Paper No. 195-12
Presentation Time: 11:15 AM
SAND ON THE WEST SHORE OF GLACIAL MENDENHALL LAKE APPEARS TO BE PETRIFIED PEAT, SILICATES AND MAGNETITE REPLACE ORGANIC CARBON
Sand on the northwest shore of Mendenhall Lake, near Juneau, Alaska, is mapped (USGS) as glacial moraine: single particles of silt, sand and gravel fractured and ground from tonalite bedrock, transported and deposited by the Mendenhall Glacier before its’ retreat, circa 1940. However, a decade of observations and more than 10 thousand detailed micro-photos show this is not likely. The sand consists of a mass of interconnected silicate and magnetite grains which appear to be molded together except where spaces between them are filled with a dark, glassy, transparent matrix. These are not individual particles and have features, e.g. ridges that continue across several adjacent grains. Grains are arranged and connected in ways unlikely to have occurred by chance or to have survived crushing and grinding glacio-fluvial transport. They are also interconnected by strands of organic material and what appears to be mineralized organic material. This sand appears to have formed in place, not a sediment but a mineralized organic deposit. A C14 date of 1950 for organic materials encapsulated in magnetite bearing grains, suggests mineralization is current and ongoing. Recent study of the Rhytidiadelphus loreus, “lanky moss,” community on the lake shore confirms this hypothesis. If near surface sand is petrified organic material, and the process is current, then sand under the moss must have formed most recently and there should be a zone between the actively growing moss and the tightly packed sand below where newly and partially formed grains are common. This appears to be the case. Newly forming silica and magnetite grains are found on the growing tips of moss plants and portions of the moss stems themselves are magnetic. Stems below the growing region show increased mineralization through a peaty layer and on down to mostly mineral sand at a depth of about 10 cm. This seems to be an unrecognized part of the carbon-silica cycle probably mediated by bacteria as they use iron for respiration and replace carbon with silicon. New ways to use digital photography to reveal opal in organic materials are presented.