GEOCHEMISTRY AND RADIOMETRIC DATING OF A MIDDLE PLEISTOCENE PEAT

'Uranium, lead, and sulphur data for a Middle Pleistocene interglacial peat deposit from Norfolk, UK, suggest that uptake of these elements was synchronous and confined to a single early diagenetic episode, pro bably coeval with peat formation. Sulphur isotope data indicate that r educing conditions have been maintained within the deposit throughout its history. Both uranium and lead concentration profiles show a marke d discontinuity near the middle of the bed, probably indicating an env ironmental change, possibly emergence. The lead isotope data an compat ible with a single lead component below the discontinuity and two comp onents above. Groundwater is thought to be the dominant source of lead with an additional airfall component present in the upper peat. The u ranium and lead concentration profiles below the discontinuity and the sulfur isotope profile throughout the pear support the view that thes e elements were sequestered from upwelling groundwaters. The organic m aterial is particularly suitable for Th-230/U-238 dating because it co ntains a negligible allogenic mineral component and very low Th-232 ac tivity. A sequence of consistent ages through the peat profile (mean 3 17 +/- 14 ka) over a wide range of uranium concentrations (7-65 mg g(- 1)), strongly suggests that a discrete, short-lived, uranium-uptake ev ent has been dated and that subsequent differential isotopic migration has not occurred. One sample, from immediately below the discontinuit y, has an infinite apparent age, but there is strong evidence for sequ estration of uranium from the peat into adjacent wood fragments found along the discontinuity. Calculated initial U-234/U-238 values of 1.2- 1.3 support a groundwater origin for the uranium, rather than a marine origin resulting from a subsequent rapid transgression. The very rest ricted range of U/Pb ratios in the lower part of the peat bed, and the heterogeneity of the initial lead isotopic composition in the upper p art, preclude U-Pb isochron dating. Po-210 measurements (as a proxy fo r Pb-210) also indicate possible post-depositional migration of Rn-222 which, if active over a significant period, would bias any U-Pb age e stimate. The Th-230/U-238 ages are consistent with deposition during o xygen isotope Stage 9. Copyright (C) 1997 Elsevier Science Ltd.