Nd and Sr Isotope Composition in the Tooth Enamel from Fe–Mn Nodules of the Cape Basin (Atlantic Ocean): Age and Sources

Resumo

Abstract—The Nd and Sr isotope compositions were determined for the first time in biogenic apatite sampled throughout the tooth section (from the base to tip) of predatory fish in the nucleus of Fe–Mn nodules from the Cape Basin. The results showed that diagenetic recrystallization of apatite does not affect the ⁸⁷Sr/⁸⁶Sr ratio in the tooth enamel, but leads to the decrease of Sr content. The age of tooth was determined using Sr isotope stratigraphy at 5.2 ± 0.2 Ma for sample 2188/4 and 6.6 ± 0.3 Ma for sample 2188/5. The calculated growth rate of Fe and Mn oxyhydroxide layers varies within 0.4–2.8 mm per 1 Ma. The ¹⁴³Nd/¹⁴⁴Nd ratio in the tooth enamel varies within single station and depends on the local Nd sources in pore water. The value of ε_Nd varies from –5.2 to –6.9 in the enamel of tooth 2188/4 and remains constant at –8.7 ± 0.1 in sample 2188/5. A change of Nd isotope composition in sample 2188/4 likely reflects temporal variations of Nd fraction from bottom and pore waters that penetrated inside the enamel during REE diffusion. The value of ε_Nd in the oxyhydroxide layers of Fe–Mn nodule 2188/4 (from –7.8 to –7.9) is homogenous for the external and internal parts of the tooth. In order to use ε_Nd in apatite enamel and authigenic Fe and Mn oxyhydroxides in sediments for paleoreconstructions of thermohaline water circulation, it is necessary to develop additional criteria for selecting diagenetically unaltered matter.