Garlichs, Thorsten Uwe (2010) Early Cretaceous seawater conditions – pH reconstruction based on boron isotopes. (Bachelor thesis), Leibniz-Institut für Meereswissenschaften, SFB754, Kiel, Germany, 46, 15 pp.

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Abstract

Oceanc-Anoxic-Events (OAE’s) are causally related to increased pCO2-values in the atmosphere. The reconstruction of the pCO2 for the OAE 1a in the Early Cretaceous (ca. 118 million years before present) is the base of this study. A reconstruction was attempted with the help of the stable isotope system of the Boron isotopes 10B and 11B. Approximately the pH-values were determined and from this the pCO2 can be calculated with different boundary conditions. The aim of this study is the reconstruction of the pH, the water conditions of the Early Cretaceous before the beginning of the OAE, during the OAE and after the OAE. For this purpose the B/Ca- and the δ11B-ratios were determined with samples from a drilling core from Southern France. Samples were taken from carbonate layers. Measurements were accomplished with the Quadrupol multi-collector-inductively-coupled-plasma mass spectrometry (Q-MC-ICP-MS or Quadrupol) and the multi-collector-inductively-coupled-plasma–mass-spectrometry with a laser ablation (LA-MC-ICP-MS). The B/Ca-ratio was determined with the Quadrupol and the δ11B-ratio with the LA-MC-ICP-MS of the carbonate samples. The carbonate samples are “bulk carbonate” sample, where the carbonate not determined in detail.
The Results are in the range of 45.1 to 178.0 [μmol B / mmol Ca] for the B/Ca-ratio. For the δ11B-ratio the results are in the range of (-10.09) to (-7.08) [δ11B vs. NIST 612]. To reconstruct the paleo-pH the formula of Yu et al. (2010) for the B/Ca-ratio and the equation of Pearson et al. (2009) δ11B-ratio were used. With the measured results it was not possible to calculated the paleo-pH. In the case of the B/Ca-ratio the results of the calculation were mostly negative. The calculation with the δ11B was not possible, because the logarithm function is mathematical not possible with negative values.
Therefore the results show that a paleo-pH reconstruction is impossible with bulk carbonate samples. A reason could be that the used equations were established for foraminifera. Hence the bulk carbonate samples consist of different types of carbonate. The measured carbonate could not exactly appropriate to their source. Also the origin of the total-sample-material was not investigated. According to it the negative values of the δ11B give only an evidence to the continental origin. Besides the Al/Ca-ratio was measured, which shows high values. These values support the assumption, that silicate and clay are present among the carbonate matrix. Therefore is the use of Boron isotopes not applicable without further investigations for the calculation of the paleo-pH.

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