Ancient Climate Change Evidence From Fossilized Plankton
October 26, 2013

Magnesium In Plankton Shells Serves As Record Of Ancient Climate Change

redOrbit Staff & Wire Reports - Your Universe Online

Climate change dating as far back as hundreds of million years ago was recorded in the shells of ancient oceanic plankton, according to new research published in the journal Earth and Planetary Sciences Letters.

Typically, scientists analyze polar ice in search of information about the planet’s temperature and atmosphere, but the oldest Antarctic ice core records only date back to approximately 800,000 years ago.

In the new study, investigators from the University of Cambridge Department of Earth Sciences managed to uncover growth bands in sea shells that could demonstrate how shell chemistry records timescale changes in water temperature occurring more than 100 million years ago.

The shells of microbial plankton are made of the mineral calcite, and as the creatures grow in ocean water, those shells capture trace amounts of chemical impurities. Sometimes only a few atoms out of a million are replaced by these so-called impurity atoms, though experts have noticed that plankton growing in warmer environments contain a higher percentage of impurities. The reason for this phenomenon is not yet clear.

The plankton fall to the mud-covered ocean floor upon their death, and since the sediments located on the floor help preserve their shells, they can be recovered. The amount of impurity contained in the fossilized plankton shells can reveal how warm the water was over the course of different periods of time, and the authors of the new study have used a large x-ray microscope located at the Lawrence Berkeley National Laboratory in Berkeley, California to measure traces of magnesium within the shells.

That microscope technology unveiled narrow nanoscale bands in the plankton shell where the magnesium content is slightly higher, at length scales roughly as small as one-hundredth that of a human hair, the researchers explained. These bands are said to be similar in nature to growth rings found in trees, but they occur every day or so instead of only representing a single year.

“These growth bands in plankton show the day by day variations in magnesium in the shell at a 30 nanometer length scale,” Professor Simon Redfern said in a statement Friday. “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.”

“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,” he added.

The research team’s measurements revealed that, based on the chemical environment of the trace elements in the plankton’s shells, the magnesium replaces calcium in the calcite instead of resting in microbial membranes found in the shells’ impurities. This discovery helps explain why temperature affects the chemistry of the plankton shells, and while warmer water results in increased magnesium content in calcite, the researchers noted.