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Monday 16 August 2010 | Christina Troelsen
- Geologists and geophysicists at the Department
of Earth Sciences, Aarhus University, have
demonstrated that movements deep in the
Earth’s mantle control some of the mechanisms
that lead to sedimentary deposits. The distribution
of these sediments has traditionally been
explained as changes in the sea level or
plate tectonic processes. This means the
geologists are now providing a completely
new explanation of why the Danish subsurface,
for example, consists of alternating layers
of clay and sand sediment.
At Møn’s Cliff,
the 75-million-year-old chalk deposits from
the Cretaceous Period bear witness to the
fact that this area of Denmark was once
covered by the sea. Sediments from Denmark’s
subsurface generally show that the Danish
area was alternately dried out or covered
by the sea. This is typically explained
by changes in the global sea level or plate
tectonic processes that raise or lower the
area. The Aarhus scientists’ model calculations
show that processes in the Earth’s mantle
are also a potentially important reason.
The researchers’ results
were published recently in the highly reputed
American journal Science. Responsible for
these remarkable results are postdoctoral
scholar Kenni Dinesen Petersen, Professor
Søren Bom Nielsen and Associate Professor
Ole Rønø Clausen, Department
of Earth Sciences, in collaboration with
colleagues in Scotland and Switzerland.
Deposits in sedimentary
sequences
Sediments are found in so-called sedimentary
basins. These are hollows in the Earth’s
surface that are filled up over millions
of years. An example is the subsurface below
Denmark – the so-called Danish Basin – which
was formed approximately 300 million years
ago in connection with powerful earthquakes
and volcanic activity, and which reaches
depths of more than ten kilometres in the
Danish area. Closer studies of sediments
in such basins show that they are often
deposited in a changing environment, where
the water level has been in a constant state
of change. Here the deposit of clay in relatively
deep water, for example, is replaced by
a sand deposit near the coast or by rivers
before being covered by clay deposits once
more. Sediments from a single period of
such variations are called a sedimentary
sequence. Sedimentary deposits in such sequences
take place over many periods – from a few
years to many millions of years.
The Aarhus scientists
are interested in the latter time scale,
because the reasons for the sedimentary
sequences here have been poorly understood.
The traditional explanation is variations
in the global sea level, caused by events
such as growth or the melting of the Earth’s
glaciers. Another explanation states that
the sequences are connected with plate tectonic
processes, where two continental plates
collide, for example. The problem with these
explanations is that the Earth’s climate
was so hot for long periods that the incidence
of glaciers was not sufficient to provide
enough changes in the global sea level,
just as there were long periods in many
areas without plate tectonic activity.
Convection moves the
surface
In the article in Science, the scientists
show how so-called small-scale mantle convection
can provide a reasonable explanation of
the way many sequences occur. Convection
is the phenomenon that takes place in a
fluid or gas when it is heated from below,
so that hot and light material rises and
cools, after which it sinks down again.
Small-scale mantle convection takes place
in the Earth’s mantle under the continental
plates, i.e. in depths below 100–150 kilometres.
Studies of earthquake waves actually show
that the Earth’s mantle is solid, but that
it behaves over a period of millions of
years like a very viscous fluid and is thus
able to convect. Convection is an effective
form of transporting heat, which contributes
to the constant cooling of the planet. The
Earth’s inner heat is caused by radioactive
isotopes that constantly decay, as well
as the original energy released in a meteor
shower that took place when the Earth was
formed 4.5 billion years ago.
The scientists use advanced
computer simulations to study the correlation
between the convection movements and overlying
sedimentation processes. The software they
use was prepared by Kenni Dinesen Petersen
during his PhD studies. The computer models
show that the convecting mantle makes the
overlying continental plate move slightly
up or down locally, thus providing a constant
change in the local sea level, so that sedimentary
sequences are formed even though the global
sea level is unaffected. The result is important
because it shows that the formation and
order of sedimentary sequences can vary
considerably within a few hundred kilometres.
This does not happen simultaneously over
large distances or even globally as is otherwise
often assumed. This is also important in
terms of searching for oil, which is often
based on accurate mapping of sedimentary
sequences, as well as understanding how
the climate impacts the global water level
via melting glaciers. In fact it has normally
been assumed that it is possible to reveal
changes in the global water level by studying
sediments in tectonically stable basins.
The new results show that no basins are
stable in this context, because convection
constantly provides more or less random
changes in local water levels.
Multidisciplinary geological collaboration
The study is a successful result of multidisciplinary
and international collaboration between
two disciplines in geology, where the focus
on surface and deeper processes is normally
kept apart. In the opinion of the researchers,
there is great potential in this type of
combination. They are therefore still going
on with their work by combining the application
of mathematical/physical modelling of geological
processes with observations made by geologists
and geophysicists based on traditional methods.
The title of the article
in Science is Small-scale mantle convection
produces stratigraphic sequences in sedimentary
basins. In addition to the three Aarhus
geologists, the research group responsible
for the article consists of Randell Stephenson
from the University of Aberdeen and Taras
Gerya from the Swiss Federal Institute of
Technology (ETH Zurich).
