03.11.08: Foraminifera

Little Climate Chroniclers Under Scrutiny

The chemical make-up of their shells records conditions of the ocean in which they live and grow. Thus, the information stored in the shells greatly contributes to understanding ocean conditions, and therefore climate, of the past. But the shells of foraminifera do not always stand the test of time: After death the calcite shells sink from the ocean surface to the sea floor, and when conditions are unfavourable, they will start to dissolve. Dissolution alters crucial chemical properties, which leads to inaccuracies in climate reconstruction based on chemical analysis of the shells. Heather Johnstone from MARUM is working on a way for scientists to draw the right conclusions from dissolution-biased data – and also use the extend of dissolution to reveal something of the chemistry of the deep ocean. A high resolution CAT scan of the little climate chroniclers provide the information needed.

Foraminifers have rather short lives. The ones shown in the pictures started theirs at the light-flooded surface of the ocean. But soon they were drawn to the deep: during the few weeks of their existence, they sank to depths of about 200 metres, all the while busily building their calcite shells. Trace metals incorporated into the calcite reflect the temperature and chemical make-up of the water they lived in. After the creatures died, the shells - by now a half a millimetre in size - sank to the floor of the ocean.

„What happens to the shells after they are deposited on the sediment depends on the water depth, and also the chemistry of the deep water which they are now exposed to.” says Heather Johnstone, a MARUM PhD student working with foraminifera. The upper ocean layers tend to be saturated with calcite. But an increase in water depth corresponds with a decrease in concentration. Deep water can still absorb more calcite and it takes it from the calcite shells of the foraminifera. These start to dissolve from the inside out. This process continues as the shells are covered by new sediment. The deeper the sediment layer in which they are found, the older the foraminifers.

The dissolution process alters two parameters important to paleo-climatologists. “The ratio of light and heavy oxygen (16O und 18O) in the calcite gives us mainly information about how much ice there was in the world at the time the forminifera was building its shell. The ratio of magnesium to calcium of the shell tells us about the water temperature” Johnstone says. “Both values are affected by the dissolution process.”

So how to tell whether the foraminifera which ended up in the lab are still in an acceptable condition? Heather Johnstone and her co-workers look right through them, literally. The tiny shells are imaged using X-ray computer tomography, a high resolution form of the CAT scanning technique used in hospitals. Now Johnstone can view them on the computer, slice by slice, to see how far dissolution has proceeded.

The results are useful in two different ways. The dissolution state of the shells does not only indicate unreliable data; it can also indicate how much carbon was stored in the waters of the deep ocean at the time the shell was deposited. This is currently of great interest to scientists as it is connected to the amount of carbon dioxide in the atmosphere.

Heather Johnstone and her co-workers have developed a dissolution index to rate the preservation state of the shells. This index can now be used to indicate the carbon chemistry of the deep water masses the foraminifera were exposed to. The next step was to use the index to try and predict the affect of dissolution on water temperatures generated from chemical analysis of the shell. It is a work in progress. “The method shows where temperatures are biased by dissolution”, Johnstone says, “but quantifying by precisely how much is more difficult.”

3D animations of foraminifers imaged using CT can be found here.