|Carn Owen: how the grey
rocks of a
Mid-Wales hillside record drastic past climate change!
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times have you read that? It is usually a sign that the person making
the statement has run
out of other arguments with which to attempt to rubbish climate
So: what's the answer?
Sure it has! The rocks here in Mid-Wales were formed at
when a remarkable period of climate
change occurred - and they record its passing. However, I should add an
afternote. The change was accompanied by the second biggest
mass-extinction in the fossil
record of the last 542 million years - from the Cambrian Period until
the present day.
rolling hills of the Cambrian Mountains that form the backbone of
Mid-Wales are made up of grey rocks that on first appearance are rather
boring - slates, shales and sandstones. It's therefore surprising to
know that they record such a drastic event. Before we go into how they
do, let's have a quick look at the geological timescale, so that we can
set the coordinates on our Tardis for a closer look at that time:
I've abbreviated the Period names because
there wasn't enough room! The relevant ones here are:
ORD = Ordovician Period, 488.3-443.7 and SIL =
Silurian Period, 443.7-416 million years ago.
We'll go up into the mountains, heading for the hill of Carn Owen that
overlooks the Nant-y-moch reservoir. Carn is Welsh for Cairn, and at
of Carn Owen there is indeed a Bronze-Age ring-cairn, looking
out over the vast landscape:
Carn Owen have been heavily quarried in the past, and two of
the smaller quarries reveal something rather interesting. The photo
below shows the western one. Straight away,
the rocks can be seen to be forming layers, and the layers are all
tilted at the same angle to the
right (eastwards). These are sedimentary rocks - they are made up of
sediment - sand, silt and mud. They were deposited underwater on an
ancient seabed, in the late Ordovician Period, about 444 million years
Because one layer of sediment gets
deposited on top of another, the higher layers in the photo are younger
than the lower layers. And because the rocks are tilted, if in this
immediate area you go from left to right (west to east) you will be
looking at younger and younger layers of sedimentary rock.
The rocks that form the Cambrian Mountains
might all look grey from a distance, but when you get close-up to them,
they are a bit more interesting! The thick layers in the quarry are
sandstone and this is what it looks like in person.
Full of small grains of white quartz, it is pale, hard and splintery.
some other places locally, it also contains rounded
quartz pebbles. Now, in order to carry coarse-grained
sediments such as sand and pebbles to their resting-place, you need
strong currents. Geologists have worked out that the sandstones were
deposited by what are called Turbidity Currents - mixtures of water,
sand and other debris, that poured from the shallower waters of what is
now East Wales, down the slopes into the deep-sea basin that occupied
Mid-Wales back then.
The next picture shows the second, or eastern quarry face. As I
explained above, we have climbed a little higher up through the
sequence of rock-strata. In fact this small rock-face marks the
boundary between the Ordovician and Silurian Periods, 443.7 million
years ago. The boundary is at a change in the rocks - can you see it?
It's right in the middle of
the rock-face. To the left are grey rather
solid-looking rocks, from the last few thousand years of the Ordovician
middle they are overlain by rather
flaky, rusty-coloured rocks. The
rusty-coloured rocks belong to the lowermost Silurian Period.
Here is a
close-up of the rocks at the top of the Ordovician - they really are
grey and boring!
But here are the rocks from the Lower
Silurian - far more to see here, just in terms of colour alone. This
rock is very fine-grained - it is a mudstone. Although dark grey where
freshly broken, it has weathered in many places to
colour. That is because the mudstone contains a lot of pyrite - iron
sulphide - that turns to rusty iron oxides when exposed to air and
moisture - just like iron does in time. The technical name for this
rock is "hemipelagite" - translated this means "deep sea mud".
The Lower Silurian rocks also
contain a lot of fossils, which makes them even more interesting
compared to the Ordovician sandstones. The commonest fossils are
graptolites (below). This specimen, with individuals 4-5cm in length,
shows how they can be beautifully preserved by pyrite that has
their remains. So what were graptolites? They were free-floating
that existed, like modern jellyfish or plankton, close to the sea
surface, drifting freely with the currents of the seas.
Pyrite is everywhere in these Lower
Silurian mudstones, as this photo taken down a special microscope
shows. Here, the pyrite shows up as bright areas, the biggest of which
is only a millimetre across. For pyrite to be this abundant, the
environment on the sea-bed must have been one where free oxygen was
scarce or absent, like the bottom of a stagnant pond, full of stinking,
sulphurous black mud.
So, in a
sequence of rocks across just a short distance on this mid-Wales
have seen a change over less than two million years from
high-energy, turbulent conditions, when sands and pebbles were
deposited, to a stagnant, deoxygenated undersea
plain where fine, muddy sediment only accumulated slowly and fossil
remains were preserved in pyrite. What happened?
THE GEOGRAPHY OF THE LATE ORDOVICIAN, 444 MILLION YEARS AGO
over the past few hundreds of millions of years
how, due to movements of the Earth's tectonic plates,
have drifted around, collided and split up through time. The globe
below is such a reconstruction: is shows how things
would have looked at the end of the Ordovician Period, when the rocks
of Carn Owen were being deposited. England, Wales and southern Ireland
were part of a block of continental crust known to geologists as
Avalonia, which lay at a low
latitude in the Southern Hemisphere and the South Pole was straddled by
the large continent of Gondwana.
Zooming in, this is what the area that is now England and Wales would
have looked like at the time of the deposition of the sandstones of
How do we know? By geological mapping - of the types of rocks and their
fossils - we can reconstruct what the environment was like at any given
time in the past. It gets much harder the further back you go, because
the very old rocks of a billion years and more in age generally don't
contain fossils and have been so messed-with by heating and pressure.
But in the Ordovician and Silurian Periods, we can get a reasonable
picture of what it was like. 440 million years ago sounds like a long
time, but when the Earth is 4,600 million years old, it doesn't sound
like so much!
The next map is just a few million years before:
And the next one is the same area in the early Silurian. Look how the
land of the Midland Platform was submerged, then it emerged from the
sea, then by the early Silurian it was being flooded again. It has been
estimated that a sea-level fall-and-rise of at least 80 metres took
place to bring this about. What caused it?
That's a good question. First let's think about the consequences of the
This sudden (in geological terms a couple of million years is sudden)
retreat and readvance of the sea had
profound effects. For a start, the retreat caused extensive areas of
shallow sea, teeming with life, to become dry land. That
was one factor in the great
mass-extinction that accompanied the changes - loss of habitat.
like that of today: the
first primitive land-plants only appeared during the Ordovician and it
was only later in the Silurian that they really became widespread. As a
consequence, with nothing much holding the ground together, loose
was readily eroded from the newly-emerged land by rainfall and rivers,
swept along to the edge of the deeper water and trundled down
into its depths by powerful submarine currents, covering the basin
floor with mud, silt, sand and, in places, pebbles.
Thus were the sandstones of Carn Owen deposited.
When sea-levels rose again in the early Silurian, flooding back over
the land, the erosion of
sediments by rainfall and rivers stopped and the deepwater area
became isolated from the sediment source, so that only the
finest muds settled out over its depths, onto which the remains of the
graptolites and other creatures settled out from the near-surface
high above. In this way, the pyrite-rich fossil-bearing mudstones came
So what caused this sudden double-flip in sea-levels? We have to look
at the rocks that were deposited over the great southern continent,
GLACIERS IN THE SAHARA? THE REMNANTS OF GONDWANA
Gondwana, the huge continent that
straddled the South Pole in the late Ordovician, has long since split
up into fragments that have drifted away in all directions. Bits of it
now make up Africa,
South America, India, Antarctica and Australia. In several of these
modern continents, late Ordovician rocks occur that have features that
can only have had
one origin - from glaciers and ice-caps. Such "direct indicators" of
ancient glaciers that are preserved in the geological record include
things like striated (scratched) rock surfaces - ice-sheets full
debris, like giant sheets of sandpaper, do a good job of grinding down
the rocks underneath. The
striking image below shows an
it illustrates an Ordovician
glacially-striated rock surface in the
Libyan part of the Sahara Desert!
There are also "indirect indicators" of ancient glaciation: the
fall (and subsequent rise) in sea-levels, recorded on Carn Owen by
changes in rock-types described above, is to be found in marine rocks
of the same age all over the world - therefore the sea-level changes
were "eustatic" - global - in nature. In other words, there was a
cooling, leading to a major ice-age, with global sea-level
falls due to so much water being locked up in ice, followed by a global
warming, melting the ice and bringing worldwide sea-levels back up
again. How, then, did it happen?
THE DRIVERS OF CLIMATE: TECTONICS, ATMOSPHERE AND SUN
several major drivers of global climate that, working together, could
have made this happen.
The geographical arrangement of the continents
due to plate tectonics affects the flow of air in the atmosphere and
the currents of water in the oceans, moving warm and cold air and water
about. The composition of
the atmosphere, in terms of greenhouse gases like carbon dioxide,
affects its ability to lose heat to outer space or retain it. The
energy output from the sun affects the
amount of energy reaching Earth.
Taking geography first, having the continent of Gondwana over the South
Pole would be advantageous if you want ice-caps to start forming: it
would have played a role similar to that of Antarctica today. The poles
are always the coldest parts of the planet and are even colder if they
have continents stuck over them. So that's
one box ticked.
atmospheric composition, carbon dioxide levels
during the Ordovician were much higher than today,
running at several thousand parts per million (ppm). Through
but very warm climate compared to today.
It seems that a fairly stable carbon-cycle
existed, with a balance between carbon sources such as volcanoes and
carbon sinks such as oceans.
However, in the late Ordovician, the stability seems to have
been disturbed. It is thought that this was because volcanic activity
died away, so there was less carbon dioxide being added to the
atmosphere. At the same time, continental collisions led to new
mountain ranges being formed in places such as what is now eastern
North America, with erosion and weathering of rocks. Rock-weathering is
a tremendous carbon dioxide sink: the gas dissolves in water, which
then falls as rain: rainwater carrying dissolved carbon dioxide is
weakly acidic and reacts with certain common minerals that make up many
rocks. The result is solutions carrying carbonate in solution plus
metals such as calcium and magnesium: in seawater, these drop out of
solution to form limestones. So the carbon dioxide gets locked away in
large amounts. There is a lot of evidence for
exactly this scenario to have happened in the late Ordovician, in
extensive areas where sediments from such new mountain ranges would
have been deposited, the resultant carbon sink causing
carbon dioxide levels to drop from about 5000ppm down to 3000ppm or
still sounds high but we haven't looked at all the major drivers of
climate yet: we still have one more to think about. What about
The Sun is a main sequence star - it behaves in a reasonably
predictable way over thousands of millions of years. Solar
energy output is thought to have
increased steadily by about 10 per cent per billion
years of Earth's history and it
in the late Ordovician, it would have been 4-5 per cent dimmer
than it is today.
What difference would that shortfall in solar energy make? Global
climate models tell us that, at the moment, polar ice can persist
when carbon dioxide
levels drop below 500 ppm (of course, they are below that at the moment
and have been for 25 million years or so).
The same models, adjusted for factors like
the the dimmer sun of the late Ordovician, predict that for glaciers to
have formed at the poles back then, carbon dioxide levels would need to
be below 2240-3920 ppm. So: combine a
dimmer sun, a
sudden drop in carbon dioxide levels and a large continent over the
South Pole and
that appears to have been enough to get the temperature down and allow
glaciers to develop, due to several major
climate drivers working together.
RAPID ENVIRONMENTAL CHANGE AND MASS-EXTINCTION
According to the fossil record, during this late Ordovician ice-age,
26 per cent of all
families and 60 percent of all genera of life
worldwide died out.
second-biggest such mass-extinction event in the fossil record - there
being five really big ones, including the K-T event that killed off the
dinosaurs and the end-Permian or P-T event that was the biggest of the
lot, when life on Earth was almost totally wiped out. The diagram below
shows how the Late Ordovician event sits with the others.
We know that
the late Ordovician mass-extinction accompanied a period of rapid
climate change - a significant cooling that lasted less than 2 million
years before a return to very warm conditions. That caused major
in sea level and changes to
ocean temperatures and chemistry. Oceanic changes are blamed for
the loss of coral-reefs: although a few corals
survived, living reefs themselves disappeared from the face of the
Earth and it took as much as 6 million years for them to reappear,
making the event the ﬁrst true ‘reef gap’ in the geological record.
There is an important lesson to be learned in this story. That is that
any major and geologically rapid climate shift in either
direction - warmer or cooler - from a stable state brings with it
drastic environmental changes that can have severe effects on
ecosystems and create the danger of extinction
events. These may occur due to actual habitat-loss, such as the
draining of the Ordovician shelf-seas, or due to changes in, for
example, oceanic chemistry that are too rapid for evolution to adapt
to. That's an important point: life can thrive in both very warm
("Hothouse") and very cold ("Icehouse") climates. It is when the rate
of transition from one to another is geologically too rapid for
adaptation that the big problems occur.
Our burning of the fossil
fuels on such a massive scale over just a couple of centuries risks
bringing about such changes unless we change the way we obtain and
consume energy, by moving more and more to renewable energy sources and
in increasing the efficiency and reducing the wastage that is currently
present in our energy-use. The "end-game" in terms of Earth's
temperature is less relevant: it is the speed at which we get there
that really matters. Several degrees in a couple of centuries: this is
a serious possibility if we carry on as usual. In geological terms this
is lightning-like in its speed. It leaves the Ordovician changes stuck
behind, still in the starting-blocks - and remember what happened back
then. Nobody wants to bring about the sixth mass-extinction, but unless
we find a way through this minefield, that scenario will not go away
It's surprising what the geology of a couple of small, grey crags on a
Mid-Wales hillside can tell us!
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