Early Diagenetic Effects on Magnetic Properties in Laminated Sediments from the Southern Gulf of California

Abstract:

Effects of early diagenesis 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 paleomagnetic record. In this study, we analyze the mineral magnetic properties and downcore magnetite contents on Late Holocene sediments from Alfonso Basin, Gulf of California. Volcanic-derived terrigenous and biogenic input represents major sediment sources, with low-oxygen environments allowing preservation of finely laminated sequences. Susceptibility logs show high values at the top sediments and small amplitude low frequency fluctuations downcore. Magnetic hysteresis loops indicate low coercivity saturation characteristic of magnetites and low-Ti titanomagnetites, with varying paramagnetic contributions. Intensity of natural remanent magnetization, saturation isothermal magnetization and coercivity parameters show similar patterns with depth. In particular, the susceptibility log shows magnetic enhancement in the upper sediments of up to 4 times the values along the core. Below 40 cm depth the exponential decay relates to diagenetic changes and magnetite authigenesis associated with organic matter oxidative decomposition during dewatering and compaction occurring above the iron reduction zone. Results support presence of recent and old dissolution fronts resulting from preservation of enriched PSD/MD magnetite in between the dissolution fronts. Oxidation/reduction conditions in turn relate to climatic and water/sediment interface factors that control dissolution processes, which occur in sub-oxic to anoxic conditions.