Magnetic characteristics and early diagenesis of Holocene sediments in the Zhujiang River Delta

Magnetic mineral diagenesis is an important early diagenetic process after the burial of sediments and its proper identification is the precondition of interpretations for the mineral magnetic properties in the sediments. This study carried out analyses of sedimentary facies, room temperature magnetic and thermomagnetic properties in a Holocene Core MZ collected in the Shunde Plain of the Zhujiang River Delta to identify the vertical changes in the assemblage of magnetic minerals, so as to explore the early diagenetic stages and possible linkage to the sedimentary facies. The results show that the Holocene sedimentary sequence of Core MZ includes tidal channel, embayment, and deltaic successions from bottom upward. The magnetic properties at room temperature lack correlation with sedimentary facies and demonstrate features of strong early diagenesis. In addition, the magnetic properties of the late Holocene sediments were strongly influenced by the human activities. The early diagenesis mainly includes the dissolution of magnetic minerals and the formation of authigenic pyrite. Greigite was also identified in the upper section of the delta-front succession and the bottom of embayment succession. The concentration of greigite increases with depth in the embayment succession. According to the magnetic mineral assemblages, we infer different formation mechanism of greigite in the two successions. We suggest that the greigite in the delta-front facies was formed in the sulfate reduction stage of early diagenesis, whilst it was formed in the anaerobic oxidation stage of methane in the embayment facies. These phenomena indicate that sedimentary environment has impacts on the early diagenetic stage of magnetic minerals by controlling the availability of organic matter and sulfate.