Trace/minor element:calcium ratios in cultured benthic foraminifera. Part II: Ontogenetic variation

Abstract

Ontogenetic (developmental stage) measurements of Mg/Ca and Sr/Ca were made on the benthic foraminifer Bulimina aculeata, which were cultured under controlled physicochemical conditions of temperature, pH, alkalinity, salinity, and trace- and minor-element concentrations. We utilized two methods of ontogenetic sampling—whole specimens progressively increasing in length and laser microdissection of a single specimen with subsequent analysis of dissected portions. A novel high-resolution laser-microdissection (HRLM) method allowed for precise (10 μm) cuts of the foraminiferal tests (shells) along the geometrically complex sutures distinguishing individual chambers. This new microdissection method limited sample loss and cross-contamination between foraminiferal chambers. Little or no variation in D Sr was observed at different foraminiferal developmental stages. Conversely, D Mg was enriched during a mid-developmental stage of whole-specimen samples (150–225 μm D Mg = 1.6 × 10 −3) compared to earlier and later stages (<150 μm, >225 μm D Mg = 8.3 × 10 −4). Further analysis of HRLM ontogenetic samples showed a larger, age-dependent D Mg signature variation. This increase in shell Mg/Ca may contribute substantially to the measured inter-individual variability in Mg/Ca temperature prediction for cultured B. aculeata. Due to relatively large Mg/Ca inter- and intra-individual variability, measuring similar-size foraminiferal samples may improve the precision of paleotemperature prediction. Additionally, partial dissolution of the highest ontogenetically Mg-enriched calcite ( D Mg = 1.3 × 10 −2–1.6 × 10 −2) may occur in undersaturated bottom-water environments or during reductive cleaning procedures. Thus, the calcite phases remaining after partial dissolution by either natural or laboratory cleaning processes may not accurately represent the calcification environment.