Dating consortium
Names and contact details for chairs:
Jørgen Peder Steffensen (Copenhagen, Denmark, +45 35 32 06 21, jps@nbi.ku.dk)
Jakob Schwander (Bern, Switzerland, Add phone number, jakob.schwander@climate.unibe.ch)
Sune Olander Rasmussen (Copenhagen, Denmark, +45 35 32 05 90, olander@nbi.ku.dk)
Point of contact and procedure for joining the consortium mailing list
Please go to https://mailman.nbi.ku.dk/listinfo to sign up for the NEEM-dating mailing list. The list is managed by the consortium chairs.
A brief description of the work of the consortium
The large ice caps covering Greenland and Antarctica comprise a fantastic archive of information about the palaeoclimate. This information has been made available through the drilling of ice cores, which represent samples of millennia of precipitation. However, the value of this information can only be fully appreciated if reliable chronologies can be established. Therefore, it's a high priority to obtain a reliable depth - age relationship (a time scale) for an ice core. Time scales can be constructed in several ways, using a variety of methods and data sources.
The following describes the status and strategy for dating the NEEM ice core.
The first time scale for the NEEM site was distributed during the November 2007 Steering committee meeting by Dorthe Dahl-Jensen. It was based on a flow model incorporating a number of age markers obtained from the well-dated NGRIP ice core by tracing layers observed in a radar-profile between the NGRIP and NEEM drill sites. Dates of selected NGRIP layers according to the so-called GICC05 time scale were transferred to NEEM depths in this way, and a model was developed to make a time scale for the NEEM core that was consistent with the age of the traced layers.
The current time scale for NEEM is called GICC05modelext-NEEM-1. The time scale was released via e-mail to NEEM collaborators on November 8, 2010. It is unpublished, and should by used within the NEEM community only. If you do not have access to the time scale and wish to use it, please contact your national NEEM SC representative or dating consortium chairman Sune Olander Rasmussen (contact details above). The time scale has been constructed as follows:
- The Greenland Ice Core Chronology 2005 (GICC05) is a Greenland annual-layer-counted dating based on data from several cores reaching about 60.2 ka back in time. Sigfus Johnsen's NGRIP ss09sea06bm model time scale shifted to younger ages by 705 years has been spliced onto the end of the GICC05 time scale, thereby forming the so-called GICC05modelext chronology.
For details on the ss09sea time scale version used, see Andersen et al. (Quat. Sci. Rev. 25, p. 3246-3257, 2006). For details on the splicing of the time scales, see Wolff et al. (Quat. Sci. Rev. 29, p. 2828-2838, 2010). Data file: txt or xls. - This chronology has been applied to NEEM via a stratigraphic match using common features based mainly on ECM data (patterns, peaks and dips), and in some sections by DEP data. Below 2186 m NEEM depth the match points are also supported by isotope data. The total number of match points is 740. There are no match points across a large part of GS-2, and there is still a data gap in the brittle zone.
- The transfer itself is made from NEEM bag depths to NGRIP depths using linear interpolation between the match points and then to ages using the published GICC05modelext depth-age relation.
The next steps are to refine the match points using FIC and CFA data and data from the brittle zone gap to produce GICC05modelext-NEEM-2. At a later stage, we will revise GICC05 for all Greenland cores in parallel to form a new version of the GICC framework.
Dorthe Dahl-Jensen and Susanne Buchardt are working on modelling the flow around NEEM. The model is a Dansgaard-Johnsen-type model that parametrizes the ice height H, the kink height h, the accumulation a, the bottom sliding factor FB, and the basal melt rate w. The parameters are determined using a Monte Carlo inversion technique in parallel by Dorthe Dahl-Jensen and Susanne Buchardt.
The age of the gas record relative to the age of the ice (the so-called Δage) will be estimated using state-of-the art models calibrated by firn air measurements.
The basic principle of the NEEM dating effort is that all time scales should be accepted by the dating consortium and if relevant by the SC before publication or circulation to prevent simultaneous use of several non-consistent time scales. Also, all time scales should have a name that reflects the nature of the time scale and allows unique identification.
Declared interests
The following nations have until now declared interests in the dating work:
- Belgium: Local (high‐order) ice flow modelling with the purpose of dating and obtaining back‐trajectories
- France: Inverse modelling for dating.
- Switzerland: Mostly firn air and Δage
- UK: Tephra-related issues
- Sweden: Cosmogenic isotopes
- Denmark: Flow modelling, annual layer counting, synchronization with other (mainly Greenland) ice cores
Please send additions or corrections to this list to the chairmen.