Measuring production—dissolution rates of marine biogenic silica by ³⁰Si‐isotope dilution using a high‐resolution sector field inductively coupled plasma mass spectrometer

Abstract

Regional and seasonal variability of the Si dissolution:production ratios in the surface ocean have not been well assessed. Here, we propose a new method for determining these rates, using the 30 Si-isotopic dilution technique with a high-resolution sector field inductively coupled plasma mass spectrometer (HR-SF-ICP-MS). Relative analytical precision of the isotopic measurement is better than 1%, similar to that obtained by thermal ionization-quadrupole mass spectrometry (TIMS). Accuracy and reproducibility of the isotopic measurements have been checked on artificial and natural solutions by intercomparison between two HR-SF-ICP-MS instruments and one TIMS. Measurements of real Si production and dissolution rates are illustrated for two contrasted situations with an average relative precision of 10%, including one from waters with low Si content (2 µmol L -1 ), which required an additional purification step by cation exchange chromatography. Si production rate from this later incubation was not significantly different from the one measured by radioactive 32 Si. The new method is faster and simpler than TIMS or isotope ratio mass spectrometry (IRMS). Its sensitivity is more than one order of magnitude better than TIMS, and it can cover the whole range of Si concentrations encountered in the ocean.

Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches

“…The isotopic dilution technique adapted by Fripiat et al (2009) from Corvaisier et al (2005) is used to simultaneously determine the rates of Si uptake (i.e., silica production) and of biogenic silica dissolution in the same seawater sample. After spiking with a solution enriched in 30 Si followed by incubation of the samples, the production rate is estimated from the change in isotopic composition of the particulate phase (increase in 30 Si).…”

“…For each depth, 5 L of seawater were sampled. One liter was subsampled to obtain a natural silicon isotopic standard (i.e., not spiked with 30 Si) to be processed along with the samples to correct for the matrix effect and mass bias during isotopic analysis (Fripiat et al, 2009). These unspiked samples were immediately filtered on 0.8 µm Nuclepore polycarbonate membranes to separate biogenic silica from silicic acid.…”

“…Preconcentration of H 4 SiO 4 in the seawater samples (for both production and dissolution measurements) was applied on board to increase the Si : salinity ratio because the maximum salinity of the solution that can be introduced into the mass spectrometer is about 2 ‰ (Fripiat et al, 2009). This step was achieved using a protocol adapted from the MAGIC method (Karl and Tien, 1992;Reynolds et al, 2006).…”

“…-compare the Si cycle dynamics in contrasting productive environments such as the southeastern Kerguelen Plateau bloom, the northeastern Kerguelen bloom in a stationary meander southward of the PF and the warmer waters located north of this front, relative to the upstream HNLC area southwest of the Kerguelen Islands, and identify controlling processes -determine and quantify the seasonal evolution of processes which drive the Si biogeochemical budget in the upper layer of the Kerguelen Plateau, using the 30 Si stable isotope method (Nelson and Goering, 1977a;Fripiat et al, 2009) applied during KEOPS-2 and other techniques of mass and isotopic balance used during KEOPS-1 ( 32 Si radiogenic tracer incubations, Mosseri et al, 2008; and natural silicon isotopic composition, δ 30 Si, Fripiat et al, 2011a), in order to fully characterize the silicon cycle above the Kerguelen Plateau -discuss the potential role of Fe in silica production/dissolution and in Si : N uptake ratios in the context of the silicic acid leakage hypothesis (Brzezinski et al, 2002;Sarmiento et al, 2004) -finally, compare our data set with previous results in other productive regions of the global ocean and discuss the different types of diatom-dominated regimes 2 Material and methods…”

Stable silicon isotope analysis on nanomole quantities using MC-ICP-MS with a hexapole gas-collision cell

“…Currently, studies on variations in stable Si isotope ratios are focused on low-temperature processes such as biogenic opal formation, [11][12][13][14] clay formation 15 and chemical weathering processes in groundwater aquifers. 16,17 Opal formation in the ocean is dominated by diatom assimilation of dissolved silicate from the surrounding aqueous phase to build up their silicified cell wall, a process which occurs in micromole Si concentration environments.…”