BRACHIOPODS – FAITHFUL RECORDERS OF OCEAN PROPERTIES? A CASE STUDY WITH NEOTHYRIS LENTICULARIS

Phanerozoic trends of temperature and/or δ¹⁸O of the ocean were largely estimated from brachiopods. Respective studies were triggered by Lowenstam (1961) according to which the shells are formed in equilibrium with δ¹⁸O_water. However, this long-standing view was rivaled by Carpenter & Lohmann (1995) and many others thereafter demonstrating that species-specific isotopic disequilibrium occurs and the hinge and certain other shell structures are affected to various degree by vital and kinetic effects. Yamamoto et al. (2011) also identified seasonal growth rate-related positive and negative offsets from expected thermodynamic equilibrium as well as ontogenetic δ¹⁸O_shell changes in two modern cold-water terebratulids. Here, we follow in the footsteps of Yamamoto et al. (2011) and tested if the same is true for another terebratulid, Neothyris lenticularis from Chatham Rise, New Zealand. Serial sampling was completed along the outer shell surface of 3 specimens collected alive in 134 m water depth. Results were contrasted to δ¹⁸O_shell time-series of a coeval bivalve mollusk, Glycymeris sp., collected at the same locality. Whereas the glycymerid δ¹⁸O_shell data fluctuated in a narrow range of +1.58 to 2.45 ‰, the brachiopod δ¹⁸O_shell showed seasonal fluctuations with an amplitude of up to 2.59 ‰ as well as a shift to lower δ¹⁸O_shell values through ontogeny (lifespan ca 5 years). Most positive δ¹⁸O_shell during age one and two closely agreed with winter data of Glycymeris sp. possibly indicating that juvenile N. lenticularis attains isotopic equilibrium during periods of slow growth. In later years of life, however, isotopic disequilibrium intensifies and δ¹⁸O_shell was ca. -2 ‰ offset from the average glycymerid δ¹⁸O_shell values. As revealed by these findings, only shell portions of this species formed during periods of slow growth during age 1 and 2 should be used for oxygen isotope-based paleoenvironmental reconstructions. Existing Phanerozoic isotope trends are most likely biased by kinetic and/or vital fractionation and should be revisited.