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Abundant Fe-Mn carbonate concretions (mainly siderite, manganosiderite, and rhodochrosite) were found in the hemipelagic claystones of Site 603 on the eastern North American continental rise. They occur as nodules, micronodules, or carbonate-replaced burrow fills and layers at a subbottom depth of between ~ 120 (Pliocene) and 1160 m (Albian-Cenomanian). In general, the Fe-Mn carbonate concretions form from CO3"~ produced by the microbiological degradation of organic matter in the presence of abundant Fe + or Mn + and very low S~ ~ concentrations. However, there is also some evidence for diagenetic replacement of preexisting calcite by siderite. The carbon isotope composition of diagenetic Fe-Mn carbonate nodules is determined by CO2 reduction during methanogenesis. Carbonate nodules in Cretaceous sediments at sub-bottom depths of 1085 and 1160 m have distinctly lower δC values (– 12.2 and 12.9%o) than Neogene siderites, associated with abundant biogenic methane in the pore space (-8.9 to 1.7%o between 330 and 780 m depth). Since no isotopic zonation could be detected within individual nodules, we assume that the isotopic composition reflects more or less geochemical conditions at the present burial depth of the carbonate nodules. Carbonates did not precipitate within the zone of sulfate reduction (approximately 0.01 to 10 m), where all of the pyrite was formed. The oxygen isotope composition indicates precipitation from seawater-derived interstitial waters. The δ 1 8 θ values decrease with increasing burial depth from + 5.1 to 1.2%o, suggesting successively higher temperatures during carbonate formation. INTRODUCTION Deep-sea drilling data have revealed that siderite is quite common in rapidly deposited, organic-matter-rich, hemipelagic mudstones and black shales along the North Atlantic continental margins. For example, siderite has been reported from the mid-Cretaceous black shales and Miocene claystones of Sites 105 and 101 (Leg 11, Lancelot et al., 1972; Jansa et al., 1979), from Cenomanian black, zeolitic, carbonaceous shales of Site 144 (Leg 14, Berger and von Rad, 1972), from the Hauterivian organic-matter-rich prodelta mudstones of Site 397 (Leg 47A, Einsele and von Rad, 1979), and from the Neogene hemipelagic claystones of Site 388 (Leg 44; Benson, Sheridan, et al., 1978; Asquith, 1979). Wada et al. (1982) described calcite, magnesian calcite, dolomite, and rhodochrosite nodules, formed in the methane-rich layer beneath an H2S/methane layer in the Middle America Trench area in the eastern Pacific (Leg 66), and Wada and Okada (1983) the same type of authigenic carbonates from the Japan Trench (Leg 56). The occurrence of abundant, complex Fe-Mn carbonate (siderite, manganosiderite, rhodochrosite) nodules, burrow fills, and layers in the middle Miocene to Pliocene hemipelagic claystones of lithostratigraphic Subunits 1 van Hinte, J. E., Wise, S. W., Jr., et al., Init. Repts. DSDP, 93: Washington (U.S. Govt. Printing Office). 2 Addresses: (von Rad) Bundesanstalt für Geowissenschaften und Rohstoffe, Postfach 51 01 53, D-3000 Hannover 51, Federal Republic of Germany; (Botz) Geologisch-Palàontologisches Institut, Universitàt Kiel, Olshausenstr. 40, D-2300 Kiel, Federal Republic of Germany. IA-IC was a spectacular discovery at Site 603, drilled on the outermost continental rise off Cape Hatteras, North Carolina. Similar Fe-Mn carbonate concretions were also found in the Eocene radiolarian claystone, in the Coniacian to Maestrichtian variegated claystone, and in the Aptian-Turonian black carbonaceous claystone (see Fig. 1). In general, siderite (FeCO3) is precipitated during "early" diagenesis at slightly negative Eh (about 0.2 to 0.4 V), at intermediate pH (6 to 8), and in the presence of abundant Fe + and high pCO2 (partial pressure of CO2), but very low S~ ~ concentrations in the pore water (Garrels and Christ, 1965; Berner, 1971). Siderite forms at low Eh and moderate to low pS~~ (Fig. 2). In marine sediments, low Eh conditions are normally not associated with high pS~~, (i.e., low S~~ concentration in pore water), because low Eh resulting from anaerobic, bacterial decomposition of organic matter almost always includes the reduction of sulfate to H2S and the precipitation of pyrite (Berner, 1971). However, below the zone of sulfate reduction, high pS~ ~ can be expected. Thus, siderite may form in marine sediments below the zone of sulfate reduction, if the pCO2 is high enough (Fig. 3). In Hole 603C, siderite starts to appear below a sub-bottom depth of around 120 m, where the early diagenetic formation of pyrite related to sulfate reduction has probably been completed. Authigenic rhodochrosite (MnC03) is the pelagic carbonate most resistant to solution. It commonly occurs in sediments deposited slightly below the calcite compensation depth (CCD) in which oxidized (reddish brown)

Authigenic Fe-Mn Carbonates in Cretaceous and Tertiary Sediments of the Continental Rise off Eastern North America, Deep Sea Drilling Project Site 603