FOR THE AGES Single-celled foraminifera helped to create the materials used in some of the world's great monuments, and are also very valuable in telling Earth's history because they produce shells that make good fossils. Michael Hesemann
Michael Hesemann
Michael Hesemann
By SEAN B. CARROLL Published: June 25, 2012
GUBBIO, Italy — North of Rome, in Umbria, a series of picturesque, ancient towns perch on the tops or sides of the foothills of the Apennine Mountains. Their placement here was a defensive imperative for successive Umbrian, Etruscan, Roman and Christian occupants over the millenniums. But these hillside locations were also of great advantage for constructing massive buildings, fortified walls and aqueducts, because of to unlimited local supplies of limestone.
Tourists flock to places like Gubbio, on the slope of Mount Ingino, to admire its impressive medieval churches and palazzos. But no one gives any thought to the tiny creatures that helped to create the materials necessary for making such spectacular, long-lived monuments.
Limestone is composed largely of crystallized calcium carbonate. Some of it comes from the skeletal remains of well-known creatures like corals, but much of the rest comes from less appreciated but truly remarkable organisms called foraminifera, or forams for short.
Forams have been called “nature’s masons,” and deservedly so. Most of the 6,000 species of these single-celled protists [ http://www.marinespecies.org/foraminifera/ ] construct surprisingly complex, ornate and beautiful shells to protect their bodies.
After forams die, their shells settle in ocean sediments — and may eventually become rocks that can be used to shelter our bodies.
While tiny relative to ourselves and most familiar marine creatures, forams are extremely large for single-celled organisms, often reaching a third of a millimeter in size. That is one hundred times larger than most bacteria and three times as large as a human egg cell, one of the largest cells in our bodies.
The forams in the limestone just outside Gubbio provided the first clues to one of the most exciting scientific discoveries in the past century.
In the 1970s, the geologist Walter Alvarez (now at the University of California, Berkeley) was studying the exquisite limestone formations around this town. Because different species with different shell shapes evolved at different times, foram fossils have been widely used to date rocks.
Dr. Alvarez was trying to figure out the ages of the rocks around Gubbio. He learned that the topmost layer of the rocks from the Cretaceous period always contained a diverse array of large fossil forams.
But the layer of rock just above it, which signaled the beginning of the later Tertiary period, lacked most of those Cretaceous species and contained only a few, much smaller species of forams. Separating the two rock layers was a thin layer of clay that appeared to lack fossils altogether.
The geologist Jan Smit, now at the VU University in Amsterdam, discovered a similar pattern in rocks in southern Spain.
The abrupt disappearance of forams in these layers of rock indicated that something had happened at the boundary between the Cretaceous and Tertiary periods. The end of the Cretaceous period also coincided with the extinction of the dinosaurs, as well as once-abundant marine animals such as ammonites.
Dr. Alvarez, Dr. Smit and their colleagues wondered: What on earth could have caused the disappearance of widespread, tiny organisms like forams, as well as much larger creatures?
As it turned out, it wasn’t something on earth but something from space. Chemical analyses of the clay marking the boundary of the two periods, carried out by these geologists and their collaborators, revealed that it contained extraordinary levels of the element iridium, a material rare on earth but more abundant in certain kinds of asteroids. The scientists proposed that the iridium was fallout from an asteroid [ http://www.sciencemag.org/content/208/4448/1095 ] that struck earth at the end of the Cretaceous period [ http://www.nature.com/nature/journal/v285/n5762/pdf/285198a0.pdf ], 65 million years ago.
The asteroid was about the size of Mount Everest and traveling at about 50,000 miles an hour when it hit the earth, drilling a 120-mile-wide crater and ejecting so much material into (and even out of) the atmosphere that food chains on land and in the oceans were disrupted for thousands of years. The impact caused one of the greatest mass extinctions in history, from the largest animals to tiny forams [ http://www.sciencemag.org/content/327/5970/1214.short ].
Eventually, forams and the oceans rebounded, and new species evolved. But today, forams are warning us of a new threat, for they are not merely witnesses to earth’s history, but critical participants in it.
Forams are a vital part of a “biological pump” that removes carbon dioxide from the atmosphere [ http://royalsociety.org/policy/publications/2005/ocean-acidification/ ]. When carbon dioxide dissolves in seawater, one reaction product is carbonate. In making their calcium carbonate shells, the large mass of so-called planktonic forams floating in the upper levels of the oceans sequester about one quarter of all carbonate produced in the oceans each year [ http://www.agu.org/pubs/crossref/2002/2001GB001459.shtml ].
The increasing levels of carbon dioxide in our planet’s atmosphere, now at a greater level than at any time in the past 400,000 years, threaten to overwhelm this biological pump by inhibiting the formation of calcium carbonate shells. As more carbon dioxide dissolves in the ocean, the waters acidify, decreasing the concentration of carbonate and making it more difficult for these organisms to form calcium carbonate shells.
At current rates of carbon dioxide production, ocean acidity is projected to increase by another three- to four-tenths of a pH unit by the end of the century, with potentially catastrophic effects on shell-forming creatures and food chains.
And in the present geologic period, guess who is at the top of those chains?