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, δ³⁰Si(OH)₄, 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 mmol L^-1, 300 m) and a relatively high (113 mmol L^-1, 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 δ³⁰Si(OH)₄ values of +1.66 per thousand for the low-concentration sample and +1.25 per thousand 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 (TM), Germany), the Nu Plasma MC-ICP-MS (Nu Instruments (TM), Wrexham, UK), and the Finnigan (TM) (now Thermo Fisher (TM), Germany) MAT 252 IRMS. Future studies analyzing δ³⁰Si(OH)₄ 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. (authors)