WAIS Divide WDC06A annual-layer thickness averages from visual stratigraphy

Using diffuse transmitted light opposite a planed surface in a light-shielded booth at the National Ice Core Lab (NICL) in Denver, CO, visual stratigraphy of the entire WAIS Divide main core was conducted by the same individual, Matthew K. Spencer (Lake Superior State University). This effort was aided by previous experience the stratigrapher gained while visually examining the entire ice core returned from Siple Dome, Antarctica (Taylor and others, 2004), using the same setup and methods.

The character of the annual signal as revealed visually in an ice core evolves throughout the depth of the core. In the firn layer the annual signal reveals itself in the contrast between the coarse-grained, low-density summertime hoar layers versus the fine-grained, higher-density winter layers (Alley and others, 1997). Below the firn layer, this grain-size contrast is retained in seasonal bubble number-density contrast (Spencer and others, 2006). At depths below which clathrates form and bubbles are increasingly absent, the stratigrapher here hypothesizes that seasonal grain-size contrast was responsible for producing the visible annual signal.

A best guess was made for the most probable visibly detectable annual signal throughout the WAIS Divide core by assuming some level of reduced variability over small depths. Therefore, actual annual layers that were significantly thinner or thicker relative to other annual layers within the same ~1-m sections of core would likely have been lumped or split, respectively. Annual layer thicknesses were averaged over ~50-m intervals, the depth range covered per logbook used. To avoid the difficult lightening conditions observed near the ends of core sections, average annual layer thickness from visual stratigraphy includes only those annual layer thicknesses contained within a 1-m logbook page.

While there is some agreement between visual stratigraphy and independent dating methods (Fitzpatrick and others, 2014), a noticeably greater mismatch appears from ~550-1000 m. The visual stratigrapher found annual layer detection increasingly difficult to detect over this depth range. Detection of annual layers become noticeably easier with decreasing bubble size and decreasing bubble number-density, with the clathrate transition beginning around 1100 m, below which seasonal grain-size variability produced transmitted light contrast significant enough to see the annual signal with somewhat greater clarity. While electrical measurements fail to reliably detect an annual signal below ~2800 m, annual layers from visual stratigraphy below 2800 m appeared as distinct as did annual layers from ~1100 to 2800 m.

References:

Alley, R.B., C.A. Shuman, D.A. Meese, A.J. Gow, K.C. Taylor, K.M. Cuffey, J.J. Fitzpatrick, P.M. Grootes, G.A. Zielinski, M. Ram, G. Spinelli, and B. Elder. 1997. Visual-stratigraphic dating of the GISP2 ice core: basis, reproducibility, and application. J. Geophys. Res., 102(C12), 26,367-26,382.

Joan J. Fitzpatrick, Donald E. Voigt, John M. Fegyveresi, Nathan T. Stevens, Matthew K. Spencer, Jihong Cole-Dai, Richard B. Alley, Gabriella Jardine, Eric Cravens, Lawrence Wilen, TJ Fudge, Joseph McConnell. 2014. Physical Properties of the WAIS Divide Ice Core. In prep for J. Glaciol.

Fudge, T. J., E. J. Steig, B. R. Markle, K. Taylor, J. R. McConnell, E. J. Brook, T. Sowers, J. W. C. White, S. W. Schoenemann , R. Alley, H. Cheng, G. D. Clow, J. Cole-Dai, H. Conway, K. M. Cuffey, J. Edwards, L. Edwards, R. Edwards, J. Fegyveresi, D. Ferris, J. Fitzpatrick, J. Johnson, G. Hargreaves, J. Lee, O. Maselli, W. Mason, K. McGwire, L. E. Mitchell, N. Mortensen, P. Neff, A. Orsi, T. Popp, A. J. Schauer, J. Severinghaus, M. Sigl, M. Spencer, B. H. Vaughn, D. Voigt, E. Waddington, G. J. Wong (2013) Onset of deglacial warming in West Antarctica driven by local orbital forcing, Nature, 500(7463), 440-444.

Taylor, K.C., R.B. Alley, D.A. Meese, M.K. Spencer, E.J. Brook, N.W. Dunbar, R.
Finkel, A.J. Gow, A.V. Kurbatov, G.W. Lamorey, P.A. Mayewski, E. Meyerson,
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Spencer, M.K., R.B. Alley and J.J. Fitzpatrick. 2006. Developing a bubble number-density paleoclimatic indicator for glacier ice. J. Glaciol. 52(178), 358-364.