Climate change in the past can tell us much about what is happening today. New research shows how plankton shells dredged from sea floors hold the information we seek.
For climate data dating back as far as 800,000 years ago, we can rely on ice cores. These ice cores are samples that are drilled out from polar ice. They have tiny bubbles that trap ancient air. But their data goes only so far back in time.
My colleagues at the University of Cambridge and I have explored how plankton shells, which are abundant in the oceans can provide information dating back further. Our results have just been published in the journal Earth and Planetary Sciences Letters.
These plankton shells show features that record historical climate, in a way that is similar to tree rings. The results could allow scientists to chart short timescale changes in ocean temperatures hundreds of millions of years ago.
As microbial plankton grow in ocean waters, their shells, made of the mineral calcite, trap tiny amounts of chemical impurities (maybe only a few atoms in a million get replaced by impure 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 recovered from the muddy sediment, which preserves the shells as they are buried. The amount of impurity, measured in fossil plankton shells, provides a record of past ocean temperature, dating back more than 100 million years ago.
We measured traces of magnesium in the shells of plankton using an X-ray microscope in Berkeley, California, at the Advanced Light Source synchrotron - a huge electron accelerator that generates X-rays to study matter in minuscule detail.
The powerful X-ray microscope has revealed narrow nanoscale (billionth of a metre) bands in the plankton shell where the amount of magnesium is slightly higher. They are growth bands, but in plankton the bands occur daily or so, rather than yearly as in trees.
The 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, revealed in the new measurements, shows that the magnesium sits in calcite crystal replacing calcium, rather than in microbial membranes in their impurities in the shell. This helps explain why temperature affects the chemistry of plankton shells - warmer waters favour increased magnesium in calcite.
These growth bands in plankton show the day by day variations in magnesium in the shell. For slow-growing plankton, a simple analysis reveals seasonal variations dating back hundreds of millions of years.
The Cambridge group is now using the UK’s Diamond Light Source synchrotron X-ray facility to measure how plankton shells grow and whether they change at all in the ocean floor sediments. Our 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.