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

The oxygen (delta(18)O) and carbon (delta(13)C) isotope ratios of 10 specie s of living Bryozoa collected from the Otago Shelf, New Zealand were analys ed to assess the extent to which isotopic equilibrium (relative to inorgani c equilibrium isotope fractionation) is attained during the precipitation o f skeletal calcium carbonate. The data reveal that whereas eight species of Bryozoa synthesise skeletal carbonate in apparent oxygen isotope equilibri um with respect to environmental conditions, two species (Celleporina grand is and Hippomonavella flexuosa) yield delta(18)O(calcite) values which indi cate significant disequilibrium oxygen isotope fractionation during calcifi cation. Sufficient data are available from one species (C, grandis) to demo nstrate that disequilibrium is probably related to kinetic factors associat ed with diffusion-controlled transport of HCO3- to the site of calcite prec ipitation. Carbon isotope signatures indicate significant departures from i norganic 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 (delta(13)C(S igma DIC)), the data suggest that two factors-kinetic fractionation and inc orporation of respiratory CO2-are important in controlling carbon isotope d isequilibrium. Where bryozoan species exhibit evidence for disequilibrium i n both oxygen and carbon isotope systems (C. grandis, H. flexuosa), it is l ikely that kinetic factors are primarily responsible for observed departure s from carbon isotope equilibrium. In contrast, the probable explanation fo r those species which display evidence for carbon isotope disequilibrium on ly, is that skeletal carbonate is precipitated from a DIC pool modified by the incorporation of respiratory CO2. Differences between the carbon isotop e composition of skeletal elements from the same species and co-existing sp ecies living in the same community suggests that significant variations may occur in the extent to which marine DIC and respiratory CO2 are utilised d uring calcification. Additional studies of carbon pathways associated with calcification are required to assess the relative effects of kinetic, metab olic, and environmental factors on the carbon isotopic composition of bryoz oan skeletal carbonate.