Timefulness

by Marcia Bjornerud

  Nier’s view was that although his lead isotope data broadly fol-

  lowed the expected pattern, some of the samples had probably

  had assimilated extra radiogenic lead (206Pb and 207Pb) derived

  from the decay of the “excess” uranium in crustal rocks and

  thus didn’t exactly track the evolution of lead isotopes for the

  whole planet.

  By the late 1940s, Arthur Holmes was a professor of geology

  at Edinburgh University, and had largely moved on to other

  major questions (such as the driving forces behind mountain

  building), but he had been following Nier’s work and saw that it

  might allow the age of the Earth to be determined at last. He was

  especially intrigued by one specific sample Nier had analyzed,

  galena from a very ancient rock sequence in Greenland that had

  both low uranium concentrations and low lead isotope ratios.

  Holmes, always a big- picture, back- of- the- envelope thinker, was

  willing to make the assumption that meticulous Nier was not—

  that the Greenland galena provided something close to primor-

  dial whole- Earth lead isotope ratios. Conceptually, calculating

  the age of the Earth was simple: one just had to determine how

  much time it would take for the ratios to evolve from that pri-

  mordial starting point to the values in younger galena deposits.

  In practice, however, the math was so difficult that Holmes had

  to purchase a mechanical computing machine to carry it out.

  After months of tedious calculations, Holmes published his

  minimum estimate for the age of the Earth: 3.35 billion years.9

  Geologists could finally relax into a luxurious abundance of time.

  But now there was a new conflict between the timescales

  envisioned by geologists and physicists. According to the ex-

  pansionary (Big Bang) theory of the Universe, which gained

  44 Ch a pter 2

  credence in the 1920s with Edwin Hubble’s observation of

  galactic redshifts, the age of the Universe can be determined

  in a remarkably simple manner— almost trivial, in fact, com-

  pared with Holmes’s lead isotope calculations for the age of

  the Earth. One just plots the velocity (distance/time) at which

  stars and galaxies are receding from Earth versus their distance

  from us. The slope of this line is called the Hubble constant, and

  the inverse of the slope, which has units of time, is the age of

  the Universe. In 1946, when Holmes declared Earth’s age to

  be more than 3 billion years, the Universe was allegedly only

  1.8 billion years old.10

  G E O C H E M I S T S TA K E T H E L E A D ( O U T )

  The embarrassing discrepancy between geologic and astro-

  nomical time remained unresolved for almost a decade, but

  as better estimates of stellar distances were made, and galaxies

  farther from Earth could be detected, the accepted value for the

  Hubble constant fell, and the age of the Universe grew. Then,

  in 1948, a young Iowa- born graduate student at the University

  of Chicago, Clair Patterson, struck upon a novel approach to

  the age of the Earth question. It was becoming clear that there

  were probably no surviving rocks that represent the original

  crust of the planet. Arthur Holmes had used the lead isotope

  ratios from the ancient Greenland galena as the closest avail-

  able approximation to primordial values, but Patterson realized

  there was an even better source of information: extraterrestrial

  rocks— meteorites.

  Meteorites represent preplanetary matter and fragments

  of ill- fated planets that formed at the same time as Earth and

  the rest of the Solar System. Unlike Earth rocks, which are in

  a constant state of modification and reincarnation through

  An atlas of time 45

  weathering, erosion, metamorphism, and melting, most

  meteorites have undergone no alteration in the vacuum of

  space since the formation of the Sun and planets. Any thin rind

  acquired from their passage through the atmosphere or time

  spent on Earth’s surface can be pared off, revealing pristine

  material from the earliest days of the Solar System.

  Patterson’s approach was to use two different types of me-

  teorites, with different compositions, to represent the original

  and modern values of lead isotopes in the Solar System, and

  then to repeat Holmes’s strenuous calculations. Iron meteor-

  ites, which contain lead but no uranium, would provide the

  true primordial values. And stony meteorites, which contain

  both lead and uranium, would provide the modern bulk- Earth

  (well- mixed milkshake) value more reliably than any Earth

  rock could (see figure 5).

  Once again, the idea seemed simple, but in practice re-

  quired Herculean effort. Patterson found that he could not

  obtain consistent enough lead isotope results from duplicate

  samples to make meaningful age determinations. After sys-

  tematically ruling out any flaws in his analytical methods, he

  realized what the problem was: there was so much ambient

  lead in the lab— on work surfaces, equipment, clothing, skin—

  that it was contaminating the meteorite samples before they

  could be analyzed. Over a period of nearly eight years, during

  which he moved to Caltech and then back to Illinois— this

  time to Argonne National Laboratory— Patterson developed

  the first “clean lab” (now an essential fixture in countless sci-

  entific and medical facilities) with a sophisticated air puri-

  fication and ventilation system. In 1956, he finally obtained

  the number that remains the accepted age of the Earth: 4.55

  billion ± 70 million years.11 ( Requiesce in pace, Darwin). After

  attaining, at age 31, the long- sought holy grail that had eluded

  46 Ch a pter 2

  F I G U R E 5 . The logic behind using meteorites to date the Earth

  geologists and physicists since the time of Hutton, Patterson

  left academe. He spent the rest of his life crusading to ban lead,

  already known then to be a neurotoxin, from paint, toys, tin

  cans, and gasoline. Reckoning the age of the Earth would seem

  to be a Nobel Prize– worthy accomplishment, but geologists

  aren’t even in the running. Patterson did receive the presti-

  gious Tyler Prize for Environmental Achievement just before

  his death, in 1995. Yet it seems understated recognition for

  An atlas of time 47

  a small- town Iowa boy who had stood up to giants: Kelvin,

  Hubble, and Big Oil.

  G E O C H R O N O L O G Y C O M E S O F A G E

  Following the pioneering work of Nier, Holmes, Patterson, and

  others, the field of geochronology— the science of determin-

  ing the age of geologic materials— expanded to include many

  other systems besides the uranium- lead decay series. Among

  the 92 naturally occurring elements, there are thousands of dif-

  ferent isotopes, and most of these are radioactive (only 254 are

  stable). But not all radioactive isotopes are useful as geologic

  time keepers. First, the half- life needs to be appropriate to the

  lengths of time being measured. Many isotopes have ha
lf- lives

  of days or seconds, and using them to measure geologic time

  would be like using a 12- inch ruler to measure the Alaska High-

  way. Also, because of the exponential nature of radioactivity,

  with half of the parent decaying away every half- life, there is

  almost no parent left after about 10 half- lives, no matter how

  much there was at the start (just as there is a limit to the number

  of times one can fold a piece of paper in half, no matter how big

  it is). Second, the parent isotope must be present in any rock

  or mineral being dated in high enough concentrations that it

  can be measured, and also yield measureable amounts of the

  daughter. The definition of “measureable” has changed over

  time, however; improvements in instrumentation now make

  it possible to detect elements that are in parts per billion and

  even parts per trillion concentrations in minerals.

  Third, the daughter element should, ideally, not be incorpo-

  rated into the mineral at the time of crystallization— the starting

  time for the isotopic clock— so that any daughter present in

  the sample is known to come from radioactive decay of the

  48 Ch a pter 2

  parent after the crystal became a closed system. This is a bit like

  the logic behind requiring students to use much- loathed “blue

  books” for exams; it ensures that they wrote all their answers

  for the test after they entered the classroom and the door was

  shut. (There are, however, mathematical techniques that can,

  in fact, correct for initial amounts of the daughter, just as an

  astute instructor might detect cheating on an exam.)

  Finally, the daughter isotope should not be too prone to es-

  caping from the mineral crystal, even though it is usually an

  ill- at- ease stranger in that setting. A parent atom, with its par-

  ticular diameter and electric charge, will generally have had a

  comfortable place in the lattice of atoms in a mineral, bonding

  harmoniously with neighbors. But after the parent undergoes

  radioactive metamorphosis to a daughter isotope, it no longer

  fits in the crystal “chrysalis.” It is a completely different ele-

  ment, with a different size and chemical proclivities. Given its

  discomfort in its parent’s home, the daughter may try to leave

  the crystal, a possibility that becomes more likely if the rock

  is reheated sometime later in its history, and the framework of

  the crystal becomes more open to diffusion. Because the ratio

  of the daughter to parent isotope is the basis for determining

  the age of the sample (table 1), any loss of the daughter isotope

  will cause isotopic ages to be too young.

  Because of these rather restrictive criteria, there only about

  a half- dozen parent- daughter isotope systems that can be used

  for dating rocks (table 2). These parent isotopes are a lasting

  legacy from the time of Earth’s formation, inherited from pre-

  cursor stars and solar systems, and some have absurdly long

  half- lives. The half- life of rubidium- 87 (87Rb), for example, at

  49 billion years, is not only greater than the age of the Earth,

  but of the Universe (which is now thought, thanks to revised

  Hubble constant estimates, to be 14 billion years). This isn’t an

  An atlas of time 49

  TA B L E 2 . 2 . Parent-daughter isotope pairs most

  commonly used for geologic dating

  Parent

  Daughter Half-life

  Parent

  Daughter

  Half-life

  isotope

  isotope

  (millions of years)

  isotope

  isotope

  (millions of years)

  238U

  206Pb

  4470

  40K

  40Ar

  1280

  235U

  207Pb

  710

  147Sm

  143Nd

  106,000

  232Th

  208Pb

  14,000

  176Lu

  176Hf

  36,000

  87Rb

  87Sr

  48,800

  187Re

  187Os

  42,300

  Source: Values from Faure, G., and Mensing, T., 2012. Isotopes: Principles and Applications.

  New York: Wiley.

  inconsistency— it simply means that only a tenth of a 87Rb half-

  life has elapsed since Earth formed, and that just a small frac-

  tion of the primordial 87Rb has so far decayed to strontium- 87

  (87Sr). But because rubidium is a common trace element in

  many minerals, both 87Rb and 87Sr occur in high enough con-

  centrations to be measureable and geologically useful.

  Some rocks, like granite, have two or more minerals that

  can each be dated using a different parent- daughter isotope

  system, and it is common to find that these minerals yield dif-

  ferent ages. This is another geologic observation that has been

  seized upon by Young- Earthers in an attempt to “debunk” the

  geologic timescale, but it would in fact be surprising if all the

  minerals in an igneous rock such as granite, formed when a

  mass of magma cools slowly deep underground, did report the

  same isotopic ages. The reason is that the closure temperature

  for each mineral— the point at which the crystal “doors” be-

  come shut to diffusion— is different for each parent element in

  each mineral species. Knowing these specific closure tempera-

  tures allows the cooling history of subsurface magma bodies—

  called plutons, for Pluto, Roman god of the Underworld— to be

  50 Ch a pter 2

  reconstructed in great detail. For example, combined U- Pb,

  Rb- Sr, and K- Ar dating of minerals from the Tuolumne granites

  in Yosemite National Park reveals that they remained above

  660°F for more than 3 million years.12 The granites that now

  form the sublime peaks of the High Sierra represent magma

  chambers that fed mighty Jurassic volcanoes, long since eroded

  away. Understanding how long a magmatic plumbing system

  may remain active is relevant to predictions about the risk

  of eruptions in places like Yellowstone, where mud pots and

  geysers hint at unrest in the Underworld.

  R A D I O C A R B O N D AY S

  The best- known isotope used for dating, carbon- 14 (14C), is in

  many ways an oddball and differs from other parent isotopes

  in several important ways. With an extremely short half- life

  of just 5730 years, it can’t be used to date anything older than

  about 60,000 years (so its use in geology is limited), and it

  doesn’t represent a primordial species— after 4.5 billion years,

  it would no longer exist. Instead, it is a cosmogenic isotope that

  is continuously regenerated in Earth’s uppermost atmosphere

  by cosmic rays— high- energy radiation from space. Cosmic rays

  are thought to come mainly from distant supernova events, in

  which old stars explode in a spectacular final extravaganza (pro-

  ducing new elements and isotopes that may be incorporated

  into future planetary systems). Because of concern over long-

  term exposure to cosmic rays, pilots and flig
ht attendants are

  typically limited to a certain number of long- haul high- altitude

  flights each year.

  Carbon- 14 is produced when a nitrogen- 14 (14N) atom high

  in the atmosphere is struck by a cosmic ray with enough energy

  to knock a proton out of the nitrogen nucleus. Some of this

  An atlas of time 51

  14C makes its way to the surface of the Earth and is taken in

  by photo synthesizers (algae, plants) and the organisms that

  consume them (fish, fungi, sheep, people). As long as a plant

  or animal lives, photosynthesizes, breathes, and/or eats, its

  blend of carbon isotopes (stable 12C and 13C, as well as radio-

  active 14C) will reflect the relative abundances of carbon in the

  environment. But when the organism dies, its carbon inventory

  becomes fixed, and the radioactive 14C gradually ticks away

  while the stable carbon isotopes remain. In contrast with other

  isotopic dating methods, in which the daughter/parent ratio is

  used to determine the age of a sample, 14C ages are based on

  the activity of the carbon present— the number of decay events

  per unit time per gram of carbon. The reason is that 14C decays

  back to 14N, a gas that will tend not to be retained in the sample.

  Carbon- 14 dating is an important tool in archeological and

  historical research and can be used to date a wide variety of ma-

  terials containing biogenic carbon: wood, bone, ivory, seeds,

  shells, linen, cotton, paper, peat, and the like. Even ocean water

  can be dated, because it has a small amount of carbon dioxide

  dissolved in it. Some deep- ocean water from the North Pacific

  yields 14C ages of 1500 years13— meaning those waters have not

  interacted with the atmosphere since before the birth of the

  prophet Muhammad.

  But the uncertainties in 14C ages are proportionally rather

  large compared with geologic age determinations because the

  rate of production of 14C in the upper atmosphere has varied

  over time owing, among other things, to fluctuations in Earth’s

  magnetic field, which partly shields the planet from cosmic

  ray bombardment. Carbon- 14 dates can be corrected using

  tree rings, those low- tech but reliable timekeepers, because

  only the outer part of a tree is actively exchanging carbon with

  the environment in a given year, and so each ring will have a

  52 Ch a pter 2

  different 14C age. By correlating the oldest rings in living trees