A rapid and precise method of establishing age model for coral skeletal radiocarbon to study surface oceanography using coupled X-ray photos and ICP-AES measurement
Miss Yuning Zeng1, Professor Yusuke Yokoyama2, Dr Shoko Hirabayashi1, Dr Yosuke Miyairi1, Dr Atsushi Suzuki2, Dr Takahiro Aze1
1Atmosphere and Ocean Research Institute, The University of Tokyo, , Japan, 2Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
The calcium carbonate skeletons of reef-building corals take up dissolved inorganic carbon from the ambient seawater and record 14C, which allows 14C in corals to be a sensitive indicator of vertical and horizontal advection of water masses.
To discuss 14C variability in corals, a high-precision age model is a critical component of the study. In previous coral-based investigations, coral skeletal bands are often used to establish age model. Summer and winter in each year can be distinguished from the dark and light coral bands on X-ray photo, because dark bands often reflect high-density growth in summer and vice versa.
However, the age model determination based on only coral bands counting may not be able to provide satisfactory precision. For example, Bautista VII et al. (2016) established the age model by coral growth band counting, though it was later testified to shift ~7 years from the actual ages according to their following analysis using X-ray computed tomography (Bautista VII et al., 2021).
Here we propose a rapid and precise method, coupling coral growth band counting using X-ray photo and Sr/Ca measurement using Inductively coupled plasma atomic emission spectroscopy (ICP-AES) to establish precise and high-resolution age model in coral-based study. Sr/Ca is a reliable proxy for sea surface temperature with insensitivity to variations in coral skeletal growth rate (Fallon et al., 2003; Hirabayashi et al., 2013; Kawakubo et al., 2014), so summer and winter (the timing with the highest and lowest temperatures) can be distinguished by peaks in Sr/Ca variation even when using corals with low growth rates.
We used the Porites sp. collected from Kikai Island, Japan. The subsampling of coral powder used in this study was conducted continuously at 400 µm interval along the major growth axis (i.e. 25 samples per year in average). Since our coral was subsampled at a fixed interval, each sample was corresponding to the position on the X-ray photo according to the distance from the top, so each peak in Sr/Ca matched the corresponding year.
Coral Sr/Ca were measured using ICP-AES (iCAP 6300 series, Thermo Fisher Scientific). In ICP-AES measurement, a pre-test was firstly conducted with standard sample Jcp-1 (Okai et al., 2002), to select a proper condition for Sr/Ca measurement using 100-150 μg coral powder.
Sr/Ca values in 1947-1982, which were obtained from this study by ICP-AES, were well correlated with those in Kawakubo et al. (2017) measured by both ICP-MS and LA-ICP-MS (a method which includes more information and has higher accuracy) with a correlation coefficient of 0.9488 (p＜0.001, n=72).
Combination of coral growth band counting and Sr/Ca ICP-AES measurement could be a lower-cost and high-precision alternative for establishing an age model in coral-based study.
ZENG Yuning is a Ph.D. student at the Graduate Program on Environmental Sciences, the University of Tokyo. Her research focuses on revealing radiocarbon variability recorded in coral from Kikai Island, Japan, to understand oceanography in the Kuroshio region and the North Western Pacific.