August GEOLOGY and GSA TODAY Media Highlights

The Earth has gone through many major cooling steps during the past 65 million years of the Cenozoic Era. Of these, one of the most prominent occurred at the Eocene-Oligocene boundary (33.7 million years ago) when large permanent ice sheets first appeared on the Antarctic continent. One major hypothesis for Antarctic cooling proposes that it was caused by the formation of the Antarctic Circumpolar Current (ACC) after Southern Ocean gateways had opened. The hypothesis states that the ACC started after the northward movement of South America and Australia opened a deep water path around Antarctica for this major ocean current. According to this hypothesis, the formation of the ACC thermally isolated Antarctica and caused its glaciation. Lyle et al. show that the ACC did not form until about 8 million years after the Antarctic ice cap formed. Thus, the ACC formation was not the driver of Antarctic cooling. Instead, a decrease in atmospheric greenhouse gases, an alternate hypothesis, may have cooled Antarctica to cause its glaciation.

Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii

J.A. Vazquez et al., California State University–Northridge, Department of Geological Sciences, 18111 Nordhoff St., Los Angeles, CA 91330-8266, USA. Pages 695-698.

Vazquez et al.’s high-resolution ion microprobe dating of zircons from basalt-hosted plutonic xenoliths provides new insight into the timing and paths of magma evolution in Hawaiian volcanoes. The ages of diorite xenoliths and the compositions of zircon-hosted melt inclusions from Hualalalai volcano reveal episodes of extreme differentiation that are only partly reflected in the volcanic stratigraphy, suggesting deep storage and fractionation of shield and post-shield basalts. In contrast, diorite xenoliths from Mauna Kea volcano formed contemporaneously with eruptions of differentiated post-shield lavas. The results indicate that the timing of magma differentiation and the assembly of plutonic complexes at Hawaiian volcanoes is variable and may be decoupled from volcanic activity.

Multiple early Eocene hyperthermals: Their sedimentary expression on the New Zealand continental margin and in the deep sea
Micah J. Nicolo et al., Rice University, Earth Science, MS-126, 6100 Main St., Houston, TX 77005, USA. Pages 699-702.

Over a seven-million-year interval of the early Paleogene (ca. 58–51 million years ago), Earth underwent a gradual warming trend. At the Paleocene/Eocene boundary (ca. 55.5 million years ago), in the midst of slowly increasing global temperatures, a phenomenally rapid carbon cycle perturbation similar to what is being anthropogenically driven today, and an associated transient extreme warming (or “hyperthermal”) event , known as the Paleocene-Eocene Thermal Maximum (PETM), severely impacted many global systems. Nicolo et al. studied sediments deposited on the New Zealand continental margin and at various deep-sea locations and suggest that the PETM was not a unique event, but rather the most pronounced of a series of similar events that punctuated the early Paleogene warming trend. These results imply that any appropriate explanation for what caused the PETM must also be able to explain multiple similar events during the early Paleogene.