Stable Isotope Values in Modern Bryozoan Carbonate: Potential for Paleoenvironmental Interpretation

Abigail M. Smith¹, Marcus M. Key², Campbell S. Nelson³, and William P. Patterson^4
1 University of Otago, Dunedin, New Zealand
² Dickinson College, Carlisle, PA
³ University of Waikato, Hamilton, New Zealand
⁴ University of Saskatchewan, Saskatoon, SK

Bryozoan carbonate contains useful geochemical evidence of temperate shelf paleoenvironments. Stable isotope values from 103 modern marine bryozoan skeletons representing 30 species from New Zealand give δ¹⁸O values ranging from -1.4 to 2.8 ‰ VPDB, while δ¹³C range from –4.5 to 2.8 ‰ VPDB (uncorrected for mineralogy). These values are distinct from those of both tropical marine skeletons and New Zealand Tertiary fossils. Most bryozoans secrete carbonate in or near isotopic equilibrium with sea water, except for Celleporina and Steginoporella. The complex and variable mineralogies of the bryozoans reported here make correction for mineralogical effects problematic. Nevertheless, mainly aragonitic forms display higher isotope values, as anticipated. Ten samples from a single branch of Cinctipora elegans from the Otago shelf cover a narrow range, with the striking difference in carbon isotope values between the endozone and exozone probably reflecting different mineralisation histories. Stable isotope results from three different laboratories on a single population from a single location are encouragingly consistent. Monomineralic bryozoans, when carefully chosen to avoid species suspected of vital fractionation, have considerable potential as geochemical paleoenvironmental indicators, particularly in temperate marine environments where bryozoans are dominant sediment-producers.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005