‘Early Pleistocene human occupation at the edge of the
boreal zone in northwest Europe’ NATURE, JULY 8
2010.
Simon A.
Parfitt, Nick M.
Ashton, Simon G.
Lewis, Richard
L. Abel, G.
Russell Coope, Mike H.
Field, Rowena
Gale, Peter G.
Hoare, Nigel R.
Larkin, Mark D.
Lewis,
Vassil
Karloukovski, Barbara
A.
Maher, Sylvia M.
Peglar, Richard
C. Preece, John E.
Whittaker & Chris
B. Stringer
THE
PALAEOMAGNETIC
DATING OF THE EARLY HUMAN
SITE AT HAPPISBURGH, EAST ANGLIA,
UK.
A key feature of the Happosburgh archaeological finds
is that they are contained within sediments which are so old that
they are palaeomagnetically REVERSED…..i.e. the sediments were
deposited before ~ 780,000 years ago, when the Earth’s magnetic
field was opposite to its present day configuration. Here, we offer some
additional information on the critical palaeomagnetic dating of the
Happisburgh site.
CENTRE FOR ENVIRONMENTAL MAGNETISM AND PALAEOMAGNETISM
LANCASTER ENVIRONMENT
CENTRE
LANCASTER UNIVERSITY
LA1 4YQ,
UK
PALAEOMAGNETIC DATING OF SEDIMENTS: AN
INTRODUCTION.
EARTH’S MAGNETIC
FIELD: The
direction of the Earth’s magnetic field is not constant; through the
geological past, the
field has reversed its direction periodically1 – in
fact, the field has reversed its polarity many times. These changes in polarity,
when correlated to an absolute timescale (eg from radiometric or
biostratigraphic dating), form a
MAGNETIC POLARITY TIMESCALE
(MPTS).
 The Earth’s present-day magnetic
field
shown
below (from Bob Butler’s
book, Paleomagnetism, 1982, Blackwell) describes the present day
field.
MAGNETIC RECORDING BY
SEDIMENTS: When
sediments are laid down, layer upon layer, at the Earth’s surface,
the direction of the magnetic field at that time can be recorded by
the small amounts of magnetic minerals present in the sediment. By this natural recording
process, the reversals of the Earth’s magnetic field are recorded in
sedimentary sequences.
SEDIMENT SAMPLING AND MAGNETIC MEASUREMENT:
Sampling a sequence of sediments, and subsequent measurement of
their palaeomagnetic record, enables dating of these sediments by
comparison with the MPTS.
PALAEOMAGNETIC SAMPLING AT HAPPISBURGH,
2005-2008.
Sediment samples were obtained from the Happisburgh
archaeological site over a number of digs and years, as shown by these
photographs.
Palaeomagnetic sampling at Happisburgh, September 2008 – here, an
undisturbed column (‘H3’ samples) of the blue-grey fine-grained
sediments in
'Bed A' (the lowest laminated sands and silts in the succession)
is obtained by careful insertion of a rectangular sampling
‘sleeve’. The ‘up’ orientation arrows can be seen on the sampler
surface; the geographic orientation of the sediment face into which
the sampler was inserted was also measured. Individual sub-samples
of the sediment column were taken upon arrival of the sediment
column at the Centre for Environmental Magnetism and
Palaeomagnetism, Lancaster Environment Centre, University of
Lancaster.
 Here,
the layers of blue-grey, fine-grained sediment and orange-grey,
coarser-grained sediment can be clearly seen, in this photograph of
the sediment column sampled in Sept. 2007 (‘HSB3’) from Bed C.
DEMAGNETISATION TO
REVEAL THE PRIMARY MAGNETIC SIGNAL: A sediment can be
subjected to more than one magnetizing event, and can contain
magnetic minerals formed at different times. To measure the magnetic
field direction recorded when the sediment was first deposited, we
apply steadily increasing demagnetizing fields, which strip away any
later ‘OVERPRINTS’.
DEMAGNETISATION OF THE
HAPPISBURGH SEDIMENTS: In all but the basal samples at
Happisburgh, a strong, NORMAL polarity overprint magnetisation is
carried by the iron sulphide,
greigite………..this
magnetic iron sulphide
mineral formed after the sediments were deposited, and we need to
remove its magnetic signal in order to see the weaker, original
magnetic polarity of these sediments.
We can draw an analogy with the
overwritten canvas of the Archimedes palimpsest shown below
( Copyright:
the Owner of the Archimedes Palimpsest. Image capture: Rochester
Institute of Technology and Johns Hopkins
University).
If the
more recent overprinting is removed:
Then the weak original signal can be seen:
If the more
recent overprinting is removed, then the weak original signal can be
seen.
When
we strip away the NORMAL magnetic overprint from the Happisburgh
samples, using incrementally higher alternating field
demagnetization, the direction of
the sample moves systematically to reveal its REVERSED
polarity.
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