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

Abstract

The oxygen (δ 18 O) and carbon (δ 13 C) 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 δ l8 O calcite 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 HCO 3 -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 Received 18 May 1999; accepted 20 December 1999 inorganic carbon (δ 13 C ΣDIC ), the data suggest that two factors-kinetic fractionation and incorporation of respiratory CO 2 -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 CO 2 . 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 CO 2 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. M99028

Influence of seawater temperature on growth bands, mineralogy and carbonate production in a bioconstructional bryozoan

“…Our observations on mineralogical differences between winter and summer bands not only corroborate the effect of seasonal changes of seawater temperature on bryozoan mineralogy (Lowenstam 1954;Crowley and Taylor 2000;Smith and Key 2004), but also show the rapidity with which ambient temperature impacts the biomineralization process. Differences in mineralogical composition within bryozoan skeletons, especially in bimineralic species where the highest levels of aragonite are usually associated with secondary calcification processes, are usually attributed to different ontogenetic stages (Carson 1978;Hayward and Ryland 1998).…”

“…Despite bryozoans exerting control over their own biomineralization, their skeletons are subject to some degree of environmental modification (Lowenstam and Weiner 1989;Smith and Key 2004;Smith et al 2006;Taylor et al 2008). It has been suggested that modern temperate bryozoan skeletons are reasonable indicators of seawater chemistry, being largely unaffected by kinetic and metabolic factors (Lowenstam and Weiner 1989;Bone and James 1993;Crowley and Taylor 2000;Smith and Key 2004). Nevertheless, taxon-specific and ontogenetic factors must be borne in mind when using mineralogical variations within bryozoan colonies, within species, among growth forms and within families to infer environmental factors (Poluzzi and Sartori 1974;Smith et al 1998;Smith and Key 2004).…”

Bryozoan Constructions in a Changing Mediterranean Sea

“…Smaller zooids are present in summer than winter growth bands, corresponding to the 'size-temperature' rule (Atkinson 1994 ). Observations on mineralogical differences between winter and summer bands not only corroborate the effect of seasonal changes of seawater temperature on bryozoan mineralogy (Lowenstam 1954 ;Crowley and Taylor 2000 ;Smith and Key 2004 ), but also show the rapidity with which ambient temperature impacts the biomineralization process. An increase in aragonite and in calcite with higher Mg content characterizes those parts of the skeleton formed in the summer, as previously observed in other bimineralic bryozoan species (Poluzzi and Sartori 1973 ;Rucker and Carver 1969 ).…”

“…Nevertheless, taxon-specifi c and ontogenetic factors also contribute to mineralogical variations seen within colonies, within species, among colony-forms and within families (Poluzzi and Sartori 1974 ;Smith et al 1998 ;Smith and Key 2004 ). Isotopically, bryozoan skeletons are generally in equilibrium with seawater (Lowenstam and Weiner 1989 ;Bone and James 1993 ;Crowley and Taylor 2000 ;Smith and Key 2004 ), although in a few instances disequilibrium may result from kinetic effects or possibly symbiosis (Crowley and Taylor 2000 ).…”

Geological imprint of methane seepage on the seabed and biota of the convergent Hikurangi Margin, New Zealand: Box core and grab carbonate results

“…Using an estimated range of −0.5 to +0.5‰ PDB for the δ 18 O composition of New Zealand sea water (Feary et al, 1991;Crowley and Taylor, 2000), and the measured values of bottom water temperatures at the box core sites (Table 4), it is possible to constrain pore fluid compositions associated with carbonate precipitation based on the equilibrium equations for calcite (Friedman and O'Neil, 1977), aragonite (Grossman and Ku, 1986) and dolomite (Fritz and Smith, 1970), as summarised in Fig. 12 (Table 4) shows that our calcite samples range from slightly depleted to slightly enriched with respect to regional sea water (Fig.…”