February Geology and GSA Today media highlights

Boulder, CO, USA - Topics include: tree ring-based reconstruction of an extinct polar forest's seasonal environment; geological vs. seismological views of paleo-earthquakes; impact of Lake St. Martin bolide on groundwater quality; analysis of earthquakes at the Cascadia plate boundary; radar images of volcanic and impact deposits on lunar Aristarchus Plateau; dynamics of methane escape into the atmosphere; use of charred plants in modeling pyroclastic density currents; and the possibility we've left the Holocene and are in the Anthropocene.

Annual patterns within tree rings of the Arctic middle Eocene (ca. 45 Ma): Isotopic signatures of precipitation, relative humidity, and deciduousness
Hope Jahren, Earth and Planetary Sciences, 301 Olin Hall, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA; and Leonel Sternberg, University of Miami, Department of Biology, Coral Gables, Florida 33124, USA. Pages 99-102.

Jahren and Sternberg's study explains that the spectacular preservation of Eocene (approximately 45-million-year-old) tree rings allowed the reconstruction of the seasonal environment of an extinct polar forest. Deciduous conifer trees experienced high levels of humidity and a fluctuating water source during the short, intense (no darkness) Arctic winter.

Stretching of fluid inclusions in calcite as an indicator of frictional heating on faults
Kohtaro Ujiie et al., Institute for Research on Earth Evolution, Japan Agency for Marin-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan. Pages 111-114.

The geological recognition of seismic slip on exhumed faults is one of those holy grails in the earth sciences. Overall, the structural geological community is conservative, with most only accepting frictional melts (pseudotachylytes as evidence of high-velocity slip). Ujiie et al. provide another line of evidence that can be invoked to recognize high-velocity slip, and begin to fill this ground between the geological and seismological views of paleo-earthquakes.

The Lake St. Martin bolide has a big impact on groundwater fluoride concentrations
Matthew I. Leybourne et al., GNS Science, Lower Hutt, New Zealand. Pages 115-118.

Meteor impact is popularly linked with catastrophic extinction events, but some effects can be harmful in a more subtle and insidious manner. As Leybourne shows, the shattering impact of a meteor on rocks results in increased groundwater-rock surface interaction. This has a direct effect on the quality of the groundwater that percolates through the fractured, melted rocks of the impact structure. A good example has been found at the Canadian town of Gypsumville, Manitoba (population 65), located near the Lake St. Martin meteor impact crater. Domestic wells in the town have elevated salinity, sulfate, and fluoride concentrations. The fluoride, which exceeds health limits, is of concern as excess intake causes mottling of teeth at moderate levels, to softening of bones and neurological damage at higher levels. The groundwater with elevated fluoride is shown to occur exclusively within the impact structure, and Leybourne's study is thought to be the first to document enhanced groundwater fluoride concentrations associated with impact structures.