OXYGEN ISOTOPE RECORD IN BIOGENIC APATITE: A TOOL FOR CHEMOSTRATIGRAPHY AND PROXY IN PALAEOCLIMATE STUDIES

Fossil bioapatite of early vertebrate and conodont microfossils, originating from Upper Silurian (Pridoli) sections of Lithuania (southeastern Baltic Basin) have been studied for their oxygen isotope composition. The ¹⁸O/¹⁶O ratios of apatite have been measured, and the first δ¹⁸O_apatite curve is now introduced for a Pridoli sequence of the eastern Baltic Basin.

The δ¹⁸O record obtained form conodont apatite ranges from 17.7 to 19.2‰ V-SMOW, suggesting low possibility of diagenetic overprinting (Joachimski et al., 2003), and reflects minor thermal alteration of the Upper Silurian strata in this part of the Baltic Basin.

Remarkable positive shift of oxygen isotope ratios is detected at the point 0.2 m. below the Formational boundary of Vievis Fm. and Lapės Fm., lower and upper Pridoli respectively. However, the δ¹⁸O values of fish dermal teeth apatite show constant discrepancy of 2.5‰ V-SMOW.

The significance of this study for Chemostratigraphy strikes on the both fossil group apatite (conodont and fish) showing comparable apatite oxygen isotope values, and parallel positive δ¹⁸O excursion. Moreover, the excursion notably matches an abrupt sea-level drop at the Formational, and Sequence boundaries in the middle Pridoli of the Baltic Basin, and also corresponds to the proposed global cooling event in the Late Silurian (Lehnert et al., 2007).

Upon this new biogeochemical record, the Sequence Boundary in the middle Pridoli of the Baltic Basin can now be verified. This also supports the further inference of the decrease in palaeoseawater temperatures, estimated from δ¹⁸O ratio excursion, as sea-level drop of a global scale (Sequence Boundary) would be generally associated with climate cooling. Therefore, biogenic apatites of Palaeozoic fossil teeth can be proposed as relevant and reliable proxies in palaeoclimate reconstructions.

References:

Joachimski, M. M., Horacek, S., Breisig, S., Buggisch, W., 2003. The oxygen isotopic composition of biogenic apatite – no evidence for a secular change in seawater δ¹⁸O. Geophysical Society, Geophysical Research Abstracts, 5: 10792.

Lehnert, O., Eriksson, M. J., Calner, M., Joachimski, M. M., Buggish, W., 2007. Concurrent sedimentary and isotopic indications for global climatic cooling in the Late Silurian. Acta Palaeontologica Sinica, 46 (Suppl.): 249-255.