Timefulness

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Timefulness Page 5

by Marcia Bjornerud

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  An atlas of time 35

  the parent, which is essentially the probability that any given

  atom will decay in a certain amount of time— analogous to one’s

  chances of winning the lottery in any year. The units of the

  decay constant are thus 1/time. Rutherford had estimated the

  decay constant for uranium from the number of radioactive

  emissions detected from a mass of uranium in a given time in-

  terval. The decay constant is inversely proportional to the more

  familiar idea of a half- life— the time it takes for half the parent

  material to decay to the daughter form. In other words, a small

  decay constant (low probability of a lottery win) means a long

  half- life (long wait to get rich), while a large decay constant

  means a short half- life (easy money!).

  By 1911, in spite of the still- rudimentary understanding of

  the phenomenon of radioactivity and rather primitive labora-

  tory facilities, Arthur Holmes had determined the absolute ages

  of a half- dozen igneous rocks whose relative ages on the fossil-

  based geologic timescale were bracketed by their relationships

  with sedimentary rocks. Three samples were from the fossil-

  iferous Paleozoic and three from the murky, undifferentiated

  Precambrian. Even though some of the lead Holmes measured

  was not from the decay of the parent uranium but from another

  radioactive element, thorium, his dates are amazingly close to

  modern values (within tens of millions of years).

  The very first rock analyzed, a granite from Norway thought

  to have formed in the Devonian Period (based on its cross-

  cutting relationships with fossil- rich sedimentary strata),

  yielded an approximate age of 370 million years— 18 times

  longer than Kelvin’s estimates of the age of the Earth. And a

  Precambrian metamorphic gneiss from Ceylon (Sri Lanka) was

  found to be 1.64 billion years old— two full orders of magnitude

  greater. Darwin’s intuition was vindicated. Holmes would go on

  to become one of the preeminent geologists of the twentieth

  36 Ch a pter 2

  century. Kelvin’s long- reigning proclamations became imme-

  diately irrelevant, because radioactivity not only provided a

  means of directly dating rocks but was also a source of internal

  heat that Kelvin had not incorporated into his calculations of

  the rate of planetary cooling. (Years later, Holmes would chal-

  lenge another of Kelvin’s fundamental assumptions, arguing

  that Earth cools mainly by convective, rather than conduc-

  tive, heat loss). Most important, the geologic timescale could

  now be calibrated. Even the deepest reaches of geologic time

  could be fathomed; the Precambrian would no longer be an

  uncharted primordial wilderness.

  T H E G E N E R A L S E D I M E N T

  In reality, it would take many more decades for the new science

  of geochronology (Earth time) to mature. The use of radio-

  active isotopes as high- precision geologic clocks required ad-

  vances in nuclear physics, cosmochemistry (which concerns

  the stellar origins of the elements), petrology (study of igneous

  and metamorphic rocks), and mineralogy, as well as the devel-

  opment of new analytical instruments, particularly mass spec-

  trometers capable of distinguishing among isotopes of a single

  element. There was also the nontrivial problem that geologic

  timescale so laboriously built by the Victorians, with fossils as

  timekeepers, was entirely based on sedimentary rocks. Any

  isotopic dates derived from these would reflect not the age of

  the sedimentary deposit but the time of crystallization of the

  igneous or metamorphic precursors from which its grains were

  derived. Assigning absolute ages to the fossil- based timescale

  has thus required finding serendipitous outcrops where sedi-

  mentary rocks of well- constrained biostratigraphic age happen

  to be interlayered with, or cut by, igneous rocks in such a way

  An atlas of time 37

  F I G U R E 4 . Cross- cutting relationships between igneous and sedimentary rocks allow calibration of the fossil- based timescale.

  that allows an isotopic age to be tied directly with the fossil

  record (figure 4). Volcanic ash layers are ideal for this purpose, since they represent fresh igneous crystals that fell from the air

  in a geologic instant and were interleaved with the sedimentary

  and paleontologic archive of their day.

  Ash layers within sedimentary strata reveal a subtle but fun-

  damental idea about the way in which the rock record is writ-

  ten. Looking at layered rocks like the extraordinary sequence

  in the Grand Canyon, one tends to imagine that each stratum

  accumulated in the manner of a snowfall, blanketing a given

  38 Ch a pter 2

  area all at once, in a well- defined period of time. But this is not

  necessarily the right way to think about rock layers. Consider

  the beautiful white, almost pure- quartz St. Peter Sandstone

  of Ordovician age, exposed along river valleys in Minnesota,

  Iowa, Wisconsin, and northern Illinois. The St. Peter forms the

  picturesque hol
low at Minnehaha Falls in Minneapolis and was

  for decades the source of silica for window glass made at the

  Ford plant in Saint Paul. During Prohibition, natural pockets

  in the St. Peter along the Mississippi River were enlarged into

  a network of caverns that housed speakeasies and secret ware-

  houses beneath the Twin Cities.

  The St. Peter Sandstone is crumbly, hardly even a proper

  rock, and when it falls apart into uniform, rounded grains in

  one’s hand, it is easy to see that this is an ancient beach sand.

  But the St. Peter is found at the surface in four states, and is

  known from drilling to continue beneath Michigan, Indiana,

  and Ohio. No beach would cover such a vast area at any partic-

  ular moment. Instead, the St. Peter records the gradual migra-

  tion of beaches across the land surface as ancient shallow seas

  waxed and waned over millions of years. One day in the Ordo-

  vician, clouds of ash from a supervolcano eruption in the infant

  Appalachian Mountains hundreds of miles away fell out of the

  air over the midcontinent seas, leaving a thin, greenish clay

  layer across the region that is like a clearly dated diary entry.

  In some places, the ash occurs near the top of the St. Peter, but

  elsewhere, the sandstone lies far below this level, having been

  buried long before by other sediments at the time the volcano

  erupted. Thus, although the unmistakable St Peter sandstone

  is a continuous layer for hundreds of miles, it is not the same

  age everywhere. The more general idea is that except for layers

  that mark sudden regional or global events, like a great eruption

  or a meteorite impact, laterally extensive sedimentary units

  An atlas of time 39

  are not strictly isochronous— markers of the same moment

  in time. Instead, they record the slow march of depositional

  settings across the Earth’s surface over time, as sea levels and

  environmental conditions changed. In geologic parlance, they

  are diachronous— that is, they transect time.

  T H E T I M E B U R E A U C R AT S

  These days the geologic timescale is not merely a chart or even

  a multivolume treatise but a gigantic digital database that is

  administered by the formidable International Commission

  on Stratigraphy (ICS), the oldest and most important body

  within the International Union of Geological Sciences. The ICS

  maintains strict rules about how geologic units are named and

  defined, and it catalogues outcrops, rock formations, fossils,

  isotopic dates, geochemical data, and analytical protocols, in

  the never- ending task of mapping geologic time at higher and

  higher resolution.

  Since the 1970s, the ICS has sought to identify specific

  sites around the world to serve as the international standards

  for the boundaries of each division of the geologic timescale.

  Such an outcrop is formally called a Global Boundary Strati-

  graphic Section and Point, or GSSP, but known colloquially

  among geologists as a “golden spike.” These sites must have

  well- exposed rocks with biostratigraphically diagnostic fossils

  that straddle the boundary between the two time intervals,

  and they must be in places that can be protected from de-

  velopment or destruction. The location of the exact stratum

  representing the boundary at a given GSSP is often described

  in charmingly idiosyncratic detail. For example, the golden

  spike outcrop for the Cenomanian division of the Upper Cre-

  taceous lies high in the French Alps and begins “36 meters

  40 Ch a pter 2

  below the top of the Marnes Bleues Formation on the south

  side of Mont Risou.”7

  The primary divisions of the geologic timescale— the eons,

  eras and periods— were largely defined by the work of Brit-

  ish geologists in the nineteenth century, and the names of the

  Paleo zoic Periods more strongly reflect that geographic influ-

  ence: Cambrian, from the Latin name for Wales; Devonian for

  the county of cream teas; Carboniferous for the coal measures

  of northern England. But the finer subdivisions— the epochs

  and ages— reveal the subsequent, wholly international nature

  of the time- mapping project: the Jiangshanian and Guzhangian

  of the Cambrian; the Eifelian and Pragian of the Devonian; the

  Moscovian and Bashkirian in the Carboniferous. The ICS is like

  a temporal counterpart to the United Nations— a parliament of

  the past, whose jurisdiction is geologic time.

  And the ICS, somewhat fussily, insists on maintaining the

  subtle but important distinction between time and the rock re-

  cord of time. Geologic time is divided into eons, eras, periods,

  epochs and ages, and the corresponding rocks into eonothems,

  erathems, systems, series, and stages. Similarly, one should say

  “Early” or “Late” Ordovician (for example) when referring to

  time, but “Lower” and “Upper” when speaking of rocks. Time

  ( chronos) could happen without rocks (representing kairos),

  but not the other way around. However, time vanishes, while

  rocks persist.

  P L U M B I N G T H E D E P T H S O F T I M E

  Arthur Holmes’s early efforts to obtain absolute ages from

  rocks, carried out before the structure of the atom and the ex-

  istence of isotopes were even known, are analogous to Darwin’s

  insights about heredity, which predated the discovery of genes

  An atlas of time 41

  and DNA. In both cases, it would take years before the rest of

  science developed the capacity to explore fully the implications

  of their visionary ideas. It was not until the 1930s that the com-

  plexity of lead isotope geochemistry was fully understood, one

  might say “plumbed.” In 1929, Ernest Rutherford showed that

  there were two different parent isotopes of uranium, 238U and

  235U, which produced two different isotopes of lead (206Pb and

  207Pb, respectively) at the end of long radioactive decay series

  with very different overall half- lives (4.47 billion and 710 mil-

  lion years, respectively). Soon after, Alfred Nier, a physicist at

  the University of Minnesota, identified another lead isotope,

  204Pb, which was nonradiogenic— that is, lead that started as

  lead, and was not the product of radioactive decay. Nier had

  developed the essential instrument in isotope analysis, the mass

  spectrometer, which allows isotopes of a single element to be

  sifted out according to their atomic weight. And with the dis-

  covery of 204Pb, Nier recognized the potential application of

  these three lead species for dating rocks, and even the Earth.

  Over geologic time, he realized, the abundances of 206Pb

  and 207Pb would have grown in a mathematically predictable

  way while the absolute amount of 204Pb remained constant. In

  particular, the comparatively short half- life of 235U would have

  caused Earth’s inventory of 207Pb to increase rapidly early in the

  planet’s history, but then flatten off, like the cumulative earn-

  ings from a savings account with a high interest rate but from

  wh
ich rapid withdrawals are made. Meanwhile, the global stock

  of 206Pb would have continued to accumulate from the slower

  decay of 238U— like the money earned at a lower interest rate in

  an account that is drawn down more slowly. (The unchanging

  amount of 204Pb would be like money hidden under a mattress).

  In 1940, Nier and his students were about to put these ideas

  to the test using geologic samples. The work was interrupted

  42 Ch a pter 2

  when Nier— the son of German immigrants— was asked by En-

  rico Fermi to work on the Manhattan Project, which required

  the separation of the fissionable isotope 235U from nonfission-

  able 238U.8 Nier’s spectrometer was the only instrument that

  could distinguish the two isotopes, and his lab was required

  to focus on the uncertain future rather than questions of the

  geologic past.

  Immediately after the war, however, Nier set about mea-

  suring the Pb isotope ratios in deposits of galena (lead sulfide,

  PbS), the primary ore of lead, of different ages from around

  the world. Galena obviously has plenty of lead in it, but the

  lead doesn’t take in uranium when it crystallizes. This means

  that lead isotope ratios in galena do not change over time and

  should instead reflect the particular mix of lead species that

  were available in the environment at the time the mineral

  formed. As Nier had predicted, the older samples had lower

  ratios of 207Pb/204Pb and 206Pb/204Pb (lead from “interest” vs.

  “mattress” lead). If the Earth had started with no 207Pb or 206Pb,

  these ratios would be enough to determine the age the planet.

  But Nier knew that at the time of its formation, Earth had al-

  most certainly inherited some “interest” lead from what had

  accumulated in the “bank accounts” of ancestor solar systems.

  Thus, determining the age of the Earth required knowing the

  primordial ratios of the various lead isotopes.

  Nier also recognized a subtler problem: even a very ancient

  galena sample would not represent the primordial lead ratios

  for the Earth as a whole. Earth is not one uniform geochemical

  reservoir, like a planetary milkshake. Instead, it has unmixed

  itself over time. In its earliest days, the planet differentiated

  into a metallic core of iron and nickel and a rocky mantle that

  got most of everything else, including virtually all Earth’s ura-

  nium. Ever since then, repeated partial melting of the mantle

  An atlas of time 43

  has generated the crust, which is much richer in uranium than

  either the bulk Earth or the mantle, in the way that butterfat

  is concentrated in the cream at the top of a bottle of raw milk.

 

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