ABSTRACT

In aragonitic coral skeletons, the Na/Ca molar ratio is clearly higher than any other minor- or trace-element/Ca molar ratio (i.e., Na/Ca > Sr/Ca ≥ S/Ca ≥ F/Ca ≈ Mg/Ca). However, little attention has been paid to the Na/Ca ratio and there is little discussion of its variability and controlling factors. In this paper, we present monthly-to-bimonthly resolved Na/Ca, Sr/Ca and Mg/Ca measurements of four Porites coral skeletal samples: two modern samples from Ishigaki Island in the Ryukyu Islands, one modern sample from Myrmidon Reef in the Great Barrier Reef, and one fossil sample (6750 ± 45 BP) from Okinawa Island in the Ryukyu Islands. The two samples from Ishigaki Island show different annual cycles of the Na/Ca ratio, while the other two samples exhibit irregular fluctuations in the Na/Ca ratio. These observations suggest that the Na/Ca ratio of coral skeletons is controlled by some factor(s) other than temperature, salinity, and seawater Na/Ca ratio. We also present a discussion about modes of Na, S and Mg incorporation into coral skeletons using published data and observations for abiogenic marine aragonites as well as coral skeletons. We suggest that the principal mechanism for the incorporation of Na, S and Mg into biogenic and abiogenic marine aragonites is ion incorporation into lattice defects and distortions and/or ion adsorption (including the adsorption at the aragonite-organic matter interface).