Preliminary Studies of Manganese-Rich Carbonate Nodules from Leg 68, Site 503, Eastern Equatorial Pacific

Nodules occur in the siliceous calcareous ooze and siliceous marl at Site 503 in the eastern equatorial Pacific.They are present below a depth of about 11 meters throughout the green-colored reduced part of the section down to 228 meters, although they are most abundant between 30 and 85 meters.They are cylindrical or barrel-shaped, up to 70 mm long, and usually have an axial channel through them or are hollow.They appear to have formed around and/or within burrows.XRD studies and microprobe analyses show that they are homogeneous and consist of calcian rhododrosite and minor calcite; Mn is present to the extent of about 30%.Isotopic analyses of the carbonate give carbon values which range from -1.2% to -3.8% 0 , and oxygen isotope compositions vary from + 4.0% 0 to + 6.0%.These values are different from those for marine-derived carbonates as exemplified by the soft sediment filling of a burrow: δ 13 C, -0.26%; δ 18 θ, + 1.05%.The carbon isotope data indicate that carbonate derived (possibly indirectly) from seawater was mixed with some produced by organic diagenesis to form the nodules.The δ 18 θ values suggest that although they formed near the sediment surface, some modification or the introduction of additional diagenetic carbonate occurred during burial.