A record of changing redox conditions in the northern Pem Basin during the Late 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 thermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium. The TIMS concentrations for nodule 62KD (63KG) vary as follows: 0.12-1.01 ppb (0.06-0.59) Th-230, 0.51-1.98 ppm (0.43-1.40) Th-232, 0.13-0.80 ppb (0.09-0.49) U-234, and 1.95-13.47 ppm (1.66-8.24) U-238. 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 ocean water value. Sections of fast nodule growth are related to those layers having high Mn/Fe ratios (up to 200) and higher densities, As a possible explanation we develop a scenario that describes similar glacial/interglacial trends in both nodules as a record of regional changes of sediment and/or deep water chemistry. (C) 1999 Elsevier Science B.V. All rights reserved.