Lagrangian Transient Firn Compaction Model With Heat Transfer
Abstract
Paleoclimate applications require knowledge of the iceage gasage offset (delta age) to correlate proxies in the ice (e.g. the temperature proxy δ18O in H2O) with trapped atmospheric gas. We develop a modular onedimensional Lagrangian transient firn compaction model to find delta age. Finite volumes proportional to surface accumulation are advected downward, as they thin vertically primarily due to increases in density. The volumes can also extend horizontally due to strain in the underlying ice sheet. The evolution of each finite volume is governed by mass conservation and a constitutive relation for firn. Firncompaction rate depends on temperature, so the compaction model is coupled to a Lagrangian heatdiffusion model, in which the thermal properties can depend on density. Upper boundary conditions for the coupled model are the histories of accumulation rate, surface density, and surface temperature. The lower boundary conditions must be provided from a larger icesheet model in which the firn model can be embedded. These conditions are the the thermal boundary condition, horizontal extension with depth, and vertical velocity at the bottom of the firn. Delta age is defined as the age of the ice where the density of bubble close of is met. The firn model can be incorporated into a larger inverse problem, creating a selfconsistent system among three lines of questioning, that have until now been treated independently: (1) deltaage (2) inferred spatial and temporal accumulation histories from dated radar layers, and (3) physically meaningful interpolations of depthage between sparse dated gas data points. This firn compaction model with a modular design, can be applied to any ice sheet given the requisite boundary conditions, further contributing to frontier work in ice sheet modeling.
 Publication:

AGU Fall Meeting Abstracts
 Pub Date:
 December 2007
 Bibcode:
 2007AGUFM.C51A0106L
 Keywords:

 0724 Ice cores (4932);
 0726 Ice sheets;
 0798 Modeling