June 2008 Geology and GSA Today media highlights

Were the past glacial ages wetter or dryer" Though it is often asked, this question does not actually make much sense because temperature and precipitation do not oscillate with the same rhythm. Glacial ages (periods of globally low temperature and a larger volume of polar ice) typically recur at 100,000 year intervals. By contrast, precipitation at several places on Earth is oscillating at 23,000 year cycles. However, the reason two different climatic parameters (temperature and rain fall) show different rhythms is not well understood. Nakagawa et al. reconstructed climate changes of the past 450,000 years using fossil pollen grains from a Japanese lake sediment. The results clearly showed that the temperatures of both continental and oceanic air masses fluctuate at 100,000 year cycles, whereas the land-ocean temperature gradient and summer rainfall oscillates at 23,000 year cycles. The land-ocean temperature gradient is the direct cause of the East Asian monsoon, which in turn is responsible for the Japanese summer rain fall. The 23,000 year climatic cycle coincides with the strongest periodicity of changes in the brightness of the sun. Based on these findings, Nakagawa et al. propose that the direct solar beam is regulating the land-ocean temperature gradient (and hence monsoon intensity and rainfall) through different heat capacities between land and oceans, and this process is essentially independent from glacial-interglacial cycles.

Cause and evolution of intraplate orogeny in Australia

S. Dyksterhuis, University of Sydney, Institute of Marine Science, School of Geosciences, Edgeworth David Building F05, NSW 2006, Australia; and R.D. Müller, School of Geosciences, Madsen Building H9, University of Sydney, NSW 2006, Australia. Pages 495-498.

Dyksterhuis and Müller demonstrate how the complex interplay between juxtaposed weak and strong geological plate elements and changes in far-field plate boundary forces can cause intraplate orogeny. The time dependence of the intraplate stress regime of the Australian continent provides a cause for an enigmatic intra-plate orogeny in southeastern Australia dubbed the "Sprigg's Orogeny." In agreement with geological observations, modeled stress directions, which rotate around geologically weaker regions and towards stronger plate elements, show ideal orientations to cause uplift along preexisting faults during two distinct stages during the past 55 million years. Dyksterhuis et al.'s work demonstrates how mountain building can occur far away from plate boundaries and has implications for the evaluation of the suitability and longevity of hazardous waste storage locations and resource exploration.

Elevated shear zone loading rate during an earthquake cluster in eastern California

M. Oskin et al., Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27516, USA. Pages 507-510.