Mass balance measurements on the Lemon Creek Glacier, Juneau Icefield, Alaska 1953-1998

Annual balance measurements on the Lemon Creek Glacier, Alaska conducted by the Juneau Icefield Research Program (JIRP) from 1953 through 1998 provide a continuous 46-year record. This is one of the nine American glaciers sel ected in a global monitoring network during the international Geophysical y ear, 1957/58. These data have been acquired primarily by employing consiste nt ground methods, conducted on similar annual dates and calculated using c omparable methodology. The results have been until now fairly precise, but of uncertain accuracy. An adjunct comparison of topographic surface maps of the glacier made in 1957 and 32 years later in 1989 provides a rough deter mination of glacier surface elevation changes which are clearly of less pre cision than the compilation of annual ground data, Airborne surface profili ng in 1995, and global positioning system leveling transects in 1996-1998 u pdate the record of surface elevation changes over the past decade. The mea n glacier ice thickness reductions suggested by these methods from 1957-198 9, from 1957-1995 and from 1957-1998 are -13.2 m, -16.4 m, and 21.7 m, resp ectively. it is of interest that the geodetic interprerations agree fairly well with the trend of sequential balances from ground-level stratigraphic measurements. To date, however, the infrequent mapping methods in this stud y have yielded specific balances averaging between 5 and 11% less than thos e resulting from our annual on-sire glaciological monitoring. For future st udies this can be an important factor. The ground data are, therefore, the ones in which we have most confidence. These show cumulative ice losses of -13.9 m (12.7 m water equivalent w.e.) from 1957-1989, of -19.0 m (-17.1 m w.e.) from 1957-1995, of -24.4 m (22 m w.e.) from 1957-1998, and -24.7 m (2 2.2 m w.e.) for the total cumulative loss over the full 46 years between 19 53 and 1998. Although the balance trend has been increasingly negative it a verages -0.48 m/a in w.e. or 0.52 m of ice loss per year. To refine the reliability of density determinations in this data set the ef fects of internal accumulation from refrozen meltwater producing diagenetic ice structures in the annual firnpack have been taken into account. An unu sual dearth of such structures within the 1997/98 firnpack provided a uniqu e opportunity to facilitate application of the probing technique over broad areas of the neve. This added to our ground truth and verified accuracy of the test-pit measurements used in these long-term mass balance computation s. The glacier's continuing negative mass balance has fueled a terminal retrea t of 800 m during the 1953-1998 period. The annual balance trend indicates that despite a higher mean elevation and a higher elevation terminus from t hinning and retreat, mean annual balance has been strongly negative since 1 977 (-0.78 m/a w.e.). Dramatically increased negative mass balances have oc curred in the 1990s, with 1996 and 1997 being the only years on record with no retained accumulation since field observations were initialed in the gl acier source areas in 1948.