PATTERNS OF TRACE ELEMENT GEOCHEMISTRY IN CULTURED NEOGLOBOQUADRINA PACHYDERMA CRUST CALCITE

Neogloboquadrina pachyderma is the species of planktonic foraminifera that dominates high latitude assemblages and is therefore the chief species utilized to study the paleoceanography of the polar regions. The calcification of many species of planktic foraminifera, including N. pachyderma, is complicated by the production of two distinct layers of calcite- the inner lamellar calcite, and a thick blocky crust. The crust calcite is distinctive from the lamellar calcite in both texture and composition and is thought to form as a result of both the downward migration of foraminifera into cooler waters and a shift in the physiological constraints on calcification. Analytical tools capable of high spatial resolution such as laser ablation ICP-MS can measure the geochemistry of discrete regions of shells, therefore allowing the lamellar and crust calcites to be analyzed separately. Additionally, live culturing of foraminifera directly links the properties of the water in which a foraminifera calcifies to the geochemistry of the shell. Here we present intrashell trace element (TE)/Ca data from N.pachyderma shells that were grown and subsequently produced a crust in culture at various temperatures. We observe marked shifts in the Mg/Ca, Ba/Ca, Zn/Ca, Al/Ca, and Mn/Ca between the lamellar and crust calcites grown in culture with no associated changes in either the environmental conditions or trace element concentrations of the seawater. The TE/Ca data were compared to the hydrologic conditions (i.e. temperature, salinity, pH, [TE]), to better understand the relationships between the crust and lamellar geochemistry and the environment in which the shells calcified. Additionally, we present a method to distinguish the TE/Ca of N. pachyderma crust and lamellar calcite, which we suggest should become the standard approach used to interpret LA-ICP-MS geochemical data. Discerning the geochemical changes that occur between the lamellar and crust calcites will inform studies on fossil N. pachyderma from the high latitudes to better understand past temperature, salinity, and meltwater influences.