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THIS WEEK IN edited by Stella Hurtley and Phil Szuromi CREDITS (TOP TO BOTTOM): PERRY ET AL.; JACOBSON ET AL. Paleocene Warming at Depth Weighing Earth’s Core The Paleocene-Eocene Thermal Maximum (PETM) was a geologi- Seismic velocities and comparisons with experiments have been cally brief period of warming that occurred about 55 million years used to infer the density of Earth’s interior, often through a linear ago. During this episode, the global carbon cycle was significantly extrapolation known as Birch’s Law. This analysis has indicated that perturbed, as reflected by a large and sudden change in the carbon Earth’s inner core is mostly an iron-nickel alloy that must contain isotopic composition of the global active carbon pool. This pertur- some light elements, such as sulfur or oxygen. However, this inferbation may have been caused by a massive release of methane hy- ence requires knowledge of how seismic velocities vary, not just drates from the sea floor, but the cause of such a release is not un- with density but also temperature. Lin et al. (p. 1892) directly derstood. Tripati and Elderfield (p. 1894) present benthic ocean measured seismic velocities in iron at high pressure and at several temperature records from intermeditemperatures. Temperatures comparable ate-depth waters in both hemispheres to those in the deep Earth reduce the and near the equator which show that sound velocity relative to the inferred A Changing Climate for all of those locations warmed roughly density. Thus, Earth’s core requires more Fish and Chips equally, by 4° to 5°C, slightly ahead of light elements than indicated from a the corresponding changes in the carlinear relation with density, a finding Climate change is well established as a pobon system. These data are consistent more consistent with other inferences. tential threat to biodiversity and the services with the id