Difference between revisions of "Category:Chronologies Working Group"

Revision as of 20:57, 15 September 2016

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.

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)
• 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.

Tie-point chronologies

Radiocarbon

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

Data standardization and reporting

Stuiver & Polach [1977] ^[1]

Reimer et al., [2004] ^[2]

As I previously mentioned, the IntCal radiocarbon community recognizes that there are more than one 'calibration' data-sets that are in the literature and utilized by the community. In an effort to not be completely prescriptive in the calibration data-set/product we strongly recommend that whatever authors do is applied equally to all of the data sets being discussed (otherwise one is in the position of mixing oranges and tangerines) and that the authors describe which calibration data set has been utilized, what software (if any) has been used to calibrate, and any corrections (eg., hard/soft water, reservoir age) that have been applied to the reported radiocarbon ages prior to calibrating or in the presentation of the data. We have the added request that whatever calibration data-set/product be peer-reviewed and published: it should not be grey literature, only a web-based algorithm, or a personal communication. This final request is simply to make it possible for reviewers/users to cross-reference and check what has been done.

Sample Information (metadata):

Sample ID, type/matrix, location (lat, lon, masl/mbsl altitude/depth, depth/height in section/core and referenced), etc.

Radiocarbon Data:

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

Method: GPC, LSC, AMS, etc

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

delta-13C ratio actual/estimated (PDB/V-PDB) †

Conventional radiocarbon age as years BP and one-sigma standard deviation ††

F14C (13C and background correction applied)

This is slightly different than Stuiver and Polach 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 Stuiver and Polach (1977).

† note, the AMS measured 13C is pretty crappy (a few per mil, compared to IRMS). Most AMS labs do NOT report the machine measured 13C value because users go off and do stupid things even if they are told not to. This should be estimated as per S&P (1977) or an IRMS-based value/estimate.

††  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. 

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:

Calibration Data-set: e.g., INTCAL13, Fairbanks, etc

Software Program/Algorithm used: eg., Bacon_v2.1, Calib_v7.x, CaliBomb_vy, CalPal, OxCal_vy, etc

As of present, the international radiocarbon community recommends the use of the INTCAL13 (Marine 13) and SHCAL13 data products.

P. J. Reimer et al., 2013. INTCAL13 and MARINE13 Radiocarbon age calibration curves, 0-50,000 years CAL BP. Radiocarbon, 55, 1869-1887.

A. G. Hogg, et al., 2013. SHCal13 Southern Hemisphere calibration, 0-50,000 years cal BP. Radiocarbon 55, 1889-1903.

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^[3] 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

Ice layers

Role of flow models.

Polls

Here are polls that the group might want to consider:

For NEW DATASETS: (this is a dummy poll; please update)

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. ↑ 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