Shell function: X-rays reveal climatic conditions

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  • Published: Nov 1, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Shell function: X-rays reveal climatic conditions

Deep time observations

This is a CT reconstruction of a foram measured at the Diamond Light Source.  Credit: Oscar Branson, University of Cambridge

The climatic condition stretching back millions of years ago are locked into the ancient skeletons of microscopic sea creatures high-power X-ray microscopy reveals. The discovery could allow researchers to map the short timescale changes in ocean temperatures hundreds of millions of years ago.

Plankton shells show features like tree rings, recording historical climate, according to the new study. As such, they point to a possible means to unravel ancient climate change and perhaps shed light on more recent events and offer a way to predict how global temperatures might change in the future. The latest discovery could outstrip the best ice cores from Antarctica, which date back to just 800,000 years.

Results published in a recent issue of the journal Earth and Planetary Sciences Letters reveal how ancient climate change, pushing back hundreds of millions of years into deep time, is recorded by the shells of oceanic plankton.

Impurity thoughts

As microbial plankton grow in ocean waters, their shells, made of the mineral calcite, trap trace amounts of chemical impurities, maybe only a few atoms in a million getting replaced by impurity atoms. Scientists have noticed that plankton growing in warmer waters contain more impurities, but it has not been clear how and why this "proxy" for temperature works. When the plankton die, they fall to the muddy ocean floor, and can be retrieved as fossilized remains by modern scientists. The amount of impurity, measured in fossil plankton shells, provides a record of past ocean temperature, dating back more than 100 million years ago.

Now, researchers from the Department of Earth Sciences at the University of Cambridge, UK, have measured traces of magnesium in the shells of plankton using X-ray facilities at Berkeley's Advanced Light Source X-ray synchrotron in California. The instrument reveals narrow nanoscale bands in the plankton shells where the amount of magnesium is very slightly higher. These growth bands are analogous to tree rings hinting at the conditions in which the plankton lived but with a resolution at the daily level rather than annual growth.

Daily growth rings

"These growth bands in plankton show the day by day variations in magnesium in the shell at a 30-nanometre length scale," explains team member Simon Redfern. "For slow-growing plankton it opens the way to seeing seasonal variations in ocean temperatures or plankton growth in samples dating back tens to hundreds of millions of years." He adds that, "Our X-ray data show that the trace magnesium sits inside the crystalline mineral structure of the plankton shell. That's important because it validates previous assumptions about using magnesium contents as a measure of past ocean temperature."

The chemical environment of the trace elements in the plankton shell, measured in the new studies, shows that the magnesium sits in calcite crystal replacing calcium, rather than in microbial membranes in the shell. This helps explain why temperature affects the chemistry of plankton shells - warmer waters favour increased magnesium in calcite.

The group is now using the UK's Diamond synchrotron X-ray facility to measure how plankton shells grow and whether they change at all in the ocean floor sediments. Their latest results could allow scientists to establish climate variability in Earth's far distant past, as well as providing new routes to measure ocean acidification and salinity in past oceans.

Related Links

Earth Planetary Sci Lett 2013, 383, 134-141: "The coordination of Mg in foraminiferal calcite"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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