Fig 1.
Effect of processes on the DIC chemistry of seawater.
Precipitation reduces DIC, alkalinity and hence pH. Titrant dosing after precipitation returns the DIC to the starting composition (black dot). CO2 invasion and outgas modify the DIC but not the alkalinity of the seawater. CO2 invasion reduces pH and triggers the dosing of titrants which return the seawater to the starting pH but at a different DIC and alkalinity composition. Contours join points of equal pH (NBS scale).
Table 1.
Chemistry of the waters used for precipitations.
[Ca2+] and [Mg2+] is estimated for artificial seawater (based upon composition) and measured (by ICP-OES) for natural seawater.
Fig 2.
The material is identified as CaCO3 based on the strong ν1 peak at ~1084 cm-1 and as aragonite based on the dual peak (v4) between 700–710 cm-1 [reference 30].
Fig 3.
Profiles of titrant volume dosed over time in duplicate unseeded experiments in artificial seawater.
a), b) Ω = 19.2 and c), d) Ω = 6.9. b) and d) showed expanded x axes to compare rates of dosing of 1–5 mLs of titrant between duplicates (T1 and T2, show data from two titrators). Measured [DIC] at the start and during 2 of the precipitations are overlaid onto the graphs.
Fig 4.
Titration profiles in seeded experiments with no added aspartic acid.
Black lines show observed profile and red dotted lines show predicted profile as explained in text. N.B. the x- and y-axes are not to the same scale for all the graphs.
Fig 5.
Predicted titrant dosing profiles assuming that all dosing replaces ions consumed in the epitaxial growth of CaCO3 over the seed.
In this example we assume a precipitation rate of 617 μmol m-2 h-1 onto a seed of surface area of 4.2 m2 g-1 using different masses of seed and 0.6 M titrants.
Fig 6.
Estimated a) aragonite precipitation rate and b) lag period at start of precipitation in experiments comparing Ω, starting seed mass and different waters. ASW = artificial seawater. Nat. = natural seawater. Error bars indicate 1 σ of duplicate precipitations. The reproducibility of estimated precipitation rates from duplicate experiments was typically 6% and was always <10%. (Reproducibility of lag periods was always better than 57%).
Fig 7.
Peak centre and FWHM of the ν1 peak in the Raman spectra of precipitates containing different proportions of seed and in vitro precipitate.
a) Peak centre and b) Full width half maxima. The signatures of the seed are shown by the point at 100% seed. Points are means of 10 spectra and error bars are 1 standard deviation.
Fig 8.
a) Aragonite precipitation rates from natural and artificial seawater in the presence of a range of concentrations of aspartic acid at Ω = 11.2. Precipitations were conducted in duplicate; points indicate mean rate and error bars indicate 1 standard deviation. The error bars are usually smaller than the symbols. b) FWHM and c) peak position of the v1 peak in the Raman spectra of aragonites precipitated in the presence of aspartic acid. Points are means of 10–11 spectra and error bars are 1 standard deviation.
Fig 9.
Scanning electron micrographs of aragonite precipitated at Ω = 11.2.
a) without aspartic acid and b) with 8.7 mM aspartic acid. Scale bars are 1 μm.