April GEOLOGY and GSA TODAY media highlights

Subduction, where one lithospheric plate is pushed beneath another, provides a key driving force for plate tectonics and is believed to have played an important role in generating the continents. However, the way that subduction begins is poorly understood due to the lack of examples. Macpherson examines magmatism in a rare, active example of a nascent subduction zone in the eastern Philippines. The depth at which magma stalls and crystallizes depends on the thickness of the upper plate. In turn, this provides insights into production of the continental crust. Compositions of melts that reach the upper crust are sensitive to variations in crystallization depth, and the eastern Philippines demonstrate that deeper crystallization produces magmas of the type required to help build continental crust.

Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison

K. Panchuk et al., 134 Edmund Park, Saskatoon, Saskatchewan S7H 0Z4, Canada. Pages 311-314.

A sudden and extreme increase in global temperatures 55 million years ago has been attributed to the release of methane, a strong greenhouse gas, due to the destabilization of seafloor methane hydrate deposits by long-term global warming. This interpretation has dire implications for the consequences of modern day climate change. Panchuk et al.’s study suggests, however, that the distribution of deep-sea calcium carbonate sediments cannot be explained by methane from hydrate deposits alone. Instead, the release of carbon related to volcanism in Greenland, and the opening of the northern Atlantic Ocean, might come closer to explaining the observation. This study represents the first time the observed spatio-temporal record of carbonate sediments has been incorporated into an Earth-system model to understand ancient climate change.

Resolving Milankovitchian controversies: The Triassic Latemar Limestone and the Eocene Green River Formation

Stephen R. Meyers, Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3315, USA. Pages 315-318.

A long-standing controversy in the field of geochronology pertains to the use of Milankovitch orbital cycles for the construction of deep-time astrochronologies. When vestiges of these cyclic perturbations to Earth's orbit are preserved in the geologic record, they provide an important high-resolution method for measuring the passage of time. Yet a major challenge to the development of accurate deep-time astrochronologies is the lack of sufficient independent time control (e.g., radiometric age data) that would reliably calibrate the stratigraphic rhythms to temporal periods, and thus directly confirm their orbital tempo. Meyers investigates two of the most controversial stratigraphic units in the field of astrochronology, using a new statistical approach that overcomes the independent time control problem. In both the Triassic Latemar Limestone and the Eocene Green River Formation, Meyers demonstrates that the null hypothesis (no orbital signal) can be rejected with an extremely high degree of confidence. These results underscore the high fidelity of the orbital chronometer, and have far-reaching consequences for the field of geochronology. In the case of the Latemar Limestone, the orbital chronometer disagrees with the existing uranium/lead zircon geochronology by a factor of 4, indicating that sources of error in the zircon-based time scale must be reconsidered. More generally, the new astrochronologic method provides an objective standard for refinement of Cenozoic-Mesozoic geochronologies, and will permit extension of orbital time scale construction into the Paleozoic.