Strontium isotope systematics of conodonts: Middle Devonian, Eifel Mountains, Germany

Abstract

The Eifel Mountains lithological profile, the stratotype for the Middle Devonian, was densely sampled for conodonts to delineate the Sr isotopic evolution of coeval sea water. The 87Sr/86Sr curve obtained shows oscillations around a mean of about 0.7078 that have a frequency in the 105–106 yr range and an amplitude in the 10−5–10−4 range. These oscillations mimic those obtained on coeval brachiopods (Diener et al., 1996), but are shifted into more radiogenic domains by up to 10−4. Model calculations suggest that the shift is due to partial (30–40%) early diagenetic equilibration of conodonts with their enclosing bulk-rock matrix. The reality of partial recrystallisation is supported by visible neoformed blocky apatite crystals on the surface of the conodonts, despite their otherwise apparently excellent preservation, with CAI values of 1.5–2.5. We therefore suggest that studies anticipating use of conodonts as a recorder of paleoceanographic events should be confined to specimens embedded in a relatively `clean' matrix, such as pure limestone. Even these results should be cross-checked by comparison with data on coeval low-Mg shells, such as brachiopods or belemnites.