快猫短视频

EARTH is made habitable thanks to its constantly changing physical, chemical
and biological cycles. Although the balance between different components of
these processes has changed throughout our planet鈥檚 history, the result has been
a moderate environment, with no extremes of temperature or aridity, for
example鈥� unlike Venus or Mars. Life has played a vital part in this
evolution, as has plate tectonics. And the key to understanding what is
happening now and in the past is to look at the concentrations of a few
important substances, recorded in the rocks.

The best-known of these cycles is the carbon cycle. Carbon exists in the
atmosphere mainly as a blanket of carbon dioxide (CO₂), which keeps the
planet warm enough for life as do methane and the other greenhouse gases (see
Inside Science No. 92). But you also find carbon in the sea, on land and in
rocks, different locations that are described as reservoirs (see Inside Science
No. 51). With the carbon cycle we can identify many complex natural processes
that move carbon between its different reservoirs.

Much of the interest in the carbon cycle today, for example, focuses on the
parts of the cycle that human activities can influence such as the role of soil,
plants and the atmosphere. But only around 20 per cent of the world鈥檚 carbon is
involved in these parts of the cycle. The remaining 80 per cent is consigned to
the rocks. In fact the Earth and its atmosphere contain about 10²³ grams of
carbon; the vast majority stored in sedimentary rocks, as organic compounds
(1.56 脳 10²² grams) and carbonates
(6.5 脳 10²² grams). And it is carbon鈥檚 role
in rocks that really matters throughout the history of the Earth. It鈥檚 the
biggest reservoir, so changes to this reflect the big story of things happening
on planet Earth.

The carbon cycle is just one of the many cyclic changes on the Earth that
redistribute rocks and the other elements that are vital for life鈥攕uch as
sulphur, nitrogen and hydrogen. Geologists rely on markers or footprints
preserved in rocks to understand how the cycles worked in the past. These
footprints take the form of the type of carbon, oxygen or other elements that
are present in particular rocks. Or to be precise, the proportions of isotopes
of these elements.

Change and decay

Isotopes and elements

ISOTOPES are different forms of the same element. They have the same atomic
number, and so the same number of protons in their nuclei. But they differ in
the number of neutrons and so in their atomic weight. The most stable isotopes
are the most abundant; they are the atomic configurations we consider typical
for each element. They are only useful if you can separate out the more common
isotopes from the more unusual ones. This is easier to do with lighter elements
because the differences in atomic weight are proportionately higher than in
heavier atoms. Hydrogen, carbon, oxygen and sulphur all have useful stable
isotopes for tracing the geological past and present.

The less stable isotopes are generally rarer and they decay by emitting
particles such as neutrons or alpha particles and/or radiation such as gamma
rays. They decay into different isotopes or different elements, depending on the
particles emitted. This is called radioactive decay鈥攇eologists find it
invaluable as it acts as a clock. An isotope鈥檚 rate of decay is constant. A
given amount of an isotope such as carbon-14 (¹⁴C) decays at a steady rate.
Carbon-14 forms when cosmic rays react with atoms such as nitrogen. It spreads
quickly through the atmosphere and is incorporated in the molecules of living
things, such as plants and animals. The proportion of ¹⁴C in the plant or
creature is the same as the proportion in the atmosphere, while an organism is
alive but it starts to decay after death. So it is possible to tell when a
creature or plant died by the amount of ¹⁴C left鈥攖he older something is,
the less ¹⁴C present. This is the principle of carbon dating, which is also
useful in archaeology as it is possible to work out the age of bones or wooden
artefacts. Similar techniques are used for rocks, which trap isotopes, for
example, when they crystallise.

Carbon in its common form has six protons and six neutrons in its nucleus.
This makes the common form of carbon, ¹²C. But carbon is also found as the
isotopes ¹³C and ¹⁴C, with seven and eight neutrons respectively. Carbon-13
occurs naturally, with about one atom of ¹³C for every 99 of ¹²C. Carbon-14 as
we know is unstable. But ¹³C and ¹⁴C behave just like 鈥渙rdinary鈥� carbon, ¹²C.
They react in the same way, dissolving in water, making gases such as CO
₂, and forming carbonate rock such as limestone (CaCO₃). But they
are different enough for the ratios of the isotopes to change when many chemical
reactions involve carbon compounds.

Carbon isotopes

Stability and decay

The ratio changes in some reactions because some atoms are heavier than
others. Imagine dry ice (frozen carbon dioxide used to make smoke or fog on
stage): it takes slightly less energy to change a molecule of CO₂
containing ¹²C from the solid to the vapour, because the molecule is slightly
lighter. Carbon dioxide containing ¹³C and ¹²C will vaporise, but there will
be a higher chance of vapour molecules using the lighter ¹²C. So whatever the
fraction of ¹²C and ¹³C in the dry ice to start with, the vapour will have
slightly more ¹²C and the solid left behind, slightly less, and proportionately
more ¹³C.

This alteration of the proportions of the two isotopes is called
fractionation. The difference in mass of ¹³C and ¹²C is very slight, but enough
for instruments called mass spectrometers to measure
(see 鈥淣ew light on a spectrum of ion beams鈥�). The changed
proportions of the isotopes, preserved in rocks, are the markers that tell us
what happened in the past.

Geologists have identified four important reservoirs in which the carbon
accumulates during the carbon cycle: the Earth鈥檚 mantle (the fluid layer beneath
the crust), the atmosphere, the carbonates such as calcite shells and limestone,
and organic carbon such as coal in rocks. The proportions of these two carbon
isotopes in these reservoirs are different, because of the different series of
reactions that the carbon undergoes to reach each one. Measured relative to a
standard, which is average Mesozoic seawater, the mantle has a steady five parts
per thousand ¹³C less, which is written as 鈭�5鈥�,
the atmosphere 鈭�7鈥�, marine
carbonates roughly zero and organic carbon 鈭�26鈥�. The value of zero for marine
carbonates shows that these rocks are taking in the isotopes in the same
proportions as they are found in seawater; in other words, there is no net
fractionation in the formation of carbonates. In contrast, there is most
fractionation in the formation of organic carbon, made from plants and animals
that eat plants. Photosynthesis, during which plants make sugars from water and
CO₂ using sunlight for energy, increases the ratio of ¹²C to ¹³C in
the carbon stored in the plant. It is the most important process that decreases
the proportion of ¹³C and these negative values reflect the importance of this
process today.

Because of the significance of photosynthesis in the balance between isotopes
in these different reservoirs, measurements of isotope ratios preserved in rocks
can show how this balance has changed through time. Roughly speaking, a more
negative &dgr;¹³C for organic carbon means more photosynthesis, and a less negative
value means less photosynthesis. So looking back through time at the isotope
ratios of these reservoirs could show, for example, when photosynthesis began,
and so when life started on Earth. The results are something of a surprise,
though.

快猫短视频s have now made more than 10 000 measurements of the carbon isotope
ratios for carbonates and organic carbon from rocks through the history of the
Earth. Astonishingly, the ratios have stayed essentially constant for the past
3800 million years. So photosynthesis has been going on for all this time. Thus
bacteria and other primitive life forms were squirrelling away as much carbon as
our lush biosphere does today. Although this early Earth would seem very
inhospitable to us, it was actually teeming with life.

Surprising though this is, it leads on to another surprising conclusion. This
change in the composition of the atmosphere paved the way for the evolution of
respiring, oxygen-breathing life such as mammals, which produce CO₂,
giving the balance we have today. But early on, as soon as there was land as
well as sea, there was also erosion by wind and water. This is called chemical
weathering and it is very efficient at taking carbon from the atmosphere.
Weathering carries CO₂ from the atmosphere to the seas, by tying it up
in reactions that break down silicates in rocks. It is deposited as carbonate in
shells and coral and lime mud. This process alone, working at the current rate,
could remove all the CO₂ in the atmosphere today in just a million
years. So as soon as there was substantial land鈥攁nd there were continents
as early as 2500 million years ago鈥攐ther process must have returned carbon
to the atmosphere, or the balance would have tipped the other way.

The most likely cause was the imposition of another cycle: plate tectonics
(see Inside Science No. 107). The constantly changing movements of the rigid
outer layer of the Earth involve movement of rocks from the crust of the Earth
into the interior at subduction zones. Although this sounds like another way to
take rock (including its carbon) out of the carbon cycle for good, in fact it is
a way of returning carbon in rocks to the atmosphere. Ocean floor sediments are
carried down into the Earth鈥檚 mantle with the slab of ocean crust. But carbonate
is not stable in the higher temperatures of the mantle; one of the first things
these rocks do is release their CO₂, together with water and other
volatile substances. These fluids are enough to make the mantle rocks above melt
and form volcanoes. And when these volcanoes erupt, they shoot CO₂ back
into the atmosphere. It takes about 60 million years for a carbon atom to
undergo the whole cycle of weathering, subduction and volcanism. So the carbon
isotope record suggests that plate tectonics in some form started long before
there is any sign of it in the conventional rock record.

In fact volcanoes contribute so much CO₂ to the atmosphere that they
are thought to be responsible for some of the warmer periods of geological
history, periods known as greenhouse climates. The plate configurations during
greenhouse climates would seem to involve more volcanism. And events such as
orogeny鈥攖he formation of mountain ranges when two plates collide鈥攃an
affect the climate, for example, by altering atmospheric circulation. It is
likely that the elevation of Tibet in the Himalayan mountain chain, for example,
has had a cooling effect on the world climate. So it is important to understand
the formation and destruction of crust.

On and off a plate

Searching the seas

There are many ways to reconstruct the movements of plates in the past. The
simplest involves working back from the configurations of plates we have now,
for example, closing up the Atlantic Ocean to put South America and Africa back
together again. That families of fossils from eons past are common to several
continents suggests that they were once in contact; mountain ranges formed by
orogenic events can be matched up across oceans that subsequently formed. But
there is also a chemical marker for some of these movements. Mountain ranges
again are the key.

When continents collide, rocks at their edges are pushed up and erode more
quickly than rocks in the stable continental interiors. Continental collision
happens very quickly by geological standards, lasting a few tens of millions of
years. Erosion is also speedy. The net result is that a major continental
collision sends lots of the material that makes up the continents into the
oceans, where they add to the sediments that accumulate on the seafloor. Some of
these are distinctive.

Continental crust originated as granite and similar igneous rocks. They are
thicker and float higher on the fluid layers of rock beneath than the ocean
crust does鈥攖hat鈥檚 why continents make land. But when molten rocks are
crystallising, certain rare elements become concentrated in particular minerals.
These elements have unusual shapes and only fit into minerals with certain
crystal structures. Some of these minerals are part of granite and so are found
in the continents and not, by and large, in ocean rocks. The levels of these
elements increase in ocean sediments when there is rapid erosion of a mountain
belt. For example, levels of strontium are relatively high in oceans nowadays,
notably higher than 100 million years ago; this can be traced to the growth and
rapid erosion of crustal rocks in the Himalayas, carried to the sea in huge
river systems such as the Brahmaputra. Measuring the concentrations of such
elements in seafloor sediments can show episodes of mountain-building followed
by erosion, however this analysis does not show where the mountain chains were.
But it does help to build up the picture of plate movements in the past.

But stable isotopes of carbon have another role to play in geological
history. Because plants take up more ¹²C in CO₂, they leave the
atmosphere richer in ¹³C. The more plants there are, the higher the &dgr;¹³C of
the atmosphere will be. In a temperate climate, plants cover much of the land
surface, so &dgr;¹³C in the atmosphere is higher. But when there is an ice age,
plants are fewer and less active, so &dgr;¹³C in the atmosphere falls. This is the
basis of a relative temperature scale for the geological past, based on carbon
isotope ratios in rocks
(see Figure 4).FIG-22047804.jpg

To take these measurements for sea sediments, researchers use planktonic
coccoliths, tiny sea creatures that float close to the sea surface. Coccoliths
grow plates of calcium carbonate which have the same carbon isotope ratio as the
atmosphere. When they die, their shells sink and become part of the limestones
forming on the seafloor. So these tiny fossil shells tell geologists about &dgr;¹³C
in the atmosphere in the past. Not all plankton behave in the same way, so
geologists pick their species carefully by considering how different types
behave today.

Oxygen and ice cores

Plumbing the depths

A similar technique has been developed using stable isotopes of oxygen: ¹⁶O,
¹⁷O and ¹⁸O. Although the proportions of ¹⁷O vary as a result of reactions
such as evaporation, most calculations involving oxygen use the ratio of ¹⁶O to
¹⁸O, because the greater mass difference between the two means that the
fractionation effects are clearer. Oxygen ratios are measured either relative to
the same standard as carbon, the Peedee belemnite
(see 鈥淣ew light on a spectrum of ion beams鈥�), or to water close
to the composition of seawater today, Standard Mean Ocean Water鈥�SMOW.
Hydrogen and its isotope deuterium also fractionate in reactions involving
water; they too are measured relative to SMOW.

Oxygen has a key role in the climate cycle because it exists as part of
water, which is involved in most significant reactions on Earth. These reactions
are all sensitive to temperature, so oxygen isotopes are especially useful in
helping us to understand the climate of the recent past. During ice ages, more
water freezes at the poles. That water comes from the oceans via rainfall and
snowfall from clouds. When water evaporates to make clouds more H₂¹⁶O
is taken up than H₂¹⁸O, so that any ice that forms is richer in ¹⁶O
and the seas become richer in ¹⁸O. Oxygen isotopes in ice cores drilled from
the polar ice sheets record temperatures over hundreds of thousands of
years鈥攄uring which ice ages have waxed and waned.

In addition, by looking at oxygen isotopes in sea sediments it is possible to
push back the temperature record even further. The oxygen isotope record has
been important in understanding some of the underlying mechanisms of climate
change, such as the ice ages.

There is a problem with applying this simple method because it assumes that
the isotope composition of seawater changes only as the temperature changes. But
other factors can alter it. Here again, plankton are the key. Some plankton live
in deep ocean water, away from the surface, where the temperature changes
little, but other factors, such as salinity change just as in the surface
waters.

The plankton from the deep water do not experience the temperature changes
that the shallow water species do. So comparing the isotope changes in deep and
shallow water species can identify the changes that correspond to the
temperature changes alone. Analysing both types of plankton from the same
sediment gives a reliable temperature scale that is widely used for
understanding the past climate.

This and similar combinations of biology, chemistry and physics create
powerful tools both for understanding our current climate and delving far back
into the geological past. The message from the chemical footprints that can be
found and followed is that our equable climate comes from the constant interplay
of many different processes, all in balance.

The global picture arises from understanding and measuring the minutest
variations between individual atoms that are a fundamental part of life on
Earth. Such slight variations are enough to track the processes essential for
life, now and in the past 3800 million years. We can see how they have waxed and
waned, but maintained a planet habitable for many plants and animals, including
ourselves. And the better we understand the cycles, the better we can ensure
that we continue to live comfortably on Earth.

* * *

New light on a spectrum of ion beams

MASS spectrometers separate a beam of ions into a spectrum of beams of
different ions, much as a prism spreads a beam of light into the different
colours. A prism spreads light into its constituent colours because the
wavelength of light and its path through glass varies with its colour. Mass
spectrometers do the same thing, but use magnetic fields to separate charged
particles or ions on the basis of their mass.

The instruments have a filament鈥攁s in a light bulb鈥攚hich glows
and heats up when a current passes through it. You place a tiny amount of a
solution containing ions on the filament. As the filament heats up, the ions
turn into vapour and strong magnetic fields focus the charged particles into a
beam. The greater the mass of the ions, the less its path is bent by the
magnetic fields, so that ions of different isotopes travel to different spots in
the instrument. Detectors register the ions as they arrive and give the
proportions of each in the original solution.

These instruments measure the proportions of the different isotopes in a
sample. But it is not enough to know the proportions of isotopes. To make sense
of all these readings, geochemists measuring isotopes in different labs around
the world have standard samples, which they use for comparison. For carbon, it
is the proportions of ¹²C and ¹³C that were originally found in a Mesozoic
fossil, a belemnite from a Cretaceous formation at Peedee in South Carolina.
Belemnites crystallise their hard parts or 鈥済uards鈥� directly from seawater, so
the values for this fossil represent the oceans of the Mesozoic. Thus all
measurements of stable carbon isotopes are relative to 鈥淧DB鈥� the Peedee
belemnite.

Isotope ratios are measured according to a complicated formula:

&dgr;¹³C =
¹³C/¹²C sample 鈭� ¹³C/¹²C
/ ¹³C/¹²C 脳 10³

The differences are measured in parts per thousand, 鈥�, equivalent to tenths
of a per cent. The symbol for the difference is &dgr;, so that a value of &dgr;¹³C_PDB = 鈭�10鈥� means carbon with 10 fewer parts per mil than the Peedee
belemnite, and thus fewer than 10 parts per mil than the Mesozoic oceans
generally. A &dgr;¹³C_PDB of zero means that the carbon in the sample was
in the same proportions as the Mesozoic ocean, and a positive &dgr;¹³C_PDB
means that the sample had more ¹³C.