Biological control of paleomagnetic remanence acquisition in carbonate framework rocks of the Tahiti coral reef

Abstract

Integrated Ocean Drilling Program (IODP) Expedition #310 recovered 632 meters of Post Last-Glacial-Max (Post-LGM) and late Pleistocene coral reef framework rocks (37 holes at 22 sites) from the current fore-reef slopes surrounding the island of Tahiti. The cores were collected primarily to constrain the timing and amplitude of rising sea levels associated with the end of the last global glaciation (10–18,000 YBP), to identify the climatic and environmental changes during that period, and to evaluate the reef response to those combined changes. The cores, which were collected in three regions (Faaa, Tiarei, and Maraa), are remarkable in that more than 60% of the total core volume is associated with microbialites rather than macroscopic coral or coralline algae assemblages. We present here paleomagnetic data from 316 oriented paleomagnetic samples collected from the Post-LGM framework rocks of the Maraa region (12 holes at 6 sites). 96% of our samples were recovered from microbialites that have filled the primary cavities of the coral/coralgal framework. All samples appear to accurately record geomagnetic field variability at Tahiti (average inclination = −30.6°, alpha 95 ~ 2.9°, site axial dipole inclination = −32.6°) and yet the samples are composed of ~ 99% precipitated calcium carbonate. Our rock magnetic data indicate that the NRM resides almost entirely in detrital titanomagnetite grains derived from the Tahiti volcanic edifice. We present magnetic and microscopic evidence that the natural remanent magnetization in the carbonate rocks was locked in through mediation of organic biofilms (mostly bacterial) that held the grains in place until they were trapped by carbonate precipitation.