The Humboldt core showed higher annual thicknesses rates around 1800 AD, possibly indicating a changed circulation pattern between 1760 to 1810 AD for the northern part of Greenland. Thus there are some periods when the part of the ice sheet represented by these cores may have experienced gains and losses over a large area, and other times when within-region variability offset overall ice sheet gains and losses. Figure 7 shows the hydrogen peroxide concentrations for Crawford Point, which provide a clear delineation of annual layers. Using the NASA-U density values for this site gives an average layer thickness of 0.47 m ice per year. This is probably a low estimate, since density could be higher at Crawford Point than at NASA-U. Figures Figure 1.
Vertical lines mark annual layers.
Comparison of annual layer thicknesses from NASA-U core 1 using calcium versus hydrogen peroxide. Annual layer thicknesses based on analysis of all parameters for NASA-U and all parameters except dust for Humboldt. Hydrogen peroxide data for Crawford Point, showing 13 annual cycles. Vertical lines mark annual layers. References D. A. Meese, A. J. Gow, P. Grootes, P. A. Mayewski, M. Ram, M. Stuiver, K. C. Taylor, E. D. Waddington, and G. A. Zeilinski.