A skeletal Sr/Ca record preserved in Dipsastraea (Favia) speciosa and implications for coral Sr/Ca thermometry in mid-latitude regions

Abstract

A core (900 mm long) of the scleractinian coral Dipsastraea (Favia) speciosa was collected from Iki Island (approximate to 33 degrees 48N), Japan, one of the highest latitude coral reefs known to exist at present, where winter monthly mean sea surface temperature (SST) drops to 13 degrees C. The Sr/Ca profile was constructed using a bulk sampling method for the uppermost 280 mm interval of the core, which grew between 1966 and 2007, to test whether it could act as a suitable proxy for SST in a harsh environmental setting where reef-building coral do not usually survive. The Sr/Ca-SST relationship derived from the annual Sr/Ca and SST extremes predicted the observed monthly averaged summer SST extremes within an error range of 1.1 degrees C (1 s.d., n=40). The obtained Sr/Ca-SST calibration was also found to be valid for subtropical Dipsastraea (Favia) corals, proving its broad applicability. However, low-amplitude winter peaks were observed in the slow-growing intervals, which we confirmed (using individual spot analysis along a continuous growth line) result from the mixing of theca grown at different times. Our bulk sampling approach, across multiple growth lines in the skeleton of D. (F.) speciosa, led to the mixing of asynchronous skeletal part. At the study site, D. (F.) speciosa grows continuously, even during the cold season, suggesting that the skeletal Sr/Ca obtained from specimens of D. (F.) speciosa can be used as an SST proxy in the northwest Pacific marginal seas.