A record of changing redox conditions in the northern Pem Basin during theLate Quaternary deduced from Mn/Fe and growth rate variations in two diagenetic manganese nodules

Two diagenetic manganese nodules from the Peru Basin were investigated by t hermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium. The TIMS concentrations for nodule 62KD (63KG) var y as follows: 0.12-1.01 ppb (0.06-0.59) Th-230, 0.51-1.98 ppm (0.43-1.40) T h-232, 0.13-0.80 ppb (0.09-0.49) U-234, and 1.95-13.47 ppm (1.66-8.24) U-23 8. Both nodules have average growth rates of similar to 110 mm per million years. However, from the variations of excess Th-230 With depth we estimate partial accumulation rates which range from 50 to 400 mm per million years . The delta(234)U dating method cannot be applied due to remobilization of U from the sediment and subsequent incorporation into the nodules' crystal lattice, reflected by decay corrected delta(234)U values far above the ocea n water value. Sections of fast nodule growth are related to those layers h aving high Mn/Fe ratios (up to 200) and higher densities, As a possible exp lanation we develop a scenario that describes similar glacial/interglacial trends in both nodules as a record of regional changes of sediment and/or d eep water chemistry. (C) 1999 Elsevier Science B.V. All rights reserved.