5:15 PM - 6:45 PM
[SEM13-P03] Characterizing Magnetic Properties among Different Organisms Forming Reef Limestone
Keywords:Limestone, Reef limestone, SQUID microscope
Radiocarbon and U/Th dating methods are commonly used to determine the age of reef limestone. However, incorporating paleomagnetic age information can contribute to a more robust chronological model. Furthermore, rock magnetic parameters obtained from reef limestone may be associated with climate changes, such as global warming and cooling, as well as sea level fluctuations. Reef limestone is made up of a variety of organisms, such as corals, coralline algae, and microorganisms, each with different calcification and sedimentation processes that significantly affect the incorporation and acquisition process of magnetic minerals. However, due to its weak remanent magnetization, measurements have been limited to bulk samples consisting of mixed organisms.
This study utilized Holocene reef limestone samples obtained from drilling cores on the main island of Okinawa. The samples were cut into discrete specimens approximately 3×3×2 mm in size, and their natural remanent magnetization (NRM) was measured using a WSGI ultrasensitive small-bore 3-component DC SQUID magnetometer. Magnetic mapping was conducted using a scanning SQUID microscope on thin sections prepared from the same areas where discrete samples were taken.
The small-bore SQUID magnetometer measurements showed that microbialite samples composed of microbially induced carbonates had a relatively strong natural remanent magnetization (NRM), while stepwise alternating field demagnetization experiments identified unstable magnetic field components (~30 mT). Some of the samples showed negative inclination, which is suspected to be originated from drilling induceed remanence. In contrast, coralline algae and coral samples had relatively weak NRM, with the remanences tending to decay linearly toward the origin, which is consistent with the expected inclination of the drilling site. Additionally, magnetic mapping using the scanning SQUID microscope revealed species-specific magnetic distributions. The microscope also identified minor magnetic field distributions caused by microbialites that secondarily filled within the coral framework.
This presentation will report on these findings.
This study utilized Holocene reef limestone samples obtained from drilling cores on the main island of Okinawa. The samples were cut into discrete specimens approximately 3×3×2 mm in size, and their natural remanent magnetization (NRM) was measured using a WSGI ultrasensitive small-bore 3-component DC SQUID magnetometer. Magnetic mapping was conducted using a scanning SQUID microscope on thin sections prepared from the same areas where discrete samples were taken.
The small-bore SQUID magnetometer measurements showed that microbialite samples composed of microbially induced carbonates had a relatively strong natural remanent magnetization (NRM), while stepwise alternating field demagnetization experiments identified unstable magnetic field components (~30 mT). Some of the samples showed negative inclination, which is suspected to be originated from drilling induceed remanence. In contrast, coralline algae and coral samples had relatively weak NRM, with the remanences tending to decay linearly toward the origin, which is consistent with the expected inclination of the drilling site. Additionally, magnetic mapping using the scanning SQUID microscope revealed species-specific magnetic distributions. The microscope also identified minor magnetic field distributions caused by microbialites that secondarily filled within the coral framework.
This presentation will report on these findings.