Ocean floor sediment could offer a way to better predict future undersea earthquakes, new research suggests.
Geologist Michael Strasser, who until 2015 was an assistant professor for sediment dynamics at ETH Zurich and is now a professor at the University of Innsbruck, traveled with colleagues to Japan to study dynamic sediment remobilization processes triggered by seismic activity.
At a depth of 7,542 meters below sea level, the team took a core sample from the Japan Trench, an 800-km-long oceanic trench in the northwestern part of the Pacific Ocean. The seismically active trench was the epicenter of the Tohoku earthquake in 2011, which made headlines when it caused the nuclear meltdown at Fukushima.
These kinds of rarthquakes wash enormous amounts of organic matter from the shallows into deeper waters. Researchers can then use the resulting sediment layers to glean information about the history of earthquakes and the carbon cycle in the deep ocean.
Dating deep ocean dirt
Researchers analyzed the carbon-rich sediments using radiocarbon dating. This method—measuring the amount of organic carbon as well as radioactive carbon (14C) in mineralized compounds—has long been a means of determining the age of individual sediment layers.
Until now, however, it has not been possible to analyze samples from deeper than 5,000 meters below the surface, because the mineralized compounds dissolve under increased water pressure.
Strasser and his team had to use new methods for their analysis. The online gas radiocarbon method, developed by the Laboratory for Ion Beam Physics and doctoral student Rui Bao and the Biogeoscience Group at ETH Zurich was one of these methods. This greatly increases efficiency, since it takes just a single core sample to make more than one hundred 14C age measurements directly on the organic matter contained within the sediment.
In addition, for the first time in the dating of deep-ocean sediment layers, the researchers applied the Ramped PyrOx measurement method (pyrolysis). The researchers used this method in cooperation with the Woods Hole Oceanographic Institute (US), which first developed it.
The process involves burning organic matter at different temperatures. Because older organic matter contains stronger chemical bonds, it requires higher temperatures to burn. The relative age variation of the individual temperature fractions between two samples distinguishes the age difference between sediment levels in the deep sea very precisely.
Past and future
Thanks to these two innovative methods, the researchers could determine the relative age of organic matter in individual sediment layers with a high degree of precision.
The core sample they tested contained older organic matter in three places, as well as higher rates of carbon export to the deep ocean. These places correspond to three historically documented yet hitherto partially imprecisely dated seismic events in the Japan Trench: the Tohoku earthquake in 2011, an unnamed earthquake in 1454, and the Sanriku earthquake in 869.
Currently, Strasser is working on a large-scale geological map of the origin and frequency of sediments in deep-ocean trenches. To do so, he is analyzing multiple core samples taken during a follow-up expedition to the Japan Trench in 2016.
“The identification and dating of tectonically triggered sediment deposits is also important for future forecasts about the likelihood of earthquakes,” Strasser says. “With our new methods, we can predict the recurrence of earthquakes with much more accuracy.”
The researchers report their findings in the journal Nature Communications.
The research initiative was organized in March 2012 by MARUM—Center for Marine Environmental Sciences.
Source: Samuel Schlaefli for ETH Zurich
