Corals at their latitudinal limits: laser ablation trace element systematics in Porites from Shirigai Bay, Japan

Abstract

The rapid analytical technique of laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) was used to measure the trace elements B, Mg, Sr, Ba and U in a high-latitude coral colony ( Porites lobata) taken from Shirigai Bay, Japan (32°N). A wide range of sea surface temperatures (SSTs 14.5–28°C) and upwelling events influenced this coral. Cold winter SSTs caused a decrease and/or cessation of skeletal extension. Measurements of U/Ca and Sr/Ca indicate an approximately linear response to SSTs above 18°C and a non-linear response below 18°C. Mg/Ca and B/Ca measurements both showed annual cycles broadly consistent with SST variations, but also exhibited intra-annual fluctuations not associated with temperature, suggesting that the incorporation of Mg and B into the coral skeleton was not simply regulated by temperature. It is shown that Ba/Ca ratios provide a proxy for wind-induced seasonal upwelling. This is inferred from the strong correlations between the strength of zonal winds ∼1 month prior to the SST minimum and the Ba/Ca maximum. Secondary upwelling events occurred during the summers of 1982, 1987, 1991 and 1992. These summers were cooler than average and were associated with El Niño Southern Oscillation events.