Chemical labelling of oyster shells used for time - calibrated high - resolution Mg / Ca 1 ratios : a tool for estimation of past seasonal temperature variations 2

18 The geochemical compositions of biogenic carbonates is increasingly used for 19 paleoenvironmental reconstructions. The skeletal ! 18 O temperature relationship is dependent 20 on water salinity, so many recent studies have focused on the Mg/Ca and Sr/Ca ratios because 21 those ratios in water do not change significantly on short time scales. Thus, those elemental 22 ratios are considered to be good paleotemperature proxies in many biominerals, although their 23 use remains ambiguous in bivalve shells. Here, we present the high-resolution Mg/Ca ratios 24 of two modern species of juvenile and adult oyster shells, Crassostrea gigas and Ostrea 25 2 edulis. These specimens were grown in controlled conditions for over one year in two 26 different locations. In situ monthly Mn-marking of the shells has been used for day 27 calibration. The daily Mg/Ca ratios in the shell have been measured with an electron 28 microprobe. The high frequency Mg/Ca variation of all specimens displays good synchronism 29 with lunar cycles, suggesting that tides strongly influence the incorporation of Mg/Ca into the 30 shells. Highly significant correlation coefficients (0.70 < r < 0.83, p < 0.0001) between the 31 Mg/Ca ratios and the seawater temperature are obtained only for juvenile C. gigas samples, 32 while metabolic control of Mg/Ca incorporation and lower shell growth rates preclude the use 33 of the Mg/Ca ratio in adult shells as a paleothermometer. Data from three juvenile C. gigas 34 shells from the two study sites are selected to establish a relationship : ! T = 3.77Mg /Ca +1.88, 35 where T is in °C and Mg/Ca in mmol/mol. 36 37