TRACE ELEMENT INCORPORATION DURING SKELETAL GROWTH

Distinguishing amongst calcium carbonate minerals (low-magnesium calcite, high-magnesium calcite, and aragonite) in the rock record is problematic. Both aragonite and high-magnesium calcite are thermodynamically unstable, and readily alter to low-magnesium calcite during early postdepositional diagenesis. Differential incorporation of trace elements into mineral phases, however, may provide a means for discriminating between polymorphs, even after diagenetic recrystallization of these thermodynamically unstable phases. If true, multi-element fingerprinting could provide a powerful tool in the diagnosis of primary mineralogy and in understanding changes in the marine trace element inventory through time.

Incorporation of trace elements into mineral phases are defined by distribution coefficients, which reflect the concentration of ions in the fluid from which the mineral precipitates, as well as parameters such as temperature and precipitation rate, which itself can vary as a function of both temperature and biological growth. This study uses two species of Echinoidea (sea urchins) as model carbonate-producing organisms to test the degree of variability within biologically-precipitated carbonate minerals and the potential extent of biotic control on trace element incorporation. Echinoids were chosen for this study because they have long been considered faithful recorders of marine chemistry, yet record substantial isotopic variation across individual organisms, suggesting the potential for a biological signal reflected in skeletal growth.

Two echinoids (Plococidaris verticillata, Echinometra mathaei) were systematically dissected to obtain skeletal elements that reflect a biological growth history. Skeletal elements were treated to remove organic matter and hand crushed into powder, and trace element concentrations were measured via ICP-MS. Results show variation in trace element concentration between skeletal elements (i.e. body, spines), but no clear trends were observed between the relative ages of skeletal elements and trace element content. Our study suggests that “vital effects” have little effect on trace element incorporation, and thus support further investigation of multi-element fingerprinting as a mineralogical proxy.