UTIG Brownbag Seminar
Unraveling Depositional and Diagenetic Signals in Magnetic Susceptibility in Ocean Drilling Records
By:
Steve Philips, UTIG postdoctoral fellow
Abstract
Magnetic susceptibility is a bulk measure that reveals variation in magnetic mineral content. High-resolution measurement of magnetic susceptibility in ocean drilling records reveals variability that can be attributed to primary depositional processes and/or secondary diagenetic processes that occur after deposition. I will discuss magnetic susceptibility measurements interpreted with integrated geochemical, physical property, and rock magnetic approaches in methane-bearing marine sediments along the Indian, Cascadia, and Japanese margins to reveal the mixed detrital and diagnetic signals contained in these records.
In the first study, using a multi-proxy approach, we investigated variation in productivity and weathering over the last 110,000 years in the northern Indian Ocean within the core Indian monsoon rainfall zone. These results reveal an increase in productivity due to reduced stratification and a decrease in weathering during the last glacial period due to a weakened summer monsoon. This work reveals a relationship between Zr/Rb and magnetic susceptibility that can be utilized to predict primary detrital magnetic susceptibility.
In the second example, we used an elemental analysis and rock magnetic approach to decouple detrital and diagenetic patterns in magnetic susceptibility at two sites in the Bay of Bengal and three sites along the Cascadia accretionary wedge. Each site reveals intervals of diagenetic loss in magnetic susceptibility that is balanced by an increase in sulfur due to dissolution of magnetite and precipitation of pyrite. The diagenetic loss of magnetic susceptibility is influenced by organic matter availability as well as upward methane flux.
In the third study, we used a rock magnetic approach to investigate the magnetic mineralogy in a deep sediment record down to ~2.5 km below the seafloor offshore northern Honshu, Japan. The magnetic susceptibility record reveals cm-scale increases that are likely the result of density sorting causing concentration of heavy minerals. The magnetic mineral assemblage is dominated by titanomagnetite with an increase in Ti-rich titanomagnetite associated with deeply buried (~2 km) coal beds. This change along the titanomagnetite solid solution series, may represent selective dissolution of Ti-poor, iron(III)-rich magnetite during microbial iron reduction since burial.
