SKELETAL CARBONATE MINERALOGY OF NEW-ZEALAND BRYOZOANS

On many cool-water carbonate shelves, both modem and ancient, bryozoan s are the dominant sediment contributor. Because skeletal carbonate mi neralogy can be related to both taxonomic and environmental controls, and has important implications for understanding the geochemistry and diagenesis in carbonate deposits, it is highly relevant to develop a c omprehensive database for bryozoan skeletal mineralogy. This X-ray dif fraction study of modern New Zealand bryozoans more than doubles the p ublished data available on bryozoan mineralogy and allows for statisti cal analysis of variability in mineralogy, particularly the hitherto u nknown background variation within individuals and within populations. Such a database allows evaluation of differences among populations, t axa, and growth forms. The 412 analyses of 49 bryozoan species fall in to four mineralogical categories: (a) 61% are single calcite, which ra nges widely from low to high wt% MgCO3 varieties; (b) 6% (in the gener a Cellaria and Macropora) comprise dual calcites, a dominant one low i n Mg ((X) over bar = 1.8 wt% MgCO3) and the other much higher (X = 9.0 wt% MgCO3) (c) 27% are mainly calcite with some aragonite; and (d) 6% are mainly aragonite with some calcite. The bimineralic calcite-arago nite skeletons can arise from different processes, including biologica lly mediated changes in precipitation and diagenetic alteration follow ing death. There is small but significant variation in Mg content in c alcite both within colonies (Mg increasing with age) and within popula tions, for a total of within-species variance of 0.526. We found, howe ver, more variation among species in both calcite/aragonite ratio and Mg content than that within species. Whereas some genera and families are reasonably consistent mineralogically (e.g., low-Mg calcite cyclos tomes such as Telopora buski), others are highly variable (e.g., Chape ria acanthina). Among the higher taxa, cyclostomes tend to be entirely or mainly calcite with a low to moderate range of MgCO3 ((X) over bar = 2.1 wt%), whereas cheilostomes are far more variable, with anascans ((X) over bar = 4.1 wt%) having a lower MgCO3 content than ascophoran s ((X) over bar = 5.6 wt%). All dual-calcite species are anascans in t he Calloporoidea. The proportions of aragonite and of Mg in calcite ar e greater in evolutionarily younger taxa, at least at the higher level s. In relation to growth forms, erect rigid varieties are calcitic exc ept for several robust-branching genera such as Adeonellopsis. Encrust ing forms, particularly unilaminar sheets, are highly variable and con tain most of the bimineralic calcite-aragonite species. Erect flexible forms are either single or dual calcite,and extremely variable in Mg content. Free-living or vagrant forms are usually aragonitic. In relat ion to latitude and temperature variation, few species show any consis tent or significant mineralogical trends, although Schizoporella unico rnis deposits increasing amounts of aragonite at lower (warmer) latitu des. Mecynoecia purpurascens includes more (but still low) Mg in its c alcite skeleton with increasing latitude, a reversal of the Mg-tempera rure relationship noted in several other skeletal carbonate producers. (C) 1998 Elsevier Science B.V. All rights reserved.