Kitazato, Hiroshi

Toyofuku, Takashi

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469Mg/Ca micro-distribution in foraminiferal test –Implication of laboratory culture experimentsTakashi Toyofuku & Hiroshi KitazatoInstitute for Research on Earth Evolution (IFREE4), Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC), 2-15, Natsushima-cho, Yokosuka,237-0061, Japan - toyofuku@jamstec.go.jpThe magnesium/calcium (Mg/Ca) ratio in the tests of foraminifera hasrapidly developed as a paleo-temperature proxy in paleoceanography becausethe magnesium content of calcite is a function of its precipitation temperature.Many studies have established the equation between Mg/Ca and temperatureusing sediment trap, plankton net, sediment core top and culture specimens.These empirical equations are useful to apply paleoenvironmental reconstruction.However, the foraminiferal uptake of elements has not fully understood yet.Pelagic foraminiferal test are consist of almost pure calcite. A generalmagnesium content of foraminiferal test is smaller than expected magnesiumcontents in calcite which is precipitated from seawater by two orders ofmagnitude, according to results of inorganic precipitation experiments. Moreover,magnesium amount in test is also different among species. Meanwhile, therelationship between Mg/Ca and temperature is different from species tospecies. These suggest that uptake of elements from seawater is positivelycontrolled by foraminiferal biology.In this study, we studied about chamber formation processes andmagnesium and calcium micro-distributions of foraminiferal test in order toinvestigate an incorporation of elements from seawater to calcareous teststhrough foraminiferal organisms. The foraminiferal chamber formation processwas observed under inverted microscope from first to last in laboratory. Theelemental measurements were carried out by electron probe microanalyzer(EPMA) on seven cultured specimens and natural populations. We used juvenilespecimens produced during asexual reproduction in the laboratory. The wholetests of cultured specimens are calcified under well-controlled physicochemicalconditions.  It is advantage of living culture in order to evaluate the influence ofambient environments on foraminiferal test chemistry because any factors arevariable systematically by an experimental design.Anuár io do Inst i tu to de Geociências -  UFRJISSN 0101-9759                        Vol. 29 - 1 / 2006  p. 469-470FORAMS  2006470Anuário do Instituto de Geociências - UFRJISSN 0101-9759 - Vol. 29 - 1 / 2006     p. 469-470FORAMS 2006Mg/Ca micro-distribution in foraminiferal test – Implication of laboratory culture experimentsTakashi Toyofuku & Hiroshi KitazatoElemental micro-mapping measurement indicated heterogeneousdistribution of magnesium. There are several magnesium concentrated bandsin both planktonic and benthic species. High magnesium bands correspond toorganic layers in the test. The intrachamber variation was also observed inaddition to high magnesium bands. The variation was not random, but showeda weak planar distributional pattern. Microscope observations documented theprecipitation of calcite does not occur evenly over the organic membrane ofthe developing chamber during their calcification process. We speculate variableMg/Ca values within their test walls, which resulted from the pattern of chamberformation and from minor variations of Mg/Ca within one chamber wall.

Mg/Ca micro-distribution in foraminiferal test: implication of laboratory culture experiments

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Mg/Ca micro-distribution in foraminiferal test: implication of laboratory culture experiments

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