New Zealand Journal of Marine and Freshwater Research abstracts

---

Stable isotope composition of modern bryozoan skeletal carbonate from the Otago Shelf, New Zealand

STEPHEN F. CROWLEY

Stable Isotope Laboratory
Department of Earth Sciences
University of Liverpool
P. O. Box 147, Liverpool L69 3BX
United Kingdom
email: sfcrow@liverpool.ac.uk

PAUL D. TAYLOR

Department of Palaeontology
Natural History Museum
Cromwell Road, London SW7 5BD
United Kingdom
email: p.taylor@nhm.ac.uk

Abstract  The oxygen (d¹⁸O) and carbon (d¹³C) isotope ratios of 10 species of living Bryozoa collected from the Otago Shelf, New Zealand were analysed to assess the extent to which isotopic equilibrium (relative to inorganic equilibrium isotope fractionation) is attained during the precipitation of skeletal calcium carbonate. The data reveal that whereas eight species of Bryozoa synthesise skeletal carbonate in apparent oxygen isotope equilibrium with respect to environmental conditions, two species (Celleporina grandis and Hippomonavella flexuosa) yield d¹⁸Ocalcite values which indicate significant disequilibrium oxygen isotope fractionation during calcification. Sufficient data are available from one species (C. grandis) to demonstrate that disequilibrium is probably related to kinetic factors associated with diffusion-controlled transport of HCO3^- to the site of calcite precipitation. Carbon isotope signatures indicate significant departures from inorganic isotope equilibrium in all but one bryozoan species (Hippomenella vellicata). Although greater uncertainties are associated with estimates of the isotopic composition of total dissolved inorganic carbon (d¹³CDIC), the data suggest that two factors--kinetic fractionation and incorporation of respiratory CO2--are important in controlling carbon isotope disequilibrium. Where bryozoan species exhibit evidence for disequilibrium in both oxygen and carbon isotope systems (C. grandis, H. flexuosa), it is likely that kinetic factors are primarily responsible for observed departures from carbon isotope equilibrium. In contrast, the probable explanation for those species which display evidence for carbon isotope disequilibrium only, is that skeletal carbonate is precipitated from a DIC pool modified by the incorporation of respiratory CO2. Differences between the carbon isotope composition of skeletal elements from the same species and co-existing species living in the same community suggests that significant variations may occur in the extent to which marine DIC and respiratory CO2 are utilised during calcification. Additional studies of carbon pathways associated with calcification are required to assess the relative effects of kinetic, metabolic, and environmental factors on the carbon isotopic composition of bryozoan skeletal carbonate.

Keywords  d¹³C; d¹⁸O; isotope fractionation; calcification; Bryozoa

M99028
Received 18 May 1999; accepted 20 December 1999

PDF file of entire paper: medium quality (1682K); (scanned from paper original: notes about this process)

---

This year's abstracts | Journal home page | All abstracts | Publishing home page