Transcript U3A-ClimChange04 2330KB Oct 27 2012 09:44:51 AM

CLIMATE CHANGE
THE GREAT DEBATE
Session 4
DEEP SEA CORES
To test the Milankovich Theory requires a
continuous proxy record of climate change
during Quaternary time
Deep sea sediments well away from land
accumulate at a very slow, nearly constant rate
and contain a wealth of microfossils
With careful site selection and with coring
equipment that can collect up to 10 meters of
unconsolidated sediment from the ocean floor,
it is possible to get complete records for the
whole Quaternary period
POLARITY REVERSALS
The polarity if the Earth refers to the direction
In which a compass points
Normal polarity occurs when a compass points
north and reverse polarity when a compass
points south. Reversals occur simultaneously
on a world-wide scale
The last reversal occurred about 780,000 years
ago and marks the transition from the reverse
Matuyama Period to the present day normal
Brunhes Period. Several reversals have
occurred during the Quaternary Ice Age
Geomagnetic Time Scale
for the last four million
years
Events provide useful time
time markers within the
Quaternary Period
REVERSALS AND DATING
The polarity of the cores is measured at
frequent intervals and reversals identified
The ages of reversals for the last 15 million
years is known very accurately from
magnetic measurements and K/Ar dating of
a sequence of basalts from Iceland. The
reversals are points of known age and can
be correlated from one core to another
Cores can also be dated directly by other
methods such as U/Th
Core depth correlated with magnetic reversals
FORAMINIFERA
They are microfossils that are usually
abundant in deep sea cores
Their skeletons are composed of
calcium carbonate containing the
elements calcium, oxygen and carbon
Oxygen contains three stable isotopes:
O16, O17 and O18. O16 is by far the
most abundant. The ratio O18/O16 varies
with that of seawater. The seawater ratio
is dependent on the amount of glacial
ice that has built up on the planet
PROXY CLIMATE RECORD
By analysing the oxygen isotope composition
of certain species of forams we can construct
a record of climate change for the whole of
the Quaternary period. It shows that there
have been many glacials and interglacials
because changes in ice volume are closely
related to temperature changes
Isotope changes are numbered backwards
from the present with odd numbers representing warm stages and even numbers
representing cold stages
Oxygen isotope record
for last 1 million years
obtained from a deep sea
core (V28-238) from the
Pacific Ocean
Oxygen isotope ratios
are measured in per mil
relative to a standard
(PDB)
TESTING ORBITAL THEORY
Deep sea cores are well dated and many of
their characteristics can be expressed in
the form of numbers
This enables the application of spectral
analysis to oxygen isotope values to detect
the presence of any cyclic variations
When this is done the orbital theory of
Milankovich is confirmed
Spectrum of climate
variation over the
last half-million years
It confirms the
Importance of the
Milankovich cycles