Paleomagnetism of a Sediment Core Taken from the Ontong-Java Plateau:
for Better Understanding of the Role of Biogenic Magnetite in
Geomagnetic Paleointensity Recording
Abstract
Marine sediments contain considerable amounts and different types of
magnetic mineral particles. Magnetic minerals in sediments may be
statistically aligned to the direction of the ambient geomagnetic field
so that sediments potentially preserve geomagnetic intensity records in
the past. However, different types of magnetic minerals should preserve
the remanent magnetization in different manners. And the compositional
variation of magnetic mineral assemblages in marine sediments may hinder
us from extracting reliable geomagnetic paleointensity records. We
studied a sediment core taken from the Ontong-Java plateau, from which
relative paleointensity (RPI) variations were estimated. The magnetic
mineral assemblages of the sediment core are principally a two-component
mixture of terrigenous and biogenic magnetite. So it provides an
opportunity to assess the influence that compositional variations in
marine sediments could bring to RPI estimations and thus to distinguish
different contributions of the biogenic and terrigenous components to
RPI recording in marine sediments. RPI obtained by normalizing natural
remanent magnetization (NRM) with anhysteretic remanent magnetization
(ARM) shows downcore decreases, and it has an inverse correlation with
the ratio of ARM susceptibility (kARM) to saturation
isothermal remanent magnetization (SIRM) (kARM/SIRM).
This indicates that the RPI signal becomes apparently weaker with
increasing proportion of biogenic magnetite. Moreover, NRM-ARM
demagnetization diagrams show concave-down curvature, which indicates
that the coercivity distributions of NRM and ARM are different. If we
assume that the magnetization of the higher coercivity interval is
mainly carried by biogenic magnetite while that of the lower coercivity
interval is mainly carried by the terrigenous component, RPI recording
efficiency of the biogenic component may be lower than that of the
terrigenous component. The validity of this assumption was investigated
by first-order reversal curve (FORC) measurements, transmission electron
microscope (TEM) observations, low-temperature measurements, and
extraction of silicate-hosted magnetic inclusion from the sediments, and
the results proved that NRM of the higher coercivity interval is carried
mainly by biogenic magnetite. But our conclusion contradicts with some
previous studies using a similar method, which suggested higher RPI
recording efficiency of the biogenic magnetic component than the
terrigenous component [Ouyang et al., 2014; Chen et al., 2017].
Different concentrations of silicate-hosted magnetic inclusions due to
different sedimentary environments might be a possible reason for the
contradiction. The contribution of silicate-hosted magnetic inclusions
to the magnetization is minor in our sediments (less than
~7% of SIRM). This contradiction remains to be studied
further. Keywords: geomagnetic paleointensity, silicate-hosted magnetic
mineral inclusion, biogenic magnetite, Ontong-Java plateau