Timefulness
Page 12
of perpetual, but directionless, cycling, the history of the at-
mosphere is a Bildungsroman about a planet reinventing itself
as it matured. Like the air in a building— smoky, moldy, well
ventilated, or heavy with cooking smells— Earth’s atmosphere
reveals much about the habits of its residents. For at least 2.5
billion years, the biosphere has altered the atmosphere at the
planetary scale, and conversely, every mass extinction and
major disruption in the biosphere has coincided with dramatic
changes in the composition of the atmosphere. While the evo-
lution of the air is linked with that of the solid Earth through
volcanism, rock weathering, and deposition of sediments, the
atmosphere is generally much nimbler than the tectonic sys-
tem, capable of quicksilver transformation. A deep dive into
the history of Earth’s invisible envelope may give us a new ap-
preciation for each breath we take.
F I R S T B R E AT H S A N D S E C O N D W I N D
Earth’s very first atmosphere was probably rocky— that is, heavy
with pulverized and vaporized rock from a constant barrage of
high- velocity extraterrestrial objects. Apart from the celebrated
Jack Hills zircons (chapter 2) there is no known Earthly re-
cord of the planet’s first 500 million years. The only extensive
source of information for this interval— the Hadean Eon, the
“hidden” or “hellish” time— are samples gathered by astronauts
98 Ch a pter 4
and cosmonauts from the Moon. Its familiar, scarred face, with
rocks as old as 4.45 billion years blanketed by shattered rock
fragments (the lunar regolith), attests to a violent regime of re-
lentless impacts as debris left over from the formation of the
solar system pummeled the young inner planets.
This debris likely included not only rocky and metallic me-
teorites but also icy comets carrying water from orbits beyond
Neptune to the infant Earth, which would have had only limited
native supplies of its own, given its proximity to the Sun. In any
case, the Jack Hills zircons suggest that within 100 million years
of its formation, some water already existed on Earth’s surface
or at least in the shallow crust— the earliest hints of what would
become its signature attribute. Yet we know from the Moon’s
surface that heavy bombardment continued until at least 3.8 bil-
lion years ago, when the great, dark maria basins of Galileo—
themselves giant craters— formed. In Hadean time, the Moon
was even closer to the Earth that it is today, and there is every
reason to think that Earth must have been similarly pelted for
its first 700 million years. It is probable, in fact, that several early
atmospheres and oceans were lost in massive impacts.1
Earth’s earliest systematic diary entries overlap with the last
pages of the Moon’s, picking up again, after a 400-million year
gap, about 4 billion years ago. Whorled metamorphic rocks
exposed near the Great Slave Lake in northern Canada—
the Acasta gneisses—are officially the oldest rocks on Earth
(not merely mineral grains), and they mark the beginning of
the Earth- based geologic time scale: the start of the Archean
Eon. However, while the august Acasta rocks (and somewhat
younger gneisses elsewhere in Canada, as well as Greenland
and southern Minnesota) speak vividly of high- temperature
upheavals deep inside the crust of the early Earth, they have
no memories to share about conditions at the planet’s surface.
Changes in the air 99
The first rocks to provide glimpses of the light of day are
the Isua supracrustals in southwestern Greenland, formed 3.8–
3.7 billion years ago, at about the time the harsh fusillade of
space debris was finally waning. The Isua sequence includes
a variety of sedimentary rocks, which are records of erosion
and deposition by surface water, as well as greenstones—
metamorphosed but still recognizable “pillow” basalts, whose
bulbous shapes are the signature of submarine eruptions. There
were oceans on this ancient Earth, and the nearness of the
Moon would have made tides significantly higher. Tides would
also have been more frequent, because the day was significantly
shorter, probably less than 18 hours (making a year of about
470 days).2 Over time, friction between the ocean- atmosphere
system and the solid Earth has acted like a soft brake that has
gradually slowed the planet’s rotation.
The Isua rocks provide indirect clues to Earth’s second atmo-
sphere. Their testimony to abundant water at Earth’s surface 3.8
billion years ago would seem to be at odds with models of stel-
lar evolution, which predict that our Sun, a yellow dwarf star,
would have been about 30% less luminous than it is today. With
so much less incoming solar energy, any water on Earth should
have been frozen. This is the faint young Sun paradox first rec-
ognized by astrophysicist Carl Sagan in 1972.3 Although there
have been many creative proposals about how to reconcile
this apparent contradiction between astrophysical theory and
the rock record (with its echoes of earlier standoffs between
physics and geology), the prevailing view is that an atmosphere
dominated by greenhouse gases could have compensated for
the dimmer Sun and made the early Earth’s climate clement
enough to keep ancient rivers rolling down to an open sea.
Based on the atmospheres of neighboring Venus and Mars— the
lingering breath of volcanoes— carbon dioxide (CO2) and water
100 Ch a pter 4
vapor are likely to have been the primary heat- trapping gases,
although methane, ethane, nitrogen, ammonia, and other com-
pounds may also have acted as additional blankets that kept the
Archean world warm. Whatever its exact greenhouse recipe,
this second atmosphere would persist for more than a billion
years, and would incubate the first Earthlings.
S I G N S O F H A B I TAT I O N
Their clearly aqueous origins make the Isua rocks an irresist-
ible hunting ground for the spoor of early life. In 1996, a group
of geologists from the United States, the United Kingdom, and
Australia announced they had detected indirect geochemical
evidence for life in graphite (carbon in mineral form) found
in an iron- rich stratum at two outcrops of the Isua sequence.4
In particular, they detected an unusual enrichment of the
lighter- weight stable (i.e., nonradioactive) carbon isotope 12C
relative to the slightly heavier 13C. Carbon- fixing organisms,
including photosynthesizing microbes and modern plants,
are picky about their carbon. It takes slightly less energy to
assimilate the lighter isotope, and so they will preferentially
select it from the available pool of carbon atoms in their en-
vironments. Biogenic carbon thus has a lower 13C/12C ratio
(by a few parts per thousand) than carbon that has not been
processed by life- forms.
Like previous claims to the oldest evidence for
life on Earth,
however, this one was attacked on many fronts. Geologists from
other research groups suggested, variously, that the rocks had
been too metamorphosed to preserve the original carbon iso-
tope signature;5 that at one site, the host rock, which appeared
to be a sedimentary formation, was in fact an igneous intru-
sion;6 and that the samples had been contaminated by recent
Changes in the air 101
organic matter.7 The number and vehemence of these critiques
reflect the stakes involved: this is our Origin story.
As a result of these uncertainties, the prize for oldest docu-
mented evidence of life was provisionally returned to a rather
similar, but 250 million- year- younger, sequence of greenstones
and sedimentary rocks on the other side of the world. The
Dresser Formation in the Warrawoona Group of north western
Australia, after all, could boast direct, visible evidence for life:
stromatolites (see figure 10).8 These finely layered, lumpy rocks (the name means “mattress” or “quilt stone,” a reference to
their hummocky surfaces) are fossilized microbial mats that
likely represent not just one species but a vertical ecosystem
of prokaryotes living in symbiotic relationships in the prime-
val ocean. Sedimentary structures diagnostic of wave agitation
indicate that stromatolites grew in shallow, sunlit waters and
suggest that the organisms in at least their upper layers were
photosynthesizers. Given their already sophisticated commu-
nal lifestyles, these stromatolite colonies cannot represent the
very first life- forms; like the Jack Hills zircons, and Hutton’s
unconformity, they point backward to still- earlier unknown
precursors. But for a time, Australia claimed not only the old-
est surviving vestiges of crust but also the first traces of the
biosphere.
Then in 2016, following two decades of discord about
whether the Isua rocks contain the chemical ghosts of ancient
organisms, a new group of geologists, including two authors of
the original carbon isotope paper, published a new study docu-
menting what appear to be plausible stromatolites in an outcrop
of carbonate (limestone- like) rock at Isua, recently exposed
as a result of the melting of an ice field.9 Inevitably, much of
the media coverage of this finding emphasized the implications
for life on Mars, rather than the more salient point that life
F I G U R E 1 0 . Stromatolites, fossilized (lower image), and alive and well at Shark Bay, Australia
Changes in the air 103
on Earth seems to have appeared, and diversified, even while
the planet was still being battered by extraterrestrial flotsam.10
From this point on, the evolution of the air would be entangled
with the saga of life on Earth.
I R O N A G E
Steel tycoon (and later, philanthropist) Andrew Carnegie—
richer in his day than Bill Gates, Sam Walton, and Warren
Buffett combined— amassed his fortune through the labor of
thousands who toiled in his mills, but he actually owed every-
thing to the work of ancient microbes. Carnegie’s steel, and
indeed almost all the steel ever produced in the world, was
made with iron from a type of rock that is, in a sense, extinct.
Most rock types— for example, the basalts that midocean ridge
volcanoes exude, or sandstones composed of the granular re-
mains of other rocks— are more or less timeless in the sense
that they form on Earth today in the same way they have for
billions of years. But the unimaginatively named sedimen-
tary rocks called “iron formations” accumulated during only
a specific interval in Earth’s history and record a one- time
revolution in the planet’s surface chemistry in the Early Pro-
terozoic Era, between about 2.5 billion and 1.8 billion years
ago. In particular, these densest of rocks testify to changes in
the air— the transition from a surface environment with no
free oxygen (O2) to a brave new world created by the rise of
oxygen- emitting photosynthetic microorganisms like blue-
green algae, or cyano bacteria (whose modern descendants
are often called, with less than due respect, “pond scum”).
This was Earth’s third atmosphere.
The iron formations, found most notably in Australia, Bra-
zil, Finland, and the Lake Superior region, are beautiful rocks
104 Ch a pter 4
with a striking color palette; fine laminae of silver hematite and
black magnetite alternate with gray chert and red jasper. They
can be many hundreds of feet thick, and are typically mined
in giant open pits like the enormous “Hull Rust” chasm (the
“Grand Canyon of the North”) in Bob Dylan’s hometown of
Hibbing, Minnesota. Apart from their metallic composition,
the iron formations have sedimentary characteristics very
similar to those of modern limestones, indicating they must
have been deposited in shallow marine environments. Yet in
today’s ocean, iron is in such short supply that it is a limiting
nutrient— an essential element whose scarcity holds biologi-
cal productivity in check. A controversial climate- engineering
scheme is even based on this fact; the idea is that if the oceans
were fertilized with iron powder, cyanobacteria would bloom
and photosynthesize enthusiastically, and (if all were to go
according to plan) sink to the ocean floor, sequestering large
amounts of carbon, without (fingers crossed) wreaking havoc
with the rest of the marine biosphere. In contrast with the trace
amounts of iron in seawater today, the tremendous volume of
the iron formations— visualize all the steel in the world’s cars,
aircraft, buildings, bridges, and railroads— attests to the great
abundance of iron in the Proterozoic oceans.
It was oxygen, the insurgent gas first produced by cyano-
bacteria, that changed the rules about what could and could
not be present in seawater. In the pre- oxygen regime, iron
spewed by deep- sea volcanic vents was able to remain dis-
solved in the open ocean, commingling invisibly with sodium,
calcium, and other ions. But when oxygen began to accumu-
late in shallow waters, it hunted down the iron atoms, bonded
itself to them, and pulled them to the seafloor, creating iron
formations. Oxygen purged the oceans of iron by literally
rusting it out.
Changes in the air 105
A N E W W O R L D O R D E R
This geochemical coup d’état is known to geologists as the Great
Oxidation Event, or GOE, and it represents a radical rewriting of
the atmosphere- hydrosphere constitution. The presence of free
oxygen changed the chemical interactions between rainwater
and rocks on land, altering the composition of lakes, rivers, and
groundwater. Certain types of cobbles that had been common in
Archean river beds— particularly chunks of pyrite and uranium-
rich minerals— disappeared from sedimentary deposits at this
time, because they were now unstable or soluble under the new
geochemica
l regulations. Conversely, modern oxide minerals—
sulfates and phosphates like gypsum and apatite— became com-
mon entries in the rock record. Upstart life- forms had forced
changes in the practices of the ancient mineral kingdom.
The presence of free oxygen (O2) at Earth’s surface also led
to the establishment of an ozone (O3) layer in the stratosphere,
which shielded the surface environment from the ravages of
ultraviolet radiation from the Sun and opened new frontiers
to settlement. Novel alliances between oxygen and other ele-
ments made previously scarce nutrients such as nitrogen more
mobile. This fueled major biological innovations, including
more efficient photosynthesis, which produced even more
oxygen. Like market opportunities created by a “disruptive”
technological advance, entirely new biogeochemical cycles
were established— global commodities exchanges mediated
by single- celled organisms, through which large volumes of
carbon, phosphorus, nitrogen, and sulfur were traded.11 And
in a strategic symbiotic merger, a tiny biological entrepreneur
that had learned to process oxygen, called a mitochondrion,
joined with a larger cell and founded the eukaryotic line that
would eventually lead to plants and animals.
106 Ch a pter 4
A continuing question about the GOE is why there was such
a long lag between the first appearance of photosynthesizing
life- forms 3.8 billion years ago and the emergence of free oxy-
gen at about 2.5 billion years. One possibility is that the or-
ganisms that formed the stromatolites in the Isua and Warra-
woona rocks performed anoxygenic (non- oxygen- producing)
photosynthesis— a seeming oxy- moron (so to speak) to those
of us familiar with plants— but a metabolic strategy that is still
practiced by some bacteria that lurk in low- oxygen haunts like
algae- clogged lakes. Rather than combining carbon dioxide
(CO2) and water (H2O) in the presence of sunlight to form
sugars (CH2O) · n (where n is 3 or greater) and release oxygen
(O2), these microbes instead forge their sugar from CO2 and
hydrogen sulfide (H2S, the “rotten egg” gas) and emit sulfur as
the waste product.
Alternatively, it could be that the stromatolite- forming
microbes did produce free oxygen, but that all of it was just