May 2008 GEOLOGY media highlights

Groundwater is not only the most important source of drinking water worldwide, but it can also be used as an archive of past environmental conditions. Using a suite of environmental tracers, Klump et al. investigated a deep aquifer system in southeastern Wisconsin. These tracers allow the residence time of the groundwater and the environmental conditions prevailing during groundwater recharge to be assessed. Results show that the aquifer contains water recharged before, during, and after the last glacial advance across the area. The findings of this study shed light on the glacially driven climatic and hydraulic changes that occurred during the late Pleistocene and the early Holocene. The work also contributes to the thorough understanding of the present and past hydrogeologic conditions of the aquifer, which is a prerequisite for the sustainable use of this important regional water resource.

Permeability control on magma fragmentation
Sebastian Mueller et al., Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS81RJ, UK. Pages 399-402.

Mueller et al. address an aspect of the geosciences that has a direct impact on lives, infrastructure, and capital-explosive volcanic eruptions. The balance and competition between gas loss via permeable flow (avoiding explosivity) and gas retention and subsequent fragmentation (generating explosivity) is a decisive factor in controlling such eruptions. Using an experimental approach, this study relates fragmentation behavior to permeability, thus taming the nagging issue of short-circuiting of explosions via gas flow. The results will enable refined modeling of various volcanological scenarios and, subsequently, lead to a more reliable hazard assessment.

Rock varnish evidence for latest Pleistocene millennial-scale wet events in the drylands of western United States
Tanzhuo Liu and Wallace S. Broecker, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964-8000, USA. Pages 403-406.

Obtaining continuous millennial-scale climate records for the world’s deserts has proven to be a difficult, but often critical, step in understanding the mechanism and spatial variations of Earth’s climate. Rock varnish is a slowly accreting (about few micrometers per thousand years), dark coating on subaerially exposed rock surfaces in arid to semiarid deserts. Because of its sedimentary origin, rock varnish often displays a layered microstratigraphy that records past climatic variations. Liu and Broecker present rock varnish evidence for latest Pleistocene (from 18,000 to 11,500 calendar years before present) millennial-scale wet events in the drylands of the western United States. Preliminary radiometric age calibration of the varnish record indicates that these wet events are largely associated with millennial-scale cooling events documented in the Greenland ice core records. This finding suggests that such wet oscillations in the western United States may be parts of regionally widespread manifestation of pervasive millennial-scale cycles of the North Atlantic climate.