Sediment cores from the Galapagos Islands (Mangini et al., 1994), from MANOP Site H (Finney et al., 1984) and along the East Pacific Rise (Walter and Stoffers, 1985) show significant variations of the Mn/Fe ratios throughout the Late Quaternary. The similarity of the profiles could suggest variable supply of hydrothermal Mn to these sediments in the past. However, it is questionable to which extent diagenetic remobilisation of Mn has altered the primary signal in the sediments. Our study of two Mn-nodules from the Peru Basin aims to answer this question, because in contrary to sediments, remobilisation of Mn in nodules is obviously negligible. The manganese nodules 63KG and 62KD were dredged in the Peru Basin at 90°42'W, 6°45'S and 90°35'W, 6°43'S, respectively, in a water depth of approximately 4250 m. Both samples were investigated by thermal ionisation mass spectrometry (TIMS) and high resolution alpha spectrometry for uranium and thorium isotopes. The isotope concentrations for nodule 63KG (62KD) vary from 0.06-0.59 ppb (0.12-1.01) 230Th, 0.43-1.40 ppm (0.51-1.98) 232Th, 0.09-0.49 ppb (0.13-0.80) 234U and 1.66-8.24 ppm (1.95-13.47) 238U.
Nodule 63KG shows a relatively smooth exponential decrease of the 230Th-excess concentration from the surface to 25 mm depth yielding a mean growth rate of 110 mm per million years (Bollhöfer et al., 1996). Nodule 62KD has a mean growth rate of about 100 mm per million years. However, because of marked variations of the 230Th-excess concentration we estimated the age depth correlation of this nodule from the partial accumulation rates calculated via the standing crop of 230Th-excess. The fast growth rates together with the high Mn/Fe ratios accentuate the diagenetic origin of the nodules.
The profiles of the concentration of manganese and iron plotted versus the age reveal the same trend in the two nodules. Mn/Fe versus time shows high values during interglacial stages 5 and 7 and lower ones during glacial stages 4, 6 and 8 (Fig. 1). From the similar variations of Mn/Fe in the nodules we conclude that the observed pattern is a record of climatically controlled changes of Mn or Mn/Fe in deep water and that it is not accidental. Furthermore the trend of Mn/Fe in the nodules is comparable with the one in sediment cores from MANOP Site H and the Galapagos Islands (but obviously at different levels of Mn/Fe). This indicates that the features observed in the sediments are to a larger extent primary. Assuming a constant intensity of the hydrothermal source, lower Mn/Fe ratios in nodules and sediments suggest enhanced transport of Mn to more remote areas due to reduced ventilation of deep water during glacial periods.
Bollhöfer, A., Eisenhauer, A., Frank, N., Pech, D., Mangini, A., Geol. Rundsch. (1996, in the press).
Finney, B.P., Lyle, M.W. & Heath, G.R., Palaeoceanography 3, 169-189 (1988).
Mangini, A., Rutsch, H.J., Frank, M., Eisenhauer, A., Eckhardt, J.D., NATO ASI Series 117, 87-104 (1994).
Walter, P. & Stoffers, P., Mar. Geol. 65, 271-287 (1985).
Fig. 1: Mn/Fe versus the age of nodules 62KD and 63KG.