Mg/Ca, Sr/Ca, and stable-isotope (DO and DC) ratio profiles from the fan mussel Pinna nobilis: Seasonal records and temperature relationships

[1] We present new annually resolved dO, dC, Mg/Ca, and Sr/Ca ratio records for two shells of the fast growing Mediterranean fan mussel Pinna nobilis, collected from proximal Spanish coast sea grass meadows. The relationship between the potential geochemical proxies and ontogenetic and environmental controlling factors is investigated. Specifically, the use of shell Mg/Ca and Sr/Ca ratios as potential calcification temperature proxies, the latter calculated from measured shell dO values, has been assessed. The dO cycles along the growth axis indicate that our P. nobilis specimens are 10.5 and 4.5 years old Shell Sr/Ca ratios do not exhibit any consistent interannual cyclicity and are not correlated to temperature. A subtle ontogenetic effect on shell Mg/Ca ratios was observed during the first 4.5 years of recorded growth but was highly evident during the organism’s later growth years. In P. nobilis shells, different mechanisms influence ontogenetic variation in shell Mg/Ca and dO records. Shell Mg/Ca ratios from the first 4.5 years of growth correlate significantly to temperature, in a best fit relationship described by the equation Mg/Ca = 17.16 ± 1.95 * exp(0.022 ± 0.004 * T). P. nobilis shell Mg/Ca records therefore are a valid temperature proxy only during an early growth phase. For the same range of temperatures, shell Mg/Ca ratios in P. nobilis are approximately 1/3 lower than those reported for inorganic calcite but 3 to 4 times higher than in another bivalve species, Mytilus trossulus, and 4 to 16 times higher than in foraminifera. We suggest these offsets are due to a higher degree of similarity between seawater and calcification-fluid composition in P. nobilis than in other bivalves and foraminifera. The observed shell Mg/Ca ratio change per C of 2.2% also is lower than that observed for inorganic and other biogenic calcites. Our findings strongly support taxonand species-specific Mg/Ca–temperature relationships for bivalves and other calcifying organisms. An appreciation of the physiology and calcification mechanisms of any biogenic carbonate archive therefore is paramount prior to the application of stable-isotope and element/Ca ratio proxies for paleotemperature reconstructions.