July 2009 Geology and GSA Today Media Highlights

The 1500-year climate oscillation in the midlatitude North Pacific during the Holocene
Dai Isono et al., Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan. Pages 591-594.

In this study by Isono et al., evidence of 1,500-year climate variability during the past 11,000 years was found in sediment cores taken from the northwestern Pacific Ocean off central Japan. Based on the periodicity and the timing, this variability is interpreted as an extension of the Dansgaard-Oeschger cycle seen in the last glacial period. The regular pacing at 1,500-year intervals seen throughout both the Holocene and the last glacial period suggests that the oscillation was a response to external forcing.

Supercontinent reconstruction from recognition of leading continental edges
J. Brendan Murphy et al., Dept. of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada. Pages 595-598.

There is a general consensus that repeated cycles of supercontinent amalgamation and dispersal have occurred for the past two billion years, and that these cycles have profoundly influenced the evolution of Earth's crust, climate, and life. Amalgamation of supercontinents is characterized by widespread mountain-building events. Breakup of supercontinents, on the other hand, is characterized by the development of continental shelves along the trailing edges of the dispersing continents. Both these events can be recognized in the geologic record and aid in the reconstruction of the changing geography of the Earth. Nevertheless, because Earth is a dynamic planet, preservation of these features is fragmentary, so that Earth's ancient geography is poorly constrained and often controversial. Pangea, which formed about 300 million years ago and began to break up about 200 million years ago, is the most recent of these supercontinents. Its geography is reasonably well constrained, but uncertainties increase substantially as we try to investigate deeper into Earth's history. Murphy et al. show that the leading edges of dispersing continents may be recognized by the diagnostic chemical composition of magmas derived from a young mantle that preferentially occurs along its margins. Using the magmatism of western North America since the breakup of Pangea as an analogue, they show that the leading edges of dispersing continents have diagnostic isotopic characteristics that can identify these margins. For example, the isotopic signatures of 600-450-million-year igneous rocks along the northern margin of Gondwana indicate that they were derived by melting a 700-million- to 1.1-billion-year mantle. This mantle must have originated in an ocean surrounding the ancient supercontinent Rodinia. The mantle was subsequently attached to northern Gondwana in response to the breakup of Rodinia, and provided a source for subsequent magmatism. Murphy et al. propose that such features should be common along the leading edges of dispersing continents following supercontinent breakup. This recognition provides an additional aid in the reconstruction of Earth's changing geography.

Mid-Pliocene Asian monsoon intensification and the onset of Northern Hemisphere glaciation
Yi Ge Zhang et al., State Laboratory for Mineral Deposits Research, Institute of Surficial Geochemistry, Dept. of Earth Sciences, Nanjing University, Nanjing 210093, China. Pages 599-602.