Determination of Bedrock Weathering Rates in the Juneau Area, Northern Southeast Alaska

Stainless steel bolts (7.94 x 38.10 mm) were drilled and glued into bedrock study sites in the Gravina, Taku, and Yukon Tanana terranes in the Auke Bay and Mendenhall Valley areas of Juneau, Alaska in the spring of 2006. These lithologies increase in metamorphic grade from the western zeolite facies basaltic-andesite flows and turbidite sequences, to greenschist facies metabasalts and turbidites, into the easternmost amphibolite facies quartzites, marbles, gabbros and sheared tonalite.

Baseline data consisting of bolthead-to-rock surface distances were collected and georeferenced with GPS to create an initial dataset from which to compare future measurements. A specially designed micrometer Rock Erosion Meter (REM) (Allred, 2004) was utilized to measure the lowering of rock surfaces adjacent to the bolts. This measuring tool was built from a Brown and Sharp 608 model micrometer by Allred. The carbide-tipped measuring rod is able to able to measure depths between 25.4-76.2 mm from the rock bolt head. Comparative studies in southern southeast on karst surfaces in Alexander terrane carbonates have yielded dissolution rates ranging from 31 mm/ka in forested terrains to 38 mm/ka in alpine settings. Runoff from acid peat bogs produced dissolution rates of 1.66 m/ka, which are amongst the highest rates in the world. We anticipate much lower rates for the weathering of newly deglaciated, mostly siliclastic rocks in the Juneau Area. Measurements will be repeated through time across this bolt array.