900, 000 years of humans at Happisburgh July 2010
‘Early Pleistocene human occupation at the edge of the boreal zone in northwest Europe’ NATURE, JULY 8 2010.
Simon A. Parfitt1,2, Nick M. Ashton3, Simon G. Lewis4, Richard L. Abel5, G. Russell Coope6, Mike H. Field7, Rowena Gale8, Peter G. Hoare9, Nigel R. Larkin10, Mark D. Lewis2, Vassil Karloukovski, Barbara A. Maher, Sylvia M. Peglar12, Richard C. Preece13, John E. Whittaker2 & Chris B. Stringer2
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.
Barbara A Maher and Vassil Karloukovski
Centre for Environmental Magnetism & Palaeomagnetism
Lancaster Environment Centre
University of Lancaster
Tel: +44 (0)1524 510268
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……this diagram (from Bob Butler’s book, Paleomagnetism, 1982, Blackwell) describes the present day field.
1. Gary A. Glatzmaier & Paul H. Roberts, "A three-dimensional self-consistent computer simulation of a geomagnetic field reversal," Nature, 377, 203-209 (1995).
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 cubes inserted into sediment face ('HB23'), Bed F (2006)
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 (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.
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.
REVERSED POLARITY SEDIMENTS AT HAPPISBURGH
In combination with the pollen and mammalian faunal content of the sediments, the REVERSED polarity of the Happisburgh sediments suggests that human occupation occurred towards the end of the Matuyama Chron between 0.99 and 0.78 Myr.