation, chemical leach (%), a/b/a, soxhlet, ultrafiltration, none, etc)

• Method: GPC, LSC, AMS, etc

• Laboratory ID# (eg. OS####; CAMS####; QUB####)

• δ13C ratio actual/estimated (PDB/V-PDB) ^[3]

• Conventional radiocarbon age as years BP and one-sigma standard deviation ^[4]

• F14C (13C and background correction applied)^[5]:

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).

• 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
    • DELTA_R value, ± one-sigma sd (or square root of the variance) uncertainty, reference/source
• hard/soft water effect: value, ± one-sigma sd (or square root of var) uncertainty, reference/source
• other corrections: value (14C years), ± one-sigma sd (or square root of var), reference/source

• Calibration or conversion to calibrated ages:

As of present, the international radiocarbon community recommends the use of the INTCAL13 (aka Marine 13[6]) and SHCAL13 [7] 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.

Lead

no known existing standard/template

U-series

Layer-counted chronologies

Comboul et al^[8] 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

Polls

Here are polls that the group might want to consider:

For LEGACY DATASETS:

References

1. ↑ Minze Stuiver and H. A. Polach, 1977. Discussion: Reporting of 14C Data. Radiocarbon 19, 3, 355-363.
2. ↑ 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.
3. ↑ 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 or an IRMS-based value/estimate.
4. ↑ Note, that by definition, conventional radiocarbon age is the Libby based age. Most AMS laboratories report 1-sigma sd of the analysis or the 1-sigma sd reproducibility for similar aged/sized material.
5. ↑ This is slightly different than Stuiver and Polach who have "D14C" as the optional parameter.
6. ↑ P. J. Reimer et al., 2013. INTCAL13 and MARINE13 Radiocarbon age calibration curves, 0-50,000 years CAL BP. Radiocarbon, 55, 1869-1887.
7. ↑ A. G. Hogg, et al., 2013. SHCal13 Southern Hemisphere calibration, 0-50,000 years cal BP. Radiocarbon 55, 1889-1903.
8. ↑ 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