Magnetic mineral diagenesis in anoxic laminated sediments from the Southern Gulf of California

Abstract

Diagenetic effects on the magnetic mineralogy in marine sediments have long been investigated, including oxidation/reduction reactions, magnetic dilution, formation of iron sulfides and oxides, magnetization acquisition mechanisms and reliability of the paleomagnetic record. This study investigates diagenetic effects in low-oxygen depositional environments characterized by recent and past magnetic mineral dissolution zones. We analyze a marine sequence from the Alfonso Basin in the southern Gulf of California in which the topmost sediments show diagenetic effects marked by high magnetic enhancement factors. The susceptibility logs show high values at the top sediments with well-defined small amplitude low frequency fluctuations down core. Magnetic hysteresis loops indicate low coercivity saturation, characteristic of magnetites and low-tititanomagnetites with varying paramagnetic contributions. Intensity of natural remanent, isothermal and anhysteretic magnetizations and coercivity parameters show similar variation patterns with depth. The anhysteretic remanence intensity-susceptibility ratio shows an inverse correlation to magnetic susceptibility, indicating varying concentration of fine grained single domain and superparamagnetic particles. The magnetic logs record diagenetic changes and magnetite authigenesis, with preserved recent and old dissolution zones marked by enriched single-domain/pseudo-singledomain/multi-domain magnetite in between the dissolution fronts. The oxidation/reduction processes relate to climatic and water/sediment interface factors controlling the dissolution processes, which occur in the Alfonso Basin anoxic conditions.