A slice of ferromanganese crust from the Pacific Ocean was analyzed using the MIT team’s SQUID microscope. The top image shows a portion of the image of the slice taken with an electron microprobe. The second image shows the magnetized regions in the slice, with red areas showing one direction of magnetization and blue the opposite direction. Credit: Ben Weiss. |
In order to date environmental events
from Earth’s history, geologists have long studied variations in slow-growing
seafloor sedimentary rocks called ferromanganese crusts that build up in layers
over the eons. The layers can be dated by various means, such as by analyzing
radioactive isotopes, but those methods don’t provide accurate dating on small
scales.
Now, a new technique pioneered by
researchers at MIT and in Japan
provides a reliable way to date these “archives” of environmental changes with
much finer precision.
Because the Earth’s magnetic field flips
to an opposite (north or south) orientation at random intervals, the layers of
rock produce a kind of natural barcode that can be used for dating past
geological changes. While the layers’ scales differ greatly, the relative thicknesses
are the same. The spacing of these bands has been determined by studying the
rapidly growing igneous rocks that form in midocean ridges where the Earth’s
crust grows rapidly, so the stripes there are broad and easy to measure and
date.
“This new instrument allows
paleomagnetism [the study of ancient magnetic fields] to be brought down to the
microscale,” says Benjamin Weiss, associate professor of planetary sciences in
MIT’s Department of Earth, Atmospheric and Planetary Sciences. Weiss is a
co-author of a paper describing the use of this new instrument that appears in Geology.
Weiss and his co-authors, from the
Geological Survey of Japan, Kochi Univ. in Japan,
and Vanderbilt Univ.
in Tennessee,
tested sedimentary ferromanganese crusts with the new technique, called
scanning SQUID (superconducting quantum interference device) microscopy which
has been developed at MIT over the last seven years. The rock samples, which
were collected in the northwest Pacific in 1996, were built up on the seafloor
at the very slow rate of 5 mm per million years, according to various
independent dating measurements. By comparing the dates the new instrument
derived with those obtained from conventional isotope ratio techniques, they
were able to prove the accuracy of the new method.
“Normally, paleomagnetists collect
hundreds of oriented rock cores and spend months analyzing them in order to
compile histories of magnetic reversals throughout time,” says Joshua Feinberg,
assistant professor of geology and geophysics at the Univ. of Minnesota. “The
ability to do this on a single 5-cm-long sample is impressive, and provides
researchers with a well-calibrated tool for dating” seafloor rock samples.
Thanks to the work of the researchers in Japan and at MIT, he says, “many of
us in the paleomagnetic community are eager to use scanning SQUID instruments
in our own research … Our imaginations are the only limitation on how useful this
method will ultimately be.”
The new technology could be especially
useful, Weiss explains, because methods of studying changes in ocean
circulation and other parameters, such as ice core sampling, only go back a few
hundred thousand years, but the new magnetic measurements can be used to date materials
going back many millions of years. Such measurements could be used, for
example, to find out how ocean circulation has changed with time, Weiss says—which
could help in understanding the potential impact of changes in circulation
patterns that might result from global climate change.
The technique can also be used to date
ancient astronomical phenomena, such as the rate of impacts of meteorites on
Earth. Such impacts may lead to deposition in terrestrial sediments of a
short-lived isotope of iron, called 60Fe; evidence for this isotope
in ancient sedimentary layers suggests the possibility of an enhanced rate of
micrometeorites several million years ago. “It’s not easy to date sedimentary
rocks,” Weiss says, so this new instrument could theoretically be used to date
“anything that’s happened” that leaves traces in these seafloor crusts.
