Temperature dependency of oxygen isotope in Koshiki coral.
The temperature dependency of oxygen isotope (δ18Oc) in Koshiki coral was
examined by comparing with the local sea surface temperature (SST) and sea surface
salinity (SSS) during the period of 1986 to 2001 (Figure S1). The ferry-based
observation data are available during 1999 and 2008 for SST (Kagoshima Prefectural
Fisheries Technology Development Center) and during 1986 and 2001 for SSS (JODC:
Japan Oceanographic Data Center; Miyazawa et al., 2009). We estimated the SST
during the 1986 to 1999 using the relationship between ferry SST and satellite SST
(AVHRR 0.25 ° x 0.25 ° gridded SST; Reynolds, et al., 2007) (Estimated SST (°C) =
1.0966 * AVHRR SST – 3.0724, r2 = 0.97).
δ18O in biogenic carbonates in general reflects change in both SST and δ18O in
seawater (δ18Osw). We calculated the δ18Osw value using the relationship between SSS
and δ18O in seawater (δ18Osw) in the East China Sea (ECS) (Oba, 1990; δ18Osw = 0.203 *
SSS – 6.76). We applied the 0.27‰ for the correction value to convert δ18Osw value
from the VSMOW to VPDB scale (Hug, 1987). We obtained the following relationship
between temperature and aragonite-water fractionation by using the regression between
summer and winter SST extremes and seasonal δ18O extremes in Koshiki coral:
δ18Oc - δ18Osw = -0.21 * SST (°C) + 0.12 (r2 =0.93, p<<0.0001)
The temperature dependency of Koshiki coral δ18O is 0.21 ‰ per 1°C which agrees
with those of other tropical corals [Galapagos; McCohnnaughey, 1989, New Caledonia;
Quinn, et al., 1996] and high-latitude coral at temperate regions [Kochi; Yamazaki et al.,
2008, Amakusa; Omata et al., 2006], aragonite mollusks [Grossman and Ku, 1986], and
abiotic calcite [Kim and O'Neil, 1997] and aragonite [Kim et al., 2007] (Figure S3 and
Table S1). Corals living at near it’s latitudial limit have been known the possibility of a
decrease and/or cessation of skeletal growth in cold winter season [Fallon, et al., 1999].
Our results show that most values between the extremes are plotted above the regression
line. This observation is coincident with the previous investigations for coral
δ18O-temperature relationship at temperate regions in Japan [Kochi; Yamazaki et al.,
2008, Amakusa; Omata et al., 2006], suggesting that coral skeleton in Koshiki island is
growing faster in summer season than in winter. The exact growth mechanism and
model in Koshiki corals are still unknown and could not eliminate the possibility for
distorting the environmental signals on winter coral record, however, the winter δ18O
profile in Koshiki coral shows large amplitude of interannul variability, suggesting that
Koshiki coral might be adapting to growth the skeletons in colder temperature than
corals living in tropic to subtropical area.
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