Experimental data on olivine dissolution

In order to determine the dissolution kinetics of olivine in sea water, commercially available ultra-mafic sand (UMS, supplied by AdL Sand- und Wasserstrahltechnik) was permitted to react with artificial seawater (ASW, Millero et al., 2008) in 250 ml polyethylene (PE) batch reactors. Three different batches with variable amounts of UMS were prepared, each containing three replicates (n = 3) to verify the reproducibility of the experiments. The amount of ASW was 200 ml for all replicates and batches. 50 ml of ambient air were included to allow for equilibration with the atmosphere. The amount of UMS was varied from 20 g for Batch1 to 10 g for Batch2 and 5 g for Batch3 in order to investigate the effect of the solid/liquid ratio. The air was partly exchanged with the laboratory atmosphere every time samples were taken. All batch reactors with artificial seawater, UMS and gas phase were subjected to constant movement in HeidolphTM Reax2© over-head shakers at ~40 rpm. The experiment was conducted under controlled laboratory conditions (p = 1 bar, 21.5 °C < T < 23.9 °C). For each sample (23 in total), two aliquots (2 ml and 5 ml) were taken for further measurements and the same volume of artificial seawater was added to ensure a constant solid/liquid ratio. For the same reason it was made sure that no UMS grains were removed via sample taking. Sampling intervals increased from minutes to hours during day one to every two weeks between day 50 and day 134. The 5 ml aliquot was filtered through a 0.2 µm cellulose membrane filter and refrigerated in 5.2 ml ZinsserTM scintillation bottles. The 2 ml aliquot was used for direct pH measurements (see below) and not filtered to ensure a pristine chemical milieu. At the end of the experiments, the ASW in the batch reactors was carefully decanted over a 0.2 µm regenerated cellulose filter to recover the greyish suspended matter that had formed in the batch reactors during the experiment. Fine particles possibly stuck to the UMS grains were eluted by refilling the batch reactor with fresh ASW, shaking and decanting it. This procedure was repeated until the supernatant was clear. Withal, it was made sure that all precipitates, which left the bottles, were recovered on the filters. Subsequently, the wet cake was rinsed with pH neutral MilliQ water (MQe, 18Ω Ohm/cm) to elute dissolved species (e.g. salinity). Furthermore, the UMS used in Batch1 was recovered, very carefully rinsed with pH neutral MQe (to only elute salinity), and dried for scanning electron microscope energy-dispersive X-ray spectroscopy (SEM−EDX).The water samples were analyzed for pH following Dickson (1993). Total alkalinity was analyzed by titration with diluted hydrochloric acid (HCl) to an end point of pH = 4.5 (Stumm & Morgan, 1996; Gieskes et al., 1991) and element concentrations were determined using inductively coupled plasma optical emission spectrometry (ICP-OES).