The effect of ice-sheet thickness change on the accumulation history inferred from GISP2 layer thicknesses

Abstract

Net accumulation rates at the Greenland summit have been inferred using layer-thickness data from the GISP2 ice core with corrections for strain using a non-linear, one-dimensional flow model of an ice sheet. The flow model accounts for thickness changes in ice-sheet in response to mass-balance variations. The model is used to investigate how net accumulation-rate changes affect the time evolution of: (1) the ice-sheet thickness. (2) the vertical strain rate, and (3) the corresponding internal annual-layer structure. The model, parameterized to fit the present net accumulation rate and thickness of the Greenland ice-sheet summit, has a characteristic time constant for adjustment to accumulation-rate changes of about 6000a and yields an ice sheet 200-400 m thinner than its present thickness during the last glacial period. Accumulation-rate histories inferred from GISP2 layer-thickness data using both a constant- and a variable-thickness model are compared. The variable-thickness model predicts accumulation rates about 25% lower than the constant-thickness model. Our results also indicate that high-frequency changes in accumulation rates (i.e. alter the Younger Dryas event) are consistent with earlier analyses. However, sensitivity tests indicate that the accumulation-rate history cannot be precisely determined. Our analysis defines an envelope of likely accumulation histories bounded above by the accumulation history inferred by the constant-thickness model. Predictions become increasingly uncertain for old ice because of (1) intrinsic difficulties associated with this inverse problem, and (2) decreased accuracy of the data.