GEOTRACES inter-calibration of the stable silicon isotope composition of dissolved silicic acid in seawater

The first inter-calibration study of the stable silicon isotope composition of dissolved silicic acid in seawater, δSi(OH), is presented as a contribution to the international GEOTRACES program. Eleven laboratories from seven countries analyzed two seawater samples from the North Pacific subtropical gyre (Station ALOHA) collected at 300 m and at 1000 m water depth. Sampling depths were chosen to obtain samples with a relatively low (9 μmol L, 300 m) and a relatively high (113 μmol L, 1000 m) silicic acid concentration as sample preparation differs for low- and high-concentration samples. Data for the 1000 m water sample were not normally distributed so the median is used to represent the central tendency for the two samples. Median δSi(OH) values of +1.66‰ for the low-concentration sample and +1.25‰ for the high-concentration sample were obtained. Agreement among laboratories is overall considered very good; however, small but statistically significant differences among the mean isotope values obtained by different laboratories were detected, likely reflecting inter-laboratory differences in chemical preparation including pre-concentration and purification methods together with different volumes of seawater analyzed, and the use of different mass spectrometers including the Neptune MC-ICP-MS (Thermo Fisher™, Germany), the Nu Plasma MC-ICP-MS (Nu Instruments™, Wrexham, UK), and the Finnigan™ (now Thermo Fisher™, Germany) MAT 252 IRMS. Future studies analyzing δSi(OH) in seawater should also analyze and report values for these same two reference waters in order to facilitate comparison of data generated among and within laboratories over time.