Mg isotope fractionation between aragonite and seawater

The detectable Mg isotope fractionation between corals and seawater and the long residence time of Mg in the ocean render it a potential thermometer to study sea surface temperature in the last 2000 years. To calibrate this thermometer, a series of inorganic precipitation experiments (free-drift experiments) in seawater (with two different Mg/Ca ratios) have been carried out from 25 to 55 oC over a range of precipitation rate at WHOI. In these experiments, results show the Mg isotope fractionation varies little with Mg/Ca ratios or with increasing precipitation rate, but decreases noticeably with increasing temperature showing a temperature sensitivity of c.a. 0.019‰/oC, slightly higher than those reported fractionation between calcite-water during speleothems formation (0.003-0.015 ‰/oC ). This temperature sensitivity is also consistent with that exhibited in a Porite coral record from Great Barrier Reef. Current high precision MC-ICP-MS techniques can detect this variation within 0.5-1.0 oC with 1sigma = 0.01-0.02 per mil analytical uncertainties, suggesting Mg isotope fractionation could extract precise SST from coral skeletons. Moreover, our precipitation experiments show that the Mg isotope composition of aragonite could be easily altered by co-precipitation of calcite and salt during early digenesis. A careful cleaning and precise element separation technique is required to apply this thermometer, and further testing on corals of different species is needed to reveal the ‘Vital effects’ on Mg isotope fractionation between corals and seawater.