Distribution of chlorine and fluorine in benthic foraminifera

Abstract

Abstract. Over the last decades a suite of inorganic proxies based on foraminiferal calcite have been developed, of which some are now widely used for paleoenvironmental reconstructions. Studies of foraminiferal shell chemistry have largely focused on cations and oxyanions, while much less is known about the incorporation of anions. The halogens fluoride and chloride are conservative in the ocean, which makes them candidates for reconstructing paleoceanographic parameters. However, their potential as a paleoproxy has hardly been explored, and fundamental insight in their incorporation is required. Here we used nano-scale secondary ion mass spectrometry (NanoSIMS) to investigate, for the first time, the distribution of Cl and F within shell walls of four benthic species of foraminifera. In the rotaliid species Ammonia tepida and Amphistegina lessonii Cl and F were highly heterogeneous and correlated within the shell walls, forming bands that were co-located with the banded distribution of phosphorus. In the miliolid species Sorites marginalis and Archaias angulatus the distribution of Cl and F was much more homogeneous without discernible bands. In these species Cl and P were correlated, whereas no correlation was observed between Cl and F or between F and P. Additionally, their F content was about an order of magnitude higher than in the rotaliid species. The high variance in the Cl and F content in the studied foraminifera could not be attributed to environmental parameters. Based on these findings we suggest that in the rotaliid species Cl and F are predominately associated with organic linings. We further propose that in the miliolid species Cl may be incorporated as a solid solution of chlorapatite or associated with organic molecules in the calcite. The high F content together with the lack of correlation between Cl and F or P in the miliolid foraminifera suggests a fundamentally different incorporation mechanism. Overall, our data clearly show that the calcification pathway employed by the studied foraminifera governs the incorporation and distribution of Cl, F, P and other elements in their calcite shells.

Fluorine incorporation into calcite, aragonite and vaterite CaCO3: Computational chemistry insights and geochemistry implications

“…As a consequence, our results lend further support to the idea that the fluorine content of biogenic carbonates may be employed as a good indicator of the partial pressure of CO 2 (pCO 2 ) in the oceans, and hence of the impacts of changes in atmospheric carbon dioxide. Since equation 1 (equivalent to reaction 5 in our schemes) is an exchange reaction, it implies that fluorine in carbonate will have sensitivity to pCO 2 , since increases in pCO 2 will result in reductions in pH and carbonate ion concentration ([ CO 2− 3 ] ) in seawater, which should result in an increase in fluorine concentration, measured in biogenic carbonates as F/Ca, in agreement with the results of Roepert et al (2019). While we can expect this to be the dominant control on fluorine contents in biogenic calcium carbonates in the oceans, especially given this reaction incorporates two fluorine atoms into the structure, we also note that reaction 4, which is only slightly more exothermic, implies F in carbonate should have some sensitivity to pO 2 in seawater.…”

“…Instead, fluorine contents seem to vary according to the preferred depth of calcification of individual foraminifera species (Opdyke et al, 1993;Rosenthal & Boyle, 1993;Rosenthal et al, 1997). A recent study of laboratorycultured benthic foraminifera suggests that the carbonate ion concentration of the growth medium is the only environmental parameter that co-varies with the fluorine content of foraminifera shells Roepert et al (2019).…”