Mineralogy of Arctic bryozoan skeletons in a global context

Abstract

Bryozoans are major carbonate producers in some ancient and Recent benthic environments, including parts of the Arctic Ocean. Seventy-six species of bryozoans from within the Arctic Circle have been studied using XRD to determine their carbonate mineralogies and the Mg content of the calcite. The majority of species were found to be calcitic, only four having bimineralic skeletons that combined calcite and aragonite, and none being entirely aragonitic. In almost all species, the calcite was of the low-(<4 mol% MgCO 3 ) or intermediate-Mg (4-11.99 mol% MgCO 3 ) varieties. Previous regional studies of bryozoan biomineralogy have found higher proportions of bimineralic and/or aragonitic species in New Zealand and the Mediterranean, with a greater number of calcitic species employing intermediate-and high-Mg calcite. The Antarctic bryozoan fauna, however, has a similar mineralogical composition to the Arctic. The lesser solubility of low-Mg calcite compared to both Mg calcite and aragonite in cold polar waters is most likely responsible for this latitudinal pattern. However, it is unknown to what extent environmental factors drive the pattern directly through eliciting an ecophenotypic response from the bryozoans concerned or the pattern reXects genetic adaptations by particular bryozoan clades.

Patterns of magnesium content in Arctic bryozoan skeletons along a depth gradient

“…The data used to test the relationship between depth and Mg content in bryozoan skeletons were obtained from Kukliński and Taylor (2009). There are several advantages of this dataset: (1) taxonomic identifications were made by one scientist, (2) uniform analytical methods of magnesium content were obtained using standard XRD methods (for details see Kukliński and Taylor 2009), (3) precise depth values and high sampling resolution were available, (4) replicates of the same species were analyzed, (5) a large number of samples ([100) were available, (6) the environmental context and ecological distributional patterns of Arctic bryozoans are quite well known, (7) the quality of the specimens (e.g., for contaminant epibionts) was carefully controlled and (8) sampling was essentially random.…”

“…There are several advantages of this dataset: (1) taxonomic identifications were made by one scientist, (2) uniform analytical methods of magnesium content were obtained using standard XRD methods (for details see Kukliński and Taylor 2009), (3) precise depth values and high sampling resolution were available, (4) replicates of the same species were analyzed, (5) a large number of samples ([100) were available, (6) the environmental context and ecological distributional patterns of Arctic bryozoans are quite well known, (7) the quality of the specimens (e.g., for contaminant epibionts) was carefully controlled and (8) sampling was essentially random. Other published datasets, for example on bryozoans (e.g., Borisenko and Gontar 1991;Smith et al 2006;Kukliński and Taylor 2008;Smith and Clark 2010), lack sampling depth data, focus on areas other than the Arctic, employ a smaller number of samples or used destructive mineralogical/geochemical analysis techniques excluding further specimen verification causing taxonomic inconsistencies.…”

“…Other published datasets, for example on bryozoans (e.g., Borisenko and Gontar 1991;Smith et al 2006;Kukliński and Taylor 2008;Smith and Clark 2010), lack sampling depth data, focus on areas other than the Arctic, employ a smaller number of samples or used destructive mineralogical/geochemical analysis techniques excluding further specimen verification causing taxonomic inconsistencies. The bryozoans studied by Kukliński and Taylor (2009) came from numerous localities (see Fig. 2), ranging from northern Norway to Greenland, Spitsbergen and the Siberian coast (Laptev Sea), and can be regarded as fairly representative for the Arctic as a whole.…”

“…Mg content was expressed as mole percentage (% mol) of magnesite (MgCO 3 ; here referred to simply as magnesium; see also Tucker and Wright 1990). The error associated with this method is estimated to be 2 % (see Kukliński and Taylor 2009). Mean values were calculated for species with more than one analysis, such as some erect species where Mg was measured at the tips and bases of the colonies.…”

“…Martin et al 2008;Smith 2009;RodolfoMetalpa et al 2010;Lombardi et al 2011a, b;Loxton et al 2013). Like echinoderms, corals and pteropods, bryozoans have calcareous skeletons potentially vulnerable to dissolution (e.g., Kukliński and Taylor 2009;Smith 2009; and literature cited therein). Their diversity (at least 6,000 species), abundance, habitat-creating abilities and significant contribution to the carbonate budget since the Ordovician (e.g., Taylor and Allison 1998) underline the importance of bryozoans among benthic organisms (Gordon et al 2009).…”