Mineralogy, O¹⁸/O¹⁶Ratios, and Strontium and Magnesium Contents of Recent and Fossil Brachiopods and Their Bearing on the History of the Oceans

Abstract

The calcareous shells of recent articulate brachiopods and fossils of the same class dating back to the Mississippian are investigated for their crystal form, $$O^{18}/O^{16}$$ ratios, and $$SrCO_{3}$$ and $$MgCO_{3}$$ contents. In the case of recent forms it is shown that temperature affects $$O^{18}/O^{16}$$ ratios, $$SrCO_{3}$$ and $$MgCO_{3}$$ contents, and consequently Sr/Ca and Mg/Ca ratios. The $$SrCO_{3}$$ and $$MgCO_{3}$$ contents are investigated in samples from waters of 35‰ ± 1.5‰ salinity over the range of temperatures from 10° C. to 26° C. As is true for the $$O^{18}$$ concentrations, the uptake of Sr and Mg in the shells is shown to be sensitive also to changes in the concentrations of the two trace elements in sea water. The $$O^{18}/O^{16}$$ ratios and the $$SrCO_{3}$$ and $$MgCO_{3}$$ contents in fossil samples of Pliocene, Cretaceous, Permian and Mississippian age were determined. Samples as old as the early Permian have been found in which the relationship of $$O^{18}/O^{16}$$ ratios and the $$SrCO_{3}$$ and $$MgCO_{3}$$ contents are similar to recent species. The relation of the $$O^{18}/O^{16}$$ ratios to the $$SrCO_{3}$$ contents in a late Mississippian sample is also similar to that in recent species. These findings make it probable that the original $$O^{18}/O^{16}$$ ratios and $$SrCO_{3}$$ and $$MgCO_{3}$$ contents in these fossils are the original ones. In other samples the relations of the relative concentrations of one, two, or all three constituents indicate diagenetic alterations by fresh water. The significance of the chemical similarity of the fossil samples to recent shells is discussed. It is concluded that the results are best explained if it is assumed that the $$O^{18}$$, Sr and Mg contents, and the Sr/Ca and Mg/Ca ratios in the oceans have remained essentially constant during the last 2.0 to $$2.5 \times 10^{8}$$ years.