Sodium, Magnesium and Strontium in the Tests of Planktonic Foraminifera

Abstract

We have determined Na/Ca, Mg/Ca, Sr/Ca, AI/Ca and CI/Na atom ratios in about sixty samples of individual species of planktonic foraminifera. Na/Ca and Sr/Ca ratios are constant in different tests of one species, but Mg/Ca ratios show considerable variability. Data indicate that Na and Sr are distributed homogeneously throughout the tests, while Mg is distributed heterogeneously. Na/Ca, Mg/Ca and Sr/Ca ratios of different species from an Upper Pleistocene assemblage from the Tasman Sea covary, with shallow-living species having the highest ratios. Atom ratios of Recent North Atlantic Globigerinoides sacculifer (Brady) and G. ruber (d'Orbigny) samples do not show any clear-cut temperature dependence. It appears that at one location atom ratios increase with temperature or a temperature-related factor, and that over a broad geographical area variables other than temperature affect trace element abundance. Sodium, magnesium and strontium in the tests of planktonic foraminifera INTRODUCTION The study of the distribution of salt major cations (herein defined as Na, K, Mg, Ca and Sr) in foraminiferal calcite is of interest for a number of reasons. First, the distribution of these elements may provide an insight into biochemical processes occurring during calcification. Second, if investigations of Recent skeletal chemistry reveal that trace element abundances vary in a quantifiable way with environmental parameters, we can use skeletal chemistry as an ecological and paleoecological index. Third, the study of fossil skeletal chemistry is our prime hope for deducing past variations in the chemistry of sea water. Our interest in these problems has prompted us to undertake a comprehensive study of foraminiferal calcite chemistry; the purpose of this paper is to summarize our results to date. Our effort builds on results of a number of earlier workers. Blackmon and Todd (1959) found that benthic foraminifera had skeletons composed dominantly of high magnesian calcite, but planktonic foraminifera were composed of calcite containing less than 2 mole percent MgCO3. Emiliani (1955), Switzer and Boucot (1955), and Mayer (1932) also found that the skeletons of planktonic foraminifera were composed of calcite. Emiliani (1955) determined the TiO2, A1203, Si02, Fe203, MnO, MgO, and SrO contents of eight samples composed of single species of planktonic foraminifera. He found that the SrO content was fairly constant at a level of 0.13%, but that all other concentration values varied sympathetically in a way that could be ascribed to the presence of contaminating detritus. Krinsley (1960) analyzed ten samples for MnO, TiO2, A1203, MgO, CuO, and NiO, and concluded (contrary to the belief of Emiliani) that the shells contained on the order of 0.1 5% by weight MgO that could not be ascribed to detrital contamination. Lipps and Ribbe (1967) reinforced this conclusion with their electron microprobe work, showing that those parts of the foraminiferal tests free of detritus contained about 0.2% by weight MgO. Savin and Douglas (1973) determined the MgCO3 contents and 5018 of foraminifera and found that the tests contained between 0.1 and 0.4% by weight MgO. With the exception of Globorotalia truncatulinoides (d'Orbigny) MgO content of foraminiferal calcite increased as isotopic temperature increased. The most extensive data in the literature on planktonic foraminiferal chemistry are the analyses of Kilbourne and Sen Gupta (1973). They found that the Mg content increased linearly with habitat temperature and that solutionsusceptible species had higher Mg contents than more resistant species. Results of Savin and Douglas (1973) also agreed with this latter conclusion, leading those authors to suggest that the Mg content controlled solution susceptibility. micropaleontology, vol. 21, no. 4, pp. 448-459, october, 1975 448 This content downloaded from 157.55.39.147 on Sun, 07 Aug 2016 05:32:01 UTC All use subject to http://about.jstor.org/terms BENDER, LORENS, and WILLIAMS