THORIUM AND URANIUM ISOTOPES IN A MANGANESE-NODULE FROM THE PERU BASIN DETERMINED BY ALPHA-SPECTROMETRY AND THERMAL IONIZATION MASS-SPECTROMETRY (TIMS) - ARE MANGANESE SUPPLY AND GROWTH-RELATED TO CLIMATE

Thorium- and uranium isotopes were measured in a diagenetic manganese nodule from the Peru basin applying alpha- and thermal ionization mass spectrometry (TIMS). Alpha-counting of 62 samples was carried out wit h a depth resolution of 0.4 mm to gain a high-resolution Th-230(excess ) profile. In addition, 17 samples were measured with TIMS to obtain p recise isotope concentrations and isotope ratios. We got values of 0.0 6-0.59 ppb (Th-230), 0.43-1.40 ppm (Th-232), 0.09-0.49 ppb (U-234) and 1.66-8.24 ppm (U-238). The uranium activity ratio in the uppermost sa mples (1-6 mm) and in two further sections in the nodule at 12.5+/-1.0 mm and 27.3-33.5 mm comes close to the present ocean wa ter value of 1.144+/-0.004. In two other sections of the nodule, this ratio is sign ificantly higher, probably reflecting incorporation of diagenetic uran ium. The upper 25 mm section of the Mn nodule shows a relatively smoot h exponential decrease in the Th-230(excess) concentration (TIMS). The slope of the best fit yields a growth rate of 110 mm/Ma up to 24.5 mm depth. The section from 25 to 30.3 mm depth shows constant Th-230(exc ess) concentrations probably due to growth rates even faster than thos e in the top section of the nodule. From 33 to 50 mm depth, the growth rate is approximately 60 mm/Ma. Two layers in the nodule with distinc t laminations (11-15 and 28-33 mm depth) probably formed during the tr ansition from isotopic stage 8 to 7 and in stage 5e, respectively. The Mn/Fe ratio shows higher values during interglacials 5 and 7, and low er ones during glacials 4 and 6. A comparison of our data with data fr om adjacent sediment cores suggests (a) a variable sb supply of hydrot hermal Mn to sediments and Mn nodules of the Peru basin or (b) suboxic conditions at the water sediment interface during periods with lower Mn/Fe ratios.