A A A
2014-02-31-stroeve

Seasonal Arctic summer ice extent still hard to forecast.

A new study lead by Prof Julienne Stroeve says year-to-year forecasts of the Arctic’s summer ice extent are not yet reliable.
More...

Bookmark and Share

26Al/10Be Dating

11 January 2014

“Late Neogene uplift of south-east England – 26Al/10Be ‘burial’ dating of marine deposits at 160 m altitude”

NERC CIAF

This is the title of an award made by NERC’s Cosmogenic Isotope Analysis Facility at East Kilbride to Tim Atkinson and John Catt (UCL Geography). As NERC ‘direct support’ the 14 analyses awarded equate to £15,420.  The project is to date shore-face sands and gravels at Little Heath, a site on the top of the Chiltern Hills that John has recently re-excavated and sampled in conjunction with the National Trust.  The significance of the sands and gravels is that they were deposited close to sea level, but have now been uplifted to 160 m altitude.  Dating them will constrain the average uplift rate since they were laid down.  The dating technique uses the decay of two radioactive isotopes, 26Al and 10Be, that have different, long half-lives.  Both isotopes were formed by cosmic ray bombardment of the parent materials of the gravels as they were weathered from rocks and exposed at the earth’s surface.  The deposits at Little Heath are thick enough to have shielded the deepest gravel layer from cosmic ray interaction since they were laid down, so that the time at which it was deposited can be calculated from the amount of differential decay of the two isotopes.  Our best guess is that the gravels will prove to be around 3 My old, as they have heavy mineral affinities with some other isolated occurrences of gravel on the Chalk hills that ring the London basin, including one at Rothamsted from which Pliocene marine fossils were recorded in 1926.

            The results of the project may assume more than local significance if they are precise enough to assess whether the average rate of uplift since the late Pliocene was significantly slower than the average rate of incision of the upper Thames over the past 700,000 years.  This comparison will provide a test for a recent theory that the uplift of Britain and other regions around the North Atlantic during the later Quaternary is due to plastic flow and thickening of the lower crust in response to the repeated changes in loading caused by rapid melting of ice sheets.  This theory predicts that uplift (and therefore river incision in response to it) should have speeded up during the last 700,000 years compared with the average for the longer period we hope to date.  Much depends on the dedication and skills of our colleagues at the CIAF, Angel Rodes and Delia Georghiou, as well as on the materials we have provided being suitable for producing precise enough dates.  Time will tell.


UCL Earth Sciences · Gower Street London WC1E 6BT · +44 (0)20 7679 2363
earthsci@ucl.ac.uk · more