Figure 4
From: A short-term in situ CO2 enrichment experiment on Heron Island (GBR)
Crustose coralline algae (CCA) dissolution rates and mineralogy.
A) CCA aragonite percentage with linear least squares correlation lines fitted using a model-2 geometric means technique59 (control R2 = 0.58, −0.06 pH treatment R2 = 0.89, −0.22 pH treatment R2 = 0.08). There is increasing weight loss with increasing % aragonite across all treatments however correlation is poor for the −0.22 pH treatment. B) mol % MgCO3 determined by x-ray diffraction (XRD) and by inductively coupled plasma atomic emission spectroscopy (ICP-AES) for four CCA dissolution samples. XRD mol % MgCO3 is determined by measuring the shift in the Mg-calcite peak relative to the calcite peak position. XRD mol % MgCO3 determines the Mg-calcite phase and allows for identification of other carbonate minerals present whereas ICP-AES measures the average mol % MgCO3 across all Mg-containing minerals. It is commonly assumed that CCA are high Mg-calcite16,60 and that the more Mg present the more susceptible the carbonate is to dissolution37. Contrary to this presumption, within the control and the −0.22 treatment the CCA samples with lower % Mg (measured with ICP-AES) had the higher weight loss (by a factor of 7). Detailed mineralogical analyses on the Mg-enriched CCA revealed that as well as Mg-calcite there is also dolomite (50 mol % MgCO3) and magnesite (100 mol % MgCO3) present46. Three samples also had aragonite present (5.6, 6.8 and 14.8 % respectively). These results suggest that for these CCA it is not the Mg-calcite mol % MgCO3 that exerts the greatest control on dissolution rates but the presence of dolomite and magnesite with aragonite adding a confounding factor.
