Cadmium,zinc, copper,and barium in foraminifera tests |
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| Authors: | Edward A Boyle |
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| Institution: | Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139U.S.A. |
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| Abstract: | The concentrations of cadmium, zinc, copper and barium have been determined on 2-mg samples of single-species foraminifera populations. Cleaning techniques were tested using North Atlantic core tops, and followed by a detailed downcore study for the last 30,000 years in South Atlantic core V22-174. Raw foram tests cleaned by crushing followed by ultrasonic removal of fine-grained material, and dissolved in a pH 5.5 acetate buffer, contain appreciable amounts of trace elements associated with ferromanganese and alumino-silicate contaminants. A reductive/complexing cleaning treatment reduces ferromanganese contamination by 1–2 orders of magnitude. Acetate buffers at pH 5.5 complex iron and raise the solubility of ferromanganese oxides; these buffers are unsuitable for separating carbonate lattice-bound trace elements from the fraction associated with ferromanganese phases. Improved mechanical and ultrasonic reductive cleaning combined with a mild dissolution in distilled water under 1 atm.PCO2 reduces contaminant levels another order of magnitude. The Cd and Zn concentrations (order 10?8 mole Cd/mole Ca and 10?5 mole Zn/mole Ca) of species with low surface area show an increase with decreasing isotopic temperatures. This increase is consistent with the increasing concentrations of these metals from low values in surface waters to higher values at depth. The variance of Cd and Zn over the last 30,000 years in the central South Atlantic is consistent with the probable variability of the dissolved trace elements at the calcification levels of the species analyzed. Cu and Ba are irreproducible and probably sensitive to residual contaminant phases. The trace element content of the tests differs from levels observed in a recent coprecipitation study. Foraminifera may be a significant vector in zinc cycling in the ocean. |
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