Magnetic Diagenesis, Organic Input, Interstitial Water Chemistry, and Paleomagnetic Record of the Carbonate Sequence on the Ontong Java Plateau

Abstract

Shipboard paleomagnetic studies on Ocean Drilling Program Leg 130 were plagued by a severe reduction in the remanence intensity, which affected sediments below a sharply defined onset at sub-bottom depths of 50 mbsf or less. This loss of intensity was accompanied by a reduction in magnetic stability and consequent loss of polarity and paleolatitude information. Viscous remanent magnetization and drilling-induced remanences contributed high-coercivity overprints, further obscuring the greatly diminished primary magnetization. The depth at which the reduction in intensity occurred correlates with the organic carbon content of the sediments, and a further relationship is present between intensity downhole and the level of sulfate reduction. This implies that loss of intensity is related to the microbial reduction of magnetite. Reduction of sulfate results in the generation of a magnetic iron sulfide, which appears to be the carrier of the high-coercivity overprints. Oxidation and drying of the sediment after sampling leads to partial destruction of this sulfide, and complete decomposition occurs at temperatures below 300°C. These features identify the magnetic sulfide as the metastable phase, greigite. Varying inputs of organic material in past times, presumably reflecting both latitude and climatic changes, resulted in varying degrees of reduction, which are now reflected in differences in magnetic intensity, degree of overprinting, and survival of primary magnetization. The highest reduction levels result in the almost total dissolution of magnetite, and the weak surviving remanence is dominated by greigite. At lower reduction states, multidomain magnetite persists together with greigite. Still-less-reducing conditions in turn cause the survival of fine-grained, single-domain magnetite, and then of hematite.An earlier paleomagnetic study of Ontong Java Plateau sediments from Deep Sea Drilling Project Leg 30 produced a paleolatitude curve that differed significantly from other records of Pacific paleolatitude. It is likely that the samples in this earlier study also carried high-coercivity overprints, which biased the results. Those samples from Leg 130 that appear to preserve a primary magnetization yielded paleolatitudes consistent with expected Pacific Plate motion.

Stylolites, porosity, depositional texture, and silicates in chalk facies sediments. Ontong Java Plateau – Gorm and Tyra fields, North Sea

“…The decline is probably partly due to sulphate-reducing bacteria giving rise to the formation of pyrite. Fe for the pyrite is partly derived from bacterially reduced magnetite (Musgrave et al, 1993) and partly derived from the clay particles of the green colour bands, which may turn purple due to colouring by disseminated pyrite (Lind et al, 1993). The content of dissolvable Fe in the sediment was found to be below 0AE001%.…”

Early Cretaceous Magnetostratigraphy and Paleolatitudes from the Mid-Pacific Mountains: Preliminary Results Bearing on Guyot Formation and Pacific Plate Translation

“…First, pyrite was observed in several cores, indicating potential complications posed by reduction diagenesis. The effect of reduction diagenesis on magnetization properties, first documented in hemipelagic regions (Karlin andLevi, 1983, 1985;Karlin, 1990;Leslie et al, 1990), has more recently been noted in pelagic sediments (Sager, 1988), including those having very high carbonate values (Musgrave et al, 1993) similar to those of Site 866. The presence of pyrite implicates sulfate reduction and the production of H 2 S, which combines with ferric iron in the magnetite lattice (Canfield and Berner, 1987).…”

Explanatory Notes

“…IRM was imparted to samples in applied fields increasing in steps up to 1.2 T. Stepwise IRM acquisition can be used to distinguish between magnetite, which is saturated in applied fields greater than about 200 mT (Banerjee and O'Reilly, 1967;Ishikawa, 1967), and hematite, which commonly remains unsaturated in fields of 1.0 T and more. Magnetic iron sulfides (pyrrhotite and greigite) may also remain unsaturated until above 0.2 T (Kligfield and Channell, 1981;Musgrave et al, 1993). IRM acquisition may also distinguish the contribution of differing magnetic carriers or differing magnetic grain sizes over the coercivity interval less than 200 mT.…”