ICE Core Agedepth Model Calculator

An age-depth model converts a position in an ice core into a calendar age by describing how annual layers thin as they are buried. In the Nye formulation the vertical velocity is assumed to fall linearly from minus the accumulation rate at the surface to zero at the bed, which makes the vertical strain rate uniform. Annual layer thickness then scales as the accumulation rate times one minus z over H, and integrating the reciprocal of that thickness gives age equal to minus H over a times the natural log of one minus z over H. Layer thickness therefore falls to half its surface value at mid-depth and the modelled age diverges toward infinity at the bed. The Dansgaard-Johnsen variant keeps uniform strain only above a basal shear layer and lets the vertical velocity decrease linearly to zero within it, which better matches ice-divide flow and yields older ages deep in the core. Realistic surface layer thicknesses run about 20 to 25 cm of ice per year at Greenland summit sites and only 2 to 3 cm per year at East Antarctic domes, thinning to sub-millimetre near the bed, which is what ultimately limits recoverable resolution. Basal melt rates of 0 to a few millimetres per year, set largely by a geothermal flux near 50 to 60 mW per square meter, remove the oldest ice entirely. The main caveats are structural: these are one-dimensional steady-state solutions valid only near an ice divide, they assume constant accumulation through glacial cycles, and they cannot represent upstream advection, basal folding, or the ice-gas age offset created by firn.

The analytical framework comes from two short papers. John Nye published the uniform-strain age-depth relation in the Journal of Glaciology in 1963, giving glaciologists a closed-form way to date depths without counting anything. Willi Dansgaard and Sigfus Johnsen followed in 1969 with a two-zone flow model built specifically to interpret the Camp Century core, drilled through 1387 m of the Greenland ice sheet to bedrock in 1966 by the US Army Cold Regions Research and Engineering Laboratory. That core, paired with Dansgaard's oxygen isotope thermometry, was the first to show abrupt glacial-age climate swings preserved in ice, and it created the demand for chronologies precise enough to compare against other archives. Subsequent practice combined analytical models with independent constraints. Volcanic sulfate horizons of known date, such as Tambora in 1815 and Laki in 1783, anchor the upper core, while multi-parameter annual layer counting produced the Greenland ice core chronology extending past 60000 years at NGRIP. In 2004 the EPICA Dome C core reached roughly 800000 years, and modern chronologies such as AICC2012 replaced single analytical models with inverse methods that combine flow models, gas and ice tie points, and orbital tuning. The Beyond EPICA project is drilling at Little Dome C in pursuit of continuous ice approaching 1.5 million years.