As countless tributes and obituaries for neurologist Oliver Sacks appear in the wake of his death, I find myself among the many fans saddened by our loss and the void that now exists in his place. As he himself recently wrote:
"When people die, they cannot be replaced... they leave holes that cannot be filled."
His Musicophilia, I think, will always remain one of my most favourite books. Indeed, his writings of human disfunctions were as much about the humans as they were about the disfunctions. And I am sure that is the reason of his extensive and mainstream popularity. In each of his patients he saw a whole person, and he possessed a wonderful—perhaps unique—ability to convey that insight to his readers. When I read his books, I find myself harbouring a rather mystifying wish to be ill myself. He makes neurological problems sound almost appealing.
Also well worth reading are his recent pieces published by the The New York Times. In one—My Periodic Table—he describes his way of dealing with loss. Even as a child he would turn to the nonhuman, to the chemical elements and numbers that he saw as his friends. And as a geologist—a physical scientist—I can relate to this approach. He writes of his collection of the elements and relates them to his age. Sacks died yesterday at the age of 82. His so-called lead (element 82 in the periodic table) birthday was his last. And so my response to his passing is to turn to this chemical, to this physical element that "has no life, but also no death".
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| The late Oliver Sacks. Credit: Adam Scourfield / AP |
It is difficult, however, to pick a single element in this way—which is not unique in its occurrence to Earth—as a Postcard from Planet Earth. And although lead is abundant and common on Earth, its geological importance is not immediately obvious (i.e., it is not normally considered as a major rock-forming element). In fact, my trusty geological dictionary has just one entry for lead:
Lead–lead dating: A radiometric dating method based on the proportion of radiogenic 207Pb and 206Pb, the former of which accumulates six times more rapidly than the latter.
Lead actually features in additional radiometric dating systems (e.g., uranium–lead and thorium–lead), but I want to stick with this lead mother/daughter system in this postcard. The most important use of the Pb–Pb dating system is to determine the age of the Earth. As I have pointed out in a previous postcard, the age of the Earth—or indeed any planet—must be considered as one of its most fundamental properties.
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| A hand sample of galena, the most common lead-bearing mineral on Earth. Credit: Fabre Minerals |
As element 82, lead has 82 protons and 82 electrons, but its number of neutrons varies such that there are four naturally occurring stable isotopes of lead on Earth (i.e., 204Pb, 206Pb, 207Pb, and 208Pb). Indeed, three of these isotopes (206Pb, 207Pb, and 208Pb) are the 'daughter' products that result from the natural radioactive decay of particular uranium and thorium isotopes. As time goes on, the final decay products of these sequences (i.e., the lead isotopes) accumulate at a constant rate and the ratio of this radiogenic lead to non-radiogenic lead (i.e., 204Pb) increases. As such, the age of a geological specimen can be determined if two factors are known: the initial radiogenic lead to non-radiogenic lead ratios, and the present-day ratios. Furthermore, if the sample has remained a closed system, a graph of 207Pb/204Pb versus 206Pb/204Pb should form a straight line.
Clair Cameron Pattern famously applied this Pb–Pb dating technique to various meteorites in 1956. He measured the lead ratios of stony and iron meteorites so that he could determine the age of the planetesimals from which they originated. The dense iron cores of planets are depleted in uranium and thorium (because they tend to stay with silicon rather than iron in rock-forming processes), whereas the more rocky parts of planets (i.e., crusts and mantles) have greater concentrations of these elements. The iron meteorites Patterson dated were pieces of planetesimal cores, and the stony meteorites derived from the outer layers of these bodies.
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| A piece (few centimetres across) of the Canyon Diablo meteorite, samples of which Patterson used to determine the age of the Earth. Credit: Meteorites Australia |
The iron meteorite U/Pb measurements were so low that no radiogenic decay was detected. These isotopic values therefore represent the primeval lead isotope composition of the solar system. In contrast, the stony meteorites had very high 207Pb/204Pb versus 206Pb/204Pb ratios. Put together, all these results define an isochron, the slope of which gives an age of 4.55 billion years for the meteorites. Furthermore, Patterson measured the isotopic composition of pelagic sediments that had been collected from Earth's ocean floor. The lead isotope values of these terrestrial samples plotted on top of the meteorite isochron, which indicates that Earth and the meteorites have the same age.
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| The lead-lead isochron obtained by Patterson (1956) to determine the age of the Earth. |
In addition to being widespread, lead is relatively easy to extract from its ores, highly malleable, and easy to smelt. As such, it has been used by humans through the millenia. Metallic lead beads dating to 6400 BCE have been discovered in Turkey. Lead was used, along with antimony and arsenic, in the early Bronze Age. And the Romans commonly used lead in their plumbing systems and building structures. Amongst its modern uses, lead is often found in battery electrodes. As well being an essential player in the determination of our Earth's age, it is clear that lead has been—and continues to be—a particularly versatile element for human activities.
Sacks wrote shortly before his death that he was sad he would not witness future breakthroughs in the physical and biological sciences. Whatever those breakthroughs turn out to be, however, I am certain that lead will play a role in at least a few of them. I would be happy, therefore, to send a piece of lead into space as a representative of Earth. For it exemplifies the human ability to utlilise and manipulate the natural resources we find around us. And although we, as a race, are poorer for no longer counting Sacks as one of our number, we are richer for the wisdom he left behind.
