Trace Elemental Distributions Within Organophosphatic Brachiopod Shells: Implications for Growth, Preservation and Paleoenvironment

• Main Author: Betzner, Tristan Joshua
• Format: Text
• Language: unknown
• Published: CU Scholar 2014
• Subjects: Arctic; diagenesis; lingulid; trace element; Paleontology; Canada; Devon Island
• Online Access: https://scholar.colorado.edu/geol_gradetds/84; https://scholar.colorado.edu/cgi/viewcontent.cgi?article=1088&context=geol_gradetds

The initial incorporation of elements into the accretionary hardparts of marine invertebrates is controlled by prevailing seawater chemistry and vital effects. While trace-element signatures of invertebrates accreting carbonate hardparts have been examined extensively, the shells of phosphatic organisms have largely been ignored. In this study, major and trace elements of fossil organophosphatic brachiopod shells are investigated to determine the fidelity with which their shell chemistry may be preserved, and to evaluate their potential as paleoenvironmental archives. Trace elements within the shells of lingulid brachiopods from the Kanguk Formation of Devon Island, Arctic Canada and the Blufftown Formation of Stewart County, Georgia, USA were analyzed via electron microprobe. Elemental maps revealed spatial distributions, and element concentrations were measured within dorso-ventral transects proceeding toward the shell interior. The distributions of Mg, Na and Sr are similar among all lingulids, and absolute values of Mg and Na were similar between all fossil specimens. Concentrations of Mg and Sr are similar between fossil lingulids and modern calcitic brachiopods, suggesting that their signatures may be preserved. Ca, Fl and P are zoned within fossil shells from Stewart County and a modern lingulid, but are heterogeneously distributed in Devon Island specimens. While Mn and Fe concentrations are orders of magnitude greater in fossil lingulids than in modern calcitic brachiopods, they exhibit opposite distributional trends between fossil sites. Among fossil assemblages, elemental concentrations along dorso-ventral transects exhibit pronounced correlations between element pairs. Fossil specimens show a positive correlation between Na--Mg (80% of shells) and Fe--Mn (60% of shells). These trends suggest that elements observed to occur in the same zones in element maps and those that commonly substitute for one another are most likely to exhibit positive correlations. Ratios of mean concentrations of Mg/Ca and Sr/Ca decline sharply with shell length, suggesting that rates of biofractionation decreased with age. The chemical signatures analyzed in this study indicate that all of the fossil lingulids are diagenetically altered, but that the Devon Island specimens are more altered than those from Stewart County. Ultimately, reliable paleoenvironmental information was not extracted from these samples.