Planktic foraminiferal I/Ca, Fe/Ca, Al/Ca and Mg/Ca ratios in core top samples from cleaning experiments and iodine stabalization experiments

Abstract

Planktic and benthic foraminifera iodine (I) to calcium (Ca) molar ratios have been proposed as an exciting new proxy to assess subsurface and bottom water oxygenation in the past. Compared to trace metals, the analysis of iodine in foraminifera calcite is more challenging, as iodine is volatile in acid solution. Here we compare previous analyses that use tertiary amine with alternative analyses using tetramethylammonium hydroxide (TMAH) and ammonium hydroxide (NH4OH). In addition, we assess the effect of sample size and cleaning on planktic and benthic foraminiferal I/Ca. Our stabilization experiments with TMAH and NH4OH show similar trends as those using tertiary amine, giving relatively low I/Ca ratios for planktic and benthic foraminifera samples from locations with either poorly oxygenated subsurface waters or bottom waters, and high ratios for locations characterized by well oxygenated subsurface or bottom waters. This suggests that both alternative methods are suitable to stabilize iodine initially dissolved in acid. In terms of number of planktic foraminifera analyzed, our results show a wide spread in I/Ca when samples contain 5 to 10 specimens. Samples containing 20 specimens or more show I/Ca values centered around the average of the smaller samples, indicated that the larger sample sizes are more representative of the average planktic foraminifera sample community. The impact of cleaning on planktic and benthic foraminifera I/Ca ratios is very similar to Mg/Ca, with the largest effect occurring during the clay removal step. Largest iodine contaminations were recorded at locations characterized by moderate to high organic carbon contents. We recommend doubling the oxidative cleaning step (4 instead of 2 steps) to ensure that all organic material is removed.