The geochemical compositions of biogenic carbonates are increasingly
used for palaeoenvironmental reconstructions. The skeletal delta O-18
temperature relationship is dependent on water salinity, so many recent
studies have focused on the Mg/Ca and Sr/Ca ratios because those ratios
in water do not change significantly on short time scales. Thus, those
elemental ratios are considered to be good palaeotemperature proxies in
many biominerals, although their use remains ambiguous in bivalve
shells. Here, we present the high-resolution Mg/Ca ratios of two modern
species of juvenile and adult oyster shells, Crassostrea gigas and
Ostrea edulis. These specimens were grown in controlled conditions for
over one year in two different locations. In situ monthly Mn-marking of
the shells has been used for day calibration. The daily Mg/Ca.ratios in
the shell have been measured with an electron microprobe. The high
frequency Mg/Ca variation of all specimens displays good synchronism
with lunar cycles, suggesting that tides strongly influence the
incorporation of Mg/Ca into the shells. Highly significant correlation
coefficients (0.70<R<0.83, p<0.0001) between the Mg/Ca ratios and the
seawater temperature are obtained only for juvenile C. gigas samples,
while metabolic control of Mg/Ca incorporation and lower shell growth
rates preclude the use of the Mg/Ca ratio in adult shells as a
palaeothermometer. Data from three juvenile C. gigas shells from the two
study sites are selected to establish a relationship: T = 3.77Mg/Ca +
1.88, where T is in degrees C and Mg/Ca in mmol/mol. (c) 2012 Elsevier
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