July 2009 Geology and GSA Today Media Highlights

The late Pliocene onset of major Northern Hemisphere glaciation (NHG) is one of the most important steps in the Cenozoic global cooling. Although most attempts have been focused on high-latitude climate feedbacks, no consensus has been reached in explaining the forcing mechanism of this dramatic climate change. Here, Zhang et al. present a key low-latitude climate record, the high-resolution Asian monsoon precipitation variability for the past five million years, reconstructed from South China Sea sediments. Their results, with supporting evidence from other records, indicate significant mid-Pliocene Asian monsoon intensification, preceding the initiation of NHG at about 2.7 million years ago. The correlation between the monsoon record and marine calcium isotopes indicates that the 1.4-million-year-long monsoon intensification probably enhanced Asian continental erosion and chemical weathering. Elevated chemical weathering and organic carbon burial probably lowered the contemporary atmospheric CO2, and the CO2 drop may have triggered the NHG onset as suggested by a recent study. As opposed to previous studies, here Zhang et al. propose a low-latitude-driven NHG glaciation scenario.

Magmatic plumbing of a large Permian caldera exposed to a depth of 25 km
J.E. Quick et al., Southern Methodist University, Dallas, Texas 75275, USA. Pages 603-606.

Large volcanic calderas, aka supervolcanoes, are enormous craters tens of kilometers in diameter produced by giant, explosive eruptions that rank among the most violent geologic events. Geophysical studies of recently active calderas and investigations of their eruption products suggest that their magmatic systems are driven by intrusion of mantle-derived basalt in the deep crust, a process commonly referred to as magmatic underplating. However, direct confirmation of this connection and our understanding of the processes involved have been limited by lack of a crustal section exposing rocks deeper than about 5 km beneath a caldera. Quick et al. report evidence for a 285-million-year-old fossil caldera, more than 13 km in diameter, in northwest Italy, situated atop a tilted crustal section that was exposed by uplift and erosion to reveal the caldera's magmatic plumbing system from the surface to a depth of greater than 25 km. This unprecedented exposure of magmatic plumbing provides a model for interpreting geophysical profiles and magmatic processes beneath active calderas, and direct confirmation of the cause-and-effect link between intrusion mantle-derived basalt in the deep crust and explosive volcanism.

Mafic dikes displacing Witwatersrand gold reefs: Evidence against metamorphic-hydrothermal ore formation
Dimitri L. Meier et al., ETH Zurich, Isotope Geochemistry and Mineral Resources, Dept. of Earth Sciences, Clausiusstrasse 25, 8092 Zurich, Switzerland. Pages 607-610.

The origin of Earth's greatest gold reefs, in the Witwatersrand sedimentary basin in South Africa, has been debated since they were discovered a hundred years ago. Meier et al. describe an new geological argument based on the nature of cross-cutting magmatic dikes, indicating that the gold deposition into the ancient sedimentary rocks probably occurred by mechanical sedimentation into loose river gravels, not by hot fluids flowing through cracks in the strata millions of years after the deposition of the gold-bearing rocks.