While tourists flock to places like Gubbio in Italy to admire its medieval churches made of limestone, no one gives much thought to the tiny creatures that help in making limestone. It is made of calcium carbonate, and some of it comes from remarkable organisms called forams, explains Sean B Carroll
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 access 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 crystallised 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 remarkable organisms called foraminifera, or forams. They have been called "nature's masons" and deservedly so.
Most of the 6,000 species of these single-celled protists construct complex, ornate and beautiful shells to protect their bodies. After forams die, their shells settle in ocean sediments - and may become rocks that can be used to shelter our bodies. While they are 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 100 times larger than most bacteria and three times as large as a human egg cell, one of the largest cells in our bodies.
First clues
The largest forams can reach a few centimetres. The most impressive of all, now extinct, were flat, disk-shaped species called nummulites (from the Latin meaning "small coins"), abundant in the limestone, and used to build Egypt's pyramids. Because they produce shells that make good fossils, and have long been abundant and widespread in the oceans, forams are valuable to geologists and paleontologists in telling us about earth's history.
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, 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.
Alvarez learned that the topmost layer of the rocks (around Gubbio) 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.
Geologist Jan Smit, now at the V U University in Amsterdam, discovered a similar pattern in southern Spain's rocks. 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 dinosaurs, and once-abundant marine animals such as ammonites. Alvarez, Smit and their colleagues wondered what could have caused the disappearance of widespread organisms like forams as well as much larger creatures. As it turned out, it was something from space.
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 iridium, a material rare on earth but more abundant in some asteroids.
Scientists proposed that the iridium was fallout from an asteroid that struck earth at the end of the Cretaceous period, 65 million years ago.
The impact crater was identified underneath the Mexican village of Chicxulub on the Yucatan Peninsula.
Warnings
The asteroid was about the size of Mount Everest and travelling at about 50,000 mph 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. Eventually, forams and the oceans rebounded, and new species evolved. Today, forams are warning us of a new threat, for they are not merely witnesses to earth's history, but critical participants in it.
They are part of a "biological pump" that removes carbon dioxide from the atmosphere. 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 in the upper levels of the oceans sequester about one quarter of all carbonate produced in the oceans each year. The increasing levels of carbon dioxide in our planet's atmosphere, now at a greater level than at any time in the past 4,00,000 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 formation of calcium carbonate shells.
The ocean surface is now about one-tenth of a pH unit more acidic than in preindustrial times. A study of forams in the Southern, or Antarctic, Ocean found that their shells are now 30 per cent to 35 per cent thinner than in the preindustrial era. 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.
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 access 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 crystallised 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 remarkable organisms called foraminifera, or forams. They have been called "nature's masons" and deservedly so.
Most of the 6,000 species of these single-celled protists construct complex, ornate and beautiful shells to protect their bodies. After forams die, their shells settle in ocean sediments - and may become rocks that can be used to shelter our bodies. While they are 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 100 times larger than most bacteria and three times as large as a human egg cell, one of the largest cells in our bodies.
First clues
The largest forams can reach a few centimetres. The most impressive of all, now extinct, were flat, disk-shaped species called nummulites (from the Latin meaning "small coins"), abundant in the limestone, and used to build Egypt's pyramids. Because they produce shells that make good fossils, and have long been abundant and widespread in the oceans, forams are valuable to geologists and paleontologists in telling us about earth's history.
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, 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.
Alvarez learned that the topmost layer of the rocks (around Gubbio) 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.
Geologist Jan Smit, now at the V U University in Amsterdam, discovered a similar pattern in southern Spain's rocks. 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 dinosaurs, and once-abundant marine animals such as ammonites. Alvarez, Smit and their colleagues wondered what could have caused the disappearance of widespread organisms like forams as well as much larger creatures. As it turned out, it was something from space.
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 iridium, a material rare on earth but more abundant in some asteroids.
Scientists proposed that the iridium was fallout from an asteroid that struck earth at the end of the Cretaceous period, 65 million years ago.
The impact crater was identified underneath the Mexican village of Chicxulub on the Yucatan Peninsula.
Warnings
The asteroid was about the size of Mount Everest and travelling at about 50,000 mph 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. Eventually, forams and the oceans rebounded, and new species evolved. Today, forams are warning us of a new threat, for they are not merely witnesses to earth's history, but critical participants in it.
They are part of a "biological pump" that removes carbon dioxide from the atmosphere. 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 in the upper levels of the oceans sequester about one quarter of all carbonate produced in the oceans each year. The increasing levels of carbon dioxide in our planet's atmosphere, now at a greater level than at any time in the past 4,00,000 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 formation of calcium carbonate shells.
The ocean surface is now about one-tenth of a pH unit more acidic than in preindustrial times. A study of forams in the Southern, or Antarctic, Ocean found that their shells are now 30 per cent to 35 per cent thinner than in the preindustrial era. 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.