Differential diagenesis of sedimentary components and the implication for strontium isotope analysis of carbonate rocks

Geochemical analyses of various components (foraminifera, coccoliths and siliciclastic fractions) of limestone and marl samples from the marine Trubi Formation (Early Pliocene) of southern Italy revealed subtle diagenetic contamination. The coccolith fraction is altered from its original value both in its trace element (Sr/Ca, Mg/Ca, Fe/Ca, Mn/Ca, Na/Ca all were higher) and isotopic (87Sr/86Sr, δ18O, δ13C) composition. Coccolith 87Sr/86Sr values (limestones 0.709010; marls 0.708951) are lower than those of coeval Early Pliocene seawater (0.709025–60 [Farrell, J.W., Clemens, S.C., Gromet, L.P., 1995. Improved chronostratigraphic reference curve of Late Neogene seawater 87Sr/86Sr. Geology 23, 403–406]) and similar to the 87Sr/86Sr values of Messinian evaporites (0.70887 to 0.70896 [Müller, D.W., Mueller, P.A., 1991. Origin and age of the Mediterranean Messinian evaporites: implications from Sr isotopes. Earth Planet. Sci. Lett. 107, 1-12]). Foraminiferal calcite is unaltered and retains its elemental and isotopic composition, with 87Sr/86Sr values (0.709052) within the range for Early Pliocene seawater. However, unaltered 87Sr/86Sr values were obtained only when the foraminifera were cleaned in acid to remove all contaminating coccoliths. Simple hand-picking and ultrasonic cleaning in water is inadequate to remove adhering coccoliths and may result in erroneous 87Sr/86Sr values being quoted.