USING GIS TO RECONSTRUCT PALEO EQUILIBRIUM-LINE ALTITUDES FROM CIRQUES IN NORTHWEST ICELAND

The purpose of this study is to use geographic information systems (GIS) to calculate the equilibrium-line altitudes (ELA) of paleo cirques on Vestfirðir, Northwest Iceland. A 20 meter resolution digital elevation model was obtained from the National Land Survey of Iceland. Three methods are used to identify the ELA of the cirques: the cirque floor method, the altitude ratio method, and the area-altitude ratio method. In ArcGIS, contour lines and surface slope are generated and used to identify cirques in the study region. Altitude ratios are developed using the toewall and the headwall of each cirque, which are identified using the derivative of the elevation profile graphs of each cirque. Two points of steepest slope along each elevation profile are used to identify the locations of the headwall and the toewall. The flattest location between the headwall and the toewall is interpreted as the cirque floor. Data from the cirque floor method is exported to Microsoft Excel and ELAs are determined from calculations using the toewall-headwall altitude ratio (THAR) method. A modified area-altitude ratio (AAR) calculation is also used to calculate the paleo ELA of cirques. The width of the cirque is determined by inflection points of contour lines where a change in concavity occurs. Area is calculated from a delineated polygon that connects inflection points to the toewall and the headwall in ArcGIS. Preliminary results from the THAR method demonstrate an arithmetic mean ELA between 387 m and 414 m. The arithmetic mean ELA using the cirque floor method is approximately 364 m. Initial results suggest that paleo ELAs are at least 186 m lower than the modern ELA of Drangajökull Ice Cap on eastern Vestfirðir. Additional analyses are currently in progress.