Intra-shell oxygen isotope ratios in the benthic foraminifera genus Amphistegina and the influence of seawater carbonate chemistry and temperature on this ratio

Abstract

Using secondary ion mass spectrometry (SIMS) we looked at the natural variability in the oxygen isotope ratio of the shallow water, symbionts-bearing foraminiferan Amphistegina lobifera. Live foraminifera were collected in February 2005 in the Gulf of Eilat, Israel. Vertical section exposing the knob area of this species represents the growth history of this species from August 2004 to February 2005. SIMS profile at a resolution of ∼15 μm (representing about 2 weeks considering the size of the knob area and the life span of ≈6 months of this foraminifera species) yielded δ 18O changes of ∼1.5‰ that are compatible with the known temperature changes for the Gulf of Eilat for this period (21–27 °C). Natural variability between primary and secondary calcite at the knob area were obtained on horizontal section of the upper knob area. This section is semi-tangential to the growth lines and exposes relatively wide belts of the primary calcite which could be analysed using the SIMS (beam size of 10 × 20 μm). The primary calcite δ 18O value is on average more than 3‰ lower than the secondary calcite that represents the bulk of the skeleton (more than 95% by weight). A vertical profile at the knob was obtained by rastering an area of 50 × 50 μm at vertical steps of roughly 1 μm. The profile revealed a narrow zone of lower δ 18O compared to the higher values above and below it. The difference between the lowest δ 18O and the highest one was also close to 2‰. The δ 18O in the margin – keel area of A. lobifera is also lower compared to the bulk secondary calcite. Specimens that were cultured in the laboratory at a constant temperature and inorganic carbon but at different pH have increased their CaCO 3 weight by roughly a factor of 8. Single specimen from each pH (ranging between 7.90 and 8.45) were investigated with the SIMS at the knob area. While there is some variability within each specimen (perhaps related to the primary calcite), the general trend was a decrease in δ 18O with increasing pH (or CO 3 2− concentration), in agreement with previous studies on planktonic foraminifera. Some other specimens grown at different temperatures (between 21 and 33 °C) were also measured with the SIMS at the knob area. For each temperature, we observed also some variability, nevertheless the trend of −0.2‰/°C in δ 18O is observed.