ABSTRACT
Results from R.V. Sonne cruises S0-79 (1992) and S0-106 (1996) are presented together with a review of older data from the Peru Basin. The extended nodule field of the Peru Basin is situated at the southern margin of the equatorial maximum of bioproduction where accumulation rates of organic carbon are relatively high, which enhances diagenetic growth of nodules. The maximum abundance of nodules (kg/m2) occurs at around 4,150 m water depth. Here a maximum of Corg in surface sediments is responsible for high nodule growth rates up to 250 mm/Ma, while mainly hydrogenetic growth in more elevated positions is characterized by accretion rates up to 50 times lower. However, due to a steep geochemical gradient between the sediment surface and a redox boundary at 5 to 10 cm depth, accretion rates of large nodules (up to 21 cm maximum diameter) show great differences between their bottoms and tops which is not observed in small nodules. Nodule size strongly determines the accretion rate, the type of internal growth, and the composition due to the different depth of immersion in sediment of large and small nodules. The layered character of nodules is mainly due to repeated biogenic lifting. Pulses of these movements are much more frequent than climate-induced oscillations, which may be superimposed on the bioturbation-induced microlayered growth pattern. Large nodules in basins tend to grow asymmetrically, and as soon as they become stuck in the stiff sediment below the redox boundary, they become buried. Parts of the nodules buried in the suboxic sediment below the redox boundary dissolve.
