A Note on the Timescales


Assigning ages to measured depths in an ice core is an evolving process, with timescales often changing to reflect new scientific developments. It is imperative, when comparing data from one ice core with another (or with data from other paleoclimate records) that differences in the timescales are taken into account. The timescales used in specific publications may not be identical to the ones provided on this CD-ROM. If there are any discrepancies, the original investigators should be contacted. Comparison of different parameters (e.g. chemical concentrations) within one ice core should always be done on a depth - depth basis.

There are three main timescales used on the CD-ROM. For both GRIP and GISP2, these timescales are in years before present (yr BP) where year 0 refers to northern hemisphere summer of the year 1950 A.D. For example, year 1989 (summer) is be referred to as the year "-39 yr BP" on this CDROM, following the convention of the radiocarbon dating method (see e.g. Stuiver and Reimer, 1993). It should be noted that, in general, GRIP publications have not used the A.D. 1950 convention.

In the case of GRIP, the currently accepted timescale is generally referred to as the ss09 timescale, and is based on a flow-modeling approach, constrained by annual layer counting and other corroborative data. Refer to the file gripage.txt, the document Dating the Grip Core and the paper by Johnsen and others (1997) for additional details.

In the case of GISP2, the official timescale, as of this writing, is the Meese/Sowers timescale. This timescale is based on annual layer counting and volcanic markers to a depth of 2431 meters (about 50,000 years), using both the shallow GISP2 "B" core and the deep GISP2 "D" core, as described in Meese and others (1994). The Meese/Sowers timescale uses the original B-core timescale plus a correction of 1.09 meters. Below 2482 meters, dating is based on comparison with the deep-sea sediment record, using the oxygen isotope concentration in atmospheric oxygen in both the GISP2 and Vostok (Antarctica) ice cores, as described in Bender and others (1994). Between 2431 and 2482 a blend of these two methods was used. Below a depth of 2808 meters (about 111,000 years ago) the dating is considered unreliable because of the disturbed stratigraphy near the base of the ice sheet (see e.g. Alley et al., 1995). Please refer to the file gisp2age.txt and the document Dating GISP2 for additional details.

The ages associated with data in each of the files on this CDROM are derived from the most commonly used timescales for each core. For some files, principal investigators provided age scales. For most GRIP data sets, we have calculated ages by linear interpolation from the ss09 timescale. For GISP2 data sets, we have calculated ages from the official Meese/Sowers timescale. Also included on the CDROM, but not generally used in the data files, is the layer-counted timescale of Meese et al. (1997), based almost entirely on annual layer counting via visual stratigraphy. This is probably the most accurate timescale to date. More details on this timescale are given in the file countage.txt.

References

Alley, R.B., A.J. Gow, S.J. Johnsen, J. Kipfstuhl, D.A. Meese, and Th. Thorsteinsson. 1995. Comparison of deep ice cores. Nature 373:393-394.

Bender, M., T. Sowers, M.L. Dickson, J. Orchardo, P.M. Grootes, P.A. Mayewski, and D.A. Meese. 1994. Climate correlations between Greenland and Antarctica during the past 100,000 years. Nature 372:663-666.

Johnsen, S.J., H.B. Clausen, W. Dansgaard, N.S. Gundestrup, C.U. Hammer, U. Andersen, K.K. Andersen, C.S. Hvidberg, D. Dahl-Jensen, J.P Steffensen, H. Shoji, A.E. Sveinbjornsdottir, J.W.C. White, J. Jouzel, and D. Fisher. 1997. The d18O record along the Greenland Ice Core Project deep ice core and the problem of possible Eemian climatic instability. Journal of Geophysical Research 102:26471-26487.

Meese, D.A., R.B. Alley, R.J. Fiacco, M.S. Germani, A.J. Gow, P.M. Grootes, M. Illing, P.A. Mayewski, M.C. Morrison, M. Ram, K.C. Taylor, Q. Yang, and G.A. Zielinski. 1994. Preliminary depth-agescale of the GISP2 ice core. Special CRREL Report 94-1, US.

Meese, D.A., A.J. Gow, R.B. Alley, P.M. Grootes, M. Ram, K.C. Taylor, G.A. Zielinski, J.F. Bolzan, P.A. Mayewski, and E.D. Waddington. 1997. The Greenland Ice Sheet Project 2 depth-age scale: Methods and results. Journal of Geophysical Research 102:26411-26423.

Stuiver, M. and P. J. Reiner. 1993. Extended 14C data base and revised CALIB 3.0 14C age calibration program. Radiocarbon 35: 215-230.