New Zealand Journal of Geology and Geophysics abstracts

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Stable isotope values in modern bryozoan carbonate from New Zealand and implications for paleoenvironmental interpretation

Abigail M. Smith

Department of Marine Science
University of Otago
P.O. Box 56
Dunedin, New Zealand
email: abby.smith@stonebow.otago.ac.nz

Campbell S. Nelson

Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand

Marcus M. Key, Jr

Department of Geology
Dickinson College
P.O. Box 1773
Carlisle, PA 17013, USA

William P. Patterson

Department of Geological Sciences
University of Saskatchewan
114 Science Place
Saskatoon, SK S7N 5E2, Canada

Abstract  Bryozoan carbonate contains useful geochemical evidence of temperate shelf paleoenvironments. Stable isotope values were determined for 103 modern marine bryozoan skeletons representing 30 species from New Zealand. δ¹⁸O values range from –1.4 to 2.8‰ VPDB, while δ¹³C range from –4.5 to 2.8‰ VPDB (values uncorrected for mineralogical variation). 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. Both temperature and salinity constrain δ¹⁸O and δ¹³C values, and vary with latitude and water depth. Ten samples from a single branch of Cinctipora elegans from the Otago shelf cover a narrow range, although the striking difference in carbon isotope values between the endozone and exozone probably reflects different mineralisation histories. Our 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.

Keywords  bryozoans; carbonate; oxygen isotopes; carbon isotopes; New Zealand shelf

G03060; Received 12 June 2003; accepted 23 February 2004; Online publication date 1 December 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 809–821
0028–8306/04/4704–0809© The Royal Society of New Zealand 2004

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