The influence of alkalilnity and pCO₂ on CaCO₃ nucleation from variable composition phanerozoic seawater /
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis eBook |
| Language: | English |
| Published: |
[College Station, Tex.] :
[Texas A&M University],
[2010]
|
| Subjects: | |
| Online Access: | Link to OAK Trust copy |
| Abstract: | There is strong evidence that variations in seawater chemistry occurred during the Phanerozoic Eon. Of particular importance are the changes in the Mg²⁺/Ca²⁺ ratio because they have been attributed to the oscillations between "calcite" and "aragonite seas" over time. In addition to the Mg²⁺/Ca²⁺ ratio variations, there were also major changes in pCO₂ levels and alkalinity that could also affect the calcium carbonate (CaCO₃) polymorph that precipitates from seawater. Experiments were conducted in seawater where the initial alkalinity and pCO₂ levels were varied and then slow degassing of CO₂ resulted in a gradual increase of saturation state with respect to CaCO₃ and eventually nucleation. The pH was continually monitored throughout the experiments and it was used in combination with the initial alkalinity to calculate the pCO₂ and saturation state of aragonite ([omega]arag) at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively.It was observed that the initial alkalinity greatly affected the nucleation pCO₂ value and the CaCO₃ polymorph that was precipitated. In seawater with Mg2+/Ca2+=1.2 and ~10 mM alkalinity and a pCO2 below 2,500 [mu]atm, calcite that was overgrown with aragonite was the dominate polymorph nucleated, while pure aragonite precipitated when the pCO2 was above 2,500 [mu]atm. Seawater with Mg²⁺/Ca²⁺ =1.2 and a wide range of initial alkalinities (5-50 mM) produced variable results. Seawater with Mg²⁺/Ca²⁺ =1.7 produced only aragonite at lower alkalinities, but calcite was nucleated when the alkalinity and pCO₂ values were exceptionally high, typically above 11 mM. These results point to the need to also consider the effects of the carbonic acid system in the "critical" Mg²⁺/Ca²⁺ region of about 1 to 2 for "calcite seas" and "aragonite seas" at various times throughout the Phanerozoic Eon. |
|---|---|
| Item Description: | "Major Subject: Oceanography" Title from author supplied metadata (automated record created 2010-03-12 12:08:51). Electronic resource. |
| Physical Description: | 1 online resource. |
| Bibliography: | Includes bibliographical references. |