Stronium and Sodium in Fossil Biogenic Calcium Carbonates

• Main Author: Graham, David
• Format: Text
• Language: unknown
• Published: DigitalCommons@URI 1980
• Subjects: Planktonic foraminifera
• Online Access: https://digitalcommons.uri.edu/theses/2075; https://doi.org/10.23860/thesis-graham-david-1980; https://digitalcommons.uri.edu/context/theses/article/3049/viewcontent/Thesis_Graham_David_1980.pdf

The trace element content of well-preserved fossil carbonates offers the best opportunity to estimate temporal changes in the chemical composition of seawater. Previous studies of cation uptake by cultured carbonate-secreting organisms allow changes in Sr/Ca and Na/Ca ratios in aragonite, and Sr/Ca ratios in calcite, to be directly related to changes in the seawater ratios. To achieve internal consistency in the paleochemical interpretation, these ratios were measured in discrete layers of the aragonitic bivalve Mercenaria and the calcitic bivalve Pecten, dating back to 15 my ago, and in the tests of deep sea fossil planktonic foraminifera dating back to 70 my ago. Sr/Ca ratios in Pecten samples are nearly constant, with values about 10% lower near 10 my ago. Na/Ca ratios in Mercenaria samples are nearly constant. The results suggest that the seawater Sr/Ca ratio was 10% less than today near 10 my ago, and that the seawater Na/Ca ratio has been constant to within a factor of two since the Miocene. The Sr/Ca ratio in well-preserved foraminifera is similar to modern values during much of the Cenozoic, with decreases of about 25% between 60-50 my and near 10 my ago. Processes believed to be important in influencing the marine Sr/Ca ratio include crustal weathering, shallow water aragonite deposition, and hydrothermal supply of ca during ocean ridge-crest metamorphism. The relative importance of these processes are estimated from the geologic record of 87Sr/86Sr Sr in foraminifera, eustatic sea level and the carbonate compensation depth, and seafloor spreading rates, respectively. Any of the three processes may have been influential in producing the Eocene Sr/Ca minimum, but increased hydrothermal supply of ca is favored for causing the minimum near 10 my ago.