Ancient Antarctic Environment Rocked by Volcanic Eruptions
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The first evidence of large volcanic eruptions that shook Antarctica around 25 million years ago has unexpectedly been discovered in rock cores retrieved from the seabed as part of an international ocean-floor drilling project in which the National Science Foundation (NSF) is a partner.
Scott Borg, who heads the geology and geophysics program for NSF's Antarctic Science Section, said the rock cores, drilled from the sea floor off the Victoria Land coast near Cape Roberts, show surprising evidence of enormous volcanic eruptions. These eruptions are believed to have significantly altered global temperatures at the time.
"The discovery of the volcanic material is really quite exciting," Borg said. "It is clearly evidence of a major eruption, several times larger than Mount St. Helens (in Washington State) and possibly comparable with the eruption that destroyed Pompeii."
The evidence of volcanic activity was gathered under the auspices of the Cape Roberts Project. For the past two Antarctic field seasons, NSF-sponsored scientific teams, in collaboration with scientists from Australia, Britain, Germany, Italy and New Zealand, have been drilling the seabed to obtain samples that would illustrate the climatic and geologic history of Antarctica during the last 100 million years. Drilling this year had reached a depth of approximately 110 meters below the seafloor when this unexpected evidence of volcanic activity was encountered.
The evidence consists of layers of volcanic debris that were erupted explosively into the atmosphere and then settled through the air and into the ocean onto the seafloor. The thickness and coarseness of the main debris layer indicates a large-volume eruption that generated an ash cloud that reached into the stratosphere. The discovery of these volcanic layers underlying the seabed demonstrates a far more spectacular history of volcanic activity than was previously suspected for the Ross Sea region of Antarctica, but it is also useful because it provides material for accurately dating the strata.
The layers of volcanic debris are encased within muddy sands, indicating a relatively quiet seafloor with occasional weak currents before and after the eruptions. This relatively quiet environment, however, was disrupted at least twice and possibly as many as four times, by large and rapid inputs of volcanic debris (mostly pumice). The debris was supplied by voluminous eruptions from a nearby source, but the exact location and characteristics of that source are still unknown.
The thickest distinct layer of volcanic debris is 1.2 meters thick, which suggests an eruption as dramatic as that of Krakatau in 1883. These layers contain volcanic pumice up to 1 cm in size, which suggests that the volcano was located within 50 to 100 kilometers of the drilling site and erupted in a style reminiscent of Vesuvius.
The eruptions recorded in this core probably had a significant impact, not only on the Antarctic environment, but also on the global environment of the time. Modern examples of similar eruptions, such as Mount Pinatubo, a much smaller event, cooled world climate by 0.5 degrees C for a year after its 1991 eruption.
Scientists will use the volcanic layers to determine the age of their strata more accurately, because volcanic debris can be dated precisely using isotopic techniques. Borg said that samples of the pumice fragments are being sent to be dated at New Mexico Tech, in Socorro, N.M., one of eight U.S. universities taking part in the Cape Roberts project.
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