[SGC41-P03] Age determination of spherical carbonate concretions by Sr isotope stratigraphy
Keywords:carbonate concretion, fossil, Sr isotope, age determination, marine sediment
The studied concretions formed in the Yatsuo Group of Miocene age in central Japan. Some formed post-mortem around tusk-shells (Fissidentaliumspp.), while other concretions have no shell fossils inside. The concretions formed under highly constrained conditions by reactions between the decay products of organic matter and Ca2+ from the marine water (Yoshida et al., 2015; 2018). And the sedimentary structure around the concretions and geochemical analyses reveal that Sr was incorporated into the CaCO3 concretions during their rapid formation.
The 87Sr/86Sr ratios of calcite in the tusk-shell concretions lie within 0.70865~0.70867 and vary little. The values are similar to those of tusk-shell aragonite, 0.70865~0.70868. In addition, the isolated carbonate concretions without shell fossils have 87Sr/86Sr ratios of 0.70865~0.70868, which are also well consistent with the tusk-shell aragonite values. Based on the Sr isotope stratigraphy (McArthur et al., 2001) and the measured 87Sr/86Sr values, the age of the tusk-shell concretions lies within a narrow range of 17.08 (+0.27, −0.28) Ma. The age determined from the 87Sr/86Sr of the tusk-shells themselves (16.86 +/− 0.34 Ma) is almost the same. The age determined from the 87Sr/86Sr of concretions without fossils, 16.95 (+0.36, −0.37) Ma, is almost the same as that of the tusk-shell concretions. Strontium isotopic stratigraphy using 87Sr/86Sr ratios of all concretions with tusk-shell and without fossils indicates an age of 17.02 +/− 0.27 Ma, with higher accuracy than the ages estimated using microfossils from the Yatsuo Group. The results imply that the 87Sr/86Sr ratio of isolated spherical carbonate concretions can be applied generally to estimate the ages of all kinds of spherical concretions in marine sediments (Yoshida et al., 2019).