Use of stable oxygen isotope determinations from brachiopod shells in palaeoenvironmental reconstruction

Abstract

Stable oxygen isotope analyses were obtained from fossil brachiopod shells from a shallow marine deposit of Upper Pleistocene age (approximately 80 ka) in New Zealand as part of a palaeoenvironmental investigation. Bulk analyses of entire shells of Calloria inconspicua yielded highly variable results (a range of 2.30‰), rendering these data of limited use as palaeoenvironmental proxies. Attempts to obtain consistent oxygen isotope values by removing areas of the brachiopod shell thought to be prone to oxygen isotope disequilibrium effects (‘vital effects’) revealed that the matching anterior portions of ventral and dorsal valves from the same individual (which were secreted at the same time) also yielded different oxygen isotope ratios. The dorsal valve δ 18O values were higher and less variable than those obtained from the ventral valve of the same individual. The investigation was then extended to living representatives of the same species from a similar shallow water habitat from the South Island of New Zealand. The pattern of dorsal valves yielding higher and less variable oxygen isotope values than matching areas of the ventral valves was repeated in seven out of eight individuals investigated, although there was considerable variation in the absolute values from specimen to specimen. Contrary to common practice therefore, it seems that in this species it is the much thinner and lighter dorsal valve that provides a more consistent isotopic measurement. If converted into absolute temperature estimates, the dorsal valve measurements correspond to a temperature midway between the mean and maximum annual temperature experienced by this species in its present-day habitat, which may make it acceptable as a palaeoenvironmental indicator. The high level of variation between individuals (including an inferred temperature from one ventral valve above the mean maximum temperature experienced) indicates that a range of individuals needs to be investigated in any palaeoenvironmental investigation to avoid misleading interpretations. However, intra- and inter-specimen variation in isotopic composition may provide valuable palaeobiological information on growth rates, metabolism and possibly also pathological conditions in the past.