Timefulness

by Marcia Bjornerud

  as efficiently consumed when they decayed. Decomposition

  is the exact converse of photosynthesis— the same chemical

  reaction, but run in reverse: the sugars and other hydrogen-

  carbon compounds built by organisms react with free oxygen

  to yield carbon dioxide and water (burning hydrocarbons, a

  favorite human activity, is just a speeded- up version of this).

  So if photosynthesis and decay are perfectly balanced, there

  will be no net accumulation of O2 in the air. This seems rather

  unlikely, however, to have been true for 1.3 billion years, given

  the tendency for at least some organic matter to be buried in

  sediments without decomposing (and eventually become those

  hydrocarbons we love to oxidize).

  Another possibility is that for more than a billion years, any

  oxygen produced through photosynthesis quickly reacted with

  oxygen- hungry volcanic gases, especially hydrogen sulfide from

  Changes in the air 107

  seafloor volcanism. Then, around the end of the Archean, there

  may have been a transition to a more modern tectonic regime,

  with gases from subduction- related arc volcanism, which are

  less reducing, gaining in importance.12 Some geologists, follow-

  ing the inborn urge for uniformitarianism, interpret Archean

  rocks like the Acasta gneisses and the Isua greenstones within

  the framework of present- day plate tectonics. A few uniformi-

  tarian zealots even argue for a modern- looking Earth back as

  far as Hadean time based on tenuous circumstantial evidence

  from the Jack Hills zircons. Others (full disclosure— I’m in this

  group) think we need to suppress Charles Lyell’s voice in our

  head and consider the possibility of a different tectonic mode

  in Archean and Hadean times.

  For one thing, the solid Earth was hotter (Lord Kelvin was

  partly right), and efficient subduction of ocean crust would have

  been unlikely. Also, while Archean rocks bear evidence of some

  sort of jostling and crumpling atop a convecting mantle, they

  don’t have the same structural styles as those deformed at today’s

  well- defined boundaries between rigid plates. Hotter, weaker

  slabs of crust might have piled up on each other and undergone

  partial melting, extracting the constituents to form granitic con-

  tinents, leaving a deep layer of dense residual rock that sank back

  into the mantle in a process unappealingly called drip tectonics.13

  But starting with rocks from the end of Archean time, we can

  recognize the elements of modern crustal architecture: conti-

  nental shelves, subduction zones, volcanic arcs, and full- fledged

  mountain belts that suggest Earth had cooled enough to form a

  brittle outer shell. So a nudge from a new tectonic system may

  have been enough to give oxygen production a slight lead over

  oxygen consumption. It seems entirely reasonable, in fact, that

  the Earth’s tectonic coming of age would coincide with profound

  changes in the chemistry of the surface environment.

  108 Ch a pter 4

  Although the Great Oxidation Event was a first- order disrup-

  tion to the old geochemical establishment, its actual magnitude

  was not as great as its name suggests. Certain metallic trace ele-

  ments in the banded iron formations such as chromium have

  stable isotopes whose behavior is highly sensitive to oxygen

  levels— Precambrian canaries, perhaps, in equally anachronis-

  tic coal mines. The ratios of these isotopes suggest that the Early

  Proterozoic atmosphere probably had only a small fraction— less

  than 0.1%— of the present level of oxygen (now 21% of the at-

  mosphere by volume).14 We Phanerozoic organisms would not

  have found this world hospitable. But the difference, in terms of

  chemical possibilities, between no free oxygen and even a little

  is greater than the difference between a little and a bit more.

  O N E B I L L I O N Y E A R S O F L A S S I T U D E

  After the upheavals of the GOE, Earth’s atmosphere seems

  to have settled into a long period of geochemical stability. Al-

  though the main period of iron formation deposition ended

  around 1.8 billion years ago, oxygen levels seem to have re-

  mained about constant, and far below the current value, for

  another billion years after that.15 Such sustained equilibrium—

  akin to a national economy that experiences no inflation, reces-

  sions, or market turmoil for decades— points to a remarkably

  fine- tuned balance between the oxygen supplied by hardy one-

  celled photosynthesizers and oxygen consumed by covetous

  metals, sulfurous volcanic gases, and decaying organic mat-

  ter. This steady state may have been enforced by a regime of

  austerity— in particular, severe limitation on the availability of

  phosphorus, an essential nutrient for all life.

  While shallow ocean waters had become oxygenated, there

  is evidence that deeper reaches remained in the transitional

  Changes in the air 109

  state of the Early Proterozoic. In these stratified conditions,

  precious phosphorus would have been continuously removed

  from deeper waters, stolen away on the surfaces of iron min-

  erals, like currency smuggled out of a poor country in the lin-

  ings of pilferers’ coats. This in turn created chronic shortages

  of phosphorus in the shallow ocean. Biological productivity

  was thus kept in check, which limited organic carbon burial

  and in turn prevented atmospheric oxygen levels from rising.16

  This lean eon encouraged organisms to pursue low-

  phosphorus lifestyles and new recycling strategies. In other ways,

  however, evolution seemed to be biding its time. The biosphere

  was diverse but still entirely unicellular; planktonic species—

  including some eukaryotic giants called acritarchs, up to 0.8 cm

  (0.3 in.) in diameter— proliferated in the oceans, and stromato-

  lites quietly blanketed coastlines around the world. This peace-

  ful stretch of the Proterozoic Eon has come to be known infor-

  mally among geologists as the “Boring Billion.” But this Homer

  Simpson– inspired designation is unfair, and misleading— akin

  to history books that focus only on war and skip over the much

  longer intervening periods of peace when “nothing happened.”

  First, maintaining such long- term equipoise is something

  that we humans in the Holocene might look to as a template for

  amending our own biogeochemical habits, since our looming

  environmental crises are the result of unchecked consumption

  of scarce resources and an extreme imbalance between the pro-

  duction and removal of an atmospheric gas. The Proterozoic

  Earth somehow “understood” the fundamental principles of

  sustainability; geochemical trading flourished, but all commod-

  ities flowed in closed loops— the waste products of one group

  of microbial manufacturers were the raw materials of another.

  Second, the Boring Billion was the period when the durable

  cores of the modern continents were assembled, as the new

  110 Ch a pter 4

  F I G U R E 1 1 . The Un
ited Plates of America— how North America was assembled plate tectonic system swept together pieces of Archean crust

  and then constructed additions in the form of volcanic arcs. The

  basement rocks beneath my feet here in Wisconsin— and buried

  under younger sediments across the Midwest and Great Plains—

  are almost entirely Proterozoic, formed by mountain- building

  events during the Boring Billion, when vast areas of continental

  crust were annexed to the old Canadian Shield (see figure 11).

  Changes in the air 111

  Boring, perhaps, but it was a productive time of infrastructure

  development— another practice we modern Earth- dwellers

  could profitably adopt.

  Maybe because Proterozoic rocks and their stories are so fa-

  miliar to me— the late great Penokee and Baraboo Ranges of the

  Lake Superior Region, the violent hotspot volcanoes of central

  Wisconsin, the immense Midcontinent Rift that nearly ripped

  North America apart— the Boring Billion doesn’t seem that

  long ago. Thus it saddens me, irrationally, to know that in the

  equivalent amount of time into the future, about 1.5 billion years

  from now, the window of habitability will have closed for the

  Earth. The Sun, which is still getting brighter (at the very mod-

  est rate of about 0.9% per 100 million years), will have grown

  so luminous that the oceans will begin to vaporize, triggering a

  “moist greenhouse runaway.”17 Solar radiation will then break

  down water molecules into hydrogen and oxygen, which will

  be lost to space. In other words, if life first became viable after

  the early bombardment era ended 3.8 billion years ago, we are

  now almost three- quarters of the way through Earth’s habitable

  period. Nevertheless, we should be grateful for the great wealth

  of time that this planet has had as a consequence of belonging to

  a yellow dwarf star with a lifetime of 10 billion years. Stars just

  50% larger than the Sun have a life expectancy of only 3 billion

  years, which on Earth would be equivalent to the time span from

  the formation of the planet to the middle of the Boring Billion.

  At that point, Earth had so much more living to do.

  T H E L O N G E S T W I N T E R

  Things might have continued indefinitely in the monoto-

  nous Proterozoic mode, except that by around 800 million

  years ago, the new tectonic system had shepherded most of

  112 Ch a pter 4

  the continental crust into one large landmass that girdled the

  equator. Geologists call this ancient supercontinent Rodinia,

  from the Russian ródina, “motherland.” Like all continents,

  Rodinia was only a temporary configuration, and it began to

  break apart through rifting by about 750 million years ago, cre-

  ating expansive new coastlines at tropical latitudes. Rivers fed

  by heavy rains would have carried sediment and rock- derived

  elements to the sea, and organisms would have thrived in these

  comparatively nutrient- rich waters. High sedimentation rates

  on the continental shelves allowed organic carbon to be bur-

  ied in significant volumes for the first time, which drew down

  atmospheric CO2 levels and set Earth on a cooling trend.

  Perennial sea ice would have begun to accumulate in the

  polar regions, increasing the albedo, or reflectivity, of the

  Earth’s surface, which in turn led to further cooling— a classic

  example of positive feedback. Even as the ice advanced far-

  ther, carbon dioxide continued to be withdrawn from the at-

  mosphere through both organic carbon burial and the intense

  chemical weathering of rocks on the low- latitude fragments of

  Rodinia (the mechanism by which the Himalaya drew down

  CO2 and cooled the Earth in the Cenozoic). Once ice cover

  reached a critical point, the albedo effect would have led the

  planet into a “snowball” state— a perpetual snow day.

  Exactly what happened during this Snowball Earth time—

  also called the Cryogenian Period, one of the few named di-

  visions of the Proterozoic in common use— generates a lot of

  heat in the geologic literature. There is no disagreement that

  something went haywire for a time with the climate system.

  The rock record makes that clear: on almost every continent,

  rocks of this age are glacial deposits— either unsorted mixes of

  boulders and clay laid down directly by ice on land, or finely lay-

  ered marine sediments punctuated by iceberg- rafted cobbles.

  Changes in the air 113

  With much of the Earth’s water locked up in glacial ice, sea

  level would have been lower by hundreds of feet, exposing large

  areas of the continents to erosion, at least until the deep ice

  age began and surface processes ground to a halt. The Great

  Unconformity in the Grand Canyon, between metamorphosed

  Proterozoic rocks like the Vishnu Schist and the first stratified

  unit, the Cambrian Tapeats Sandstone, is a record of Snowball

  Earth time in absentia. So while there is no question that an

  exceptional cold snap occurred at the end of the Proterozoic,

  specifics like how deep the freeze was, how the biosphere sur-

  vived, and how the Earth emerged from its hypothermic state

  stoke the fires of academic debate.

  S P R I N G O F L I F E

  But, clearly, the Earth did warm up again. Maybe the breath of

  volcanoes, which would have continued to erupt while other

  geologic processes had stopped, gradually coaxed Earth back

  from its cold coma over many thousands of years. Or perhaps

  a sudden, rude belch of long- sequestered biogenic methane

  from the seafloor transformed the icy planet into a hothouse in

  a matter of months or years. The resolution of the rock record

  and the precision of our dating methods are not fine enough

  for us to distinguish between these possibilities.

  In any case, the end of Snowball Earth marks what could be

  called the Great Aeration, the second big step in free oxygen

  levels and the emergence of Earth’s fourth, and current, atmo-

  sphere. Oxygen- sensitive trace elements in sedimentary rocks

  finally started behaving in the modern way, indicating that O2

  levels jumped from a fraction of a percent to something close

  to the present value. But the details of how the long- reigning

  quasi- oxygenated realm of the Proterozoic was overthrown

  114 Ch a pter 4

  are not known. Perhaps it was a large influx into the oceans

  of phosphorus, from rock powder ground up by Snowball

  glaciers, that kick- started marine life.18 Or it might have been

  the energetic mixing of shallow and deep- ocean waters in the

  transition between ice- bound and greenhouse worlds that fi-

  nally broke the geochemical stratification that had prevailed

  for 1.5 billion years.

  Once oxygen levels rose even a bit higher, organisms that

  evolved to use it in their metabolic processes were significantly

  more efficient in extracting energy from the environment and

  were able to grow larger than any had before. Within a million

  years of the end of Snowball Earth, a strange new macroscop
ic

  ecosystem of puffy organisms called the Ediacaran fauna ap-

  pears in the fossil record at sites around the world, including

  southern Australia, the White Sea region of Russia, Leicester-

  shire in England, and Newfoundland in Canada. These bizarre

  down- parka- like organisms were shaped like Frisbees and fern

  fronds, the latter up to 1 m (3 ft) high, with holdfasts to anchor

  them to the seafloor. They had neither guts nor mineralized

  shells, suggesting their world was a peaceable kingdom of os-

  motic nutrition without threat of predation. Some may have

  been precursors to later, more familiar marine lineages such as

  the brachiopods, or lampshells. But others seem to have been

  early evolutionary experiments in building bigger life- forms

  that left no modern descendants.

  The Ediacarans’ moment in the avant- garde was brief, how-

  ever. Within about 40 million years, the seafloor had become

  the venue for the period of frenzied anatomical tinkering called

  the Cambrian explosion, when the first carnivores set off an

  arms race between predator and prey. Like Wile E. Coyote

  and the Road Runner, they’ve been trying to outwit each other

  ever since. Hard protective shells of calcium carbonate became

  Changes in the air 115

  obligatory for bite- sized organisms; specialized swimming gear

  and killing apparatuses de rigueur for the big meat eaters.

  The pace of evolution in the Cambrian explosion continues

  to be a topic of some controversy, pitting paleontologists

  against biologists who use genomic approaches to determine

  when different branches of the tree of life first emerged. The

  fossil record suggests that the interval between about 540

  and 520 million years ago was a time of unprecedented, and

  never to be repeated, biological innovation. But this is at odds

  with various molecular clocks, which are based on the assump-

  tion that protein- coding genes accumulate substitutions at a

  constant rate in evolutionary lineages. Most of the molecular

  analyses suggest that Kingdom Animalia, whose first members

  were probably sponges, was founded in the late Proterozoic,

  750– 800 million years ago and that the Cambrian “Explosion”

  may instead have been a slow- burning fuse.19 This, however,

  puts our infancy in the bleak and frigid time of Snowball Earth,

  which would seem an unlikely nursery. The disagreement re-