Sensitivity of planktic foraminiferal test bulk density to ocean acidification

Abstract

The anthropogenic CO 2 accumulating in the ocean is lowering seawater carbonate ion concentration and may reduce calcification rates of marine calcareous organisms. Several proxies based on test weights of planktic foraminifera have been used to evaluate the impact of ocean acidification on these organisms. Unfortunately, because of the absence of a method to evaluate the bulk density of a test, the impact of seawater carbonate chemistry on test calcification is still not fully understood. In this study, we measured bulk densities of living Globigerina bulloides (planktic foraminifera) tests with an X-ray micro-computed tomography (XMCT) scanner and compared them with ambient seawater characteristics. Results demonstrated that test bulk densities were controlled by ambient seawater carbonate ion concentrations and that changes of test bulk densities were accompanied by changes in micron to submicron scale porosity of internal ultrastructure. These results suggest that alteration of the bulk density of foraminiferal tests due to acidification of ambient seawater can be directly observed by XMCT scanning. A useful metric of calcification intensity would therefore be physical measurements of test densities with XMCT.

Micro‐CT Scanning of Tests of Three Planktic Foraminiferal Species to Clarify DissolutionProcess and Progress

“…In contrast, the test area density (μg μm −2 ) of foraminifera, which is the ratio of the weight of a test to its projected area, is a more rigorous size‐standardized proxy than SNW, and it is also employed as a proxy of calcification intensity controlled by sea surface conditions (e.g., Moy et al, ; Weinkauf et al, ; Zarkogiannis et al, ). Sea surface conditions where foraminifera calcify can influence the thickness of foraminiferal test walls (Marshall et al, ) and bulk density (Iwasaki et al, ), which implies that the initial weights of foraminiferal tests differ depending on growth conditions as suggested in Barker and Elderfield (); thus, the test weight proxies SNW and test area density may be affected by not only dissolution at the deep seafloor but also calcification at the sea surface. The assessment of carbonate dissolution based on foraminiferal test weight measurements has been performed on the assumption that the sea surface conditions were uniform during glacial‐interglacial periods, despite these uncertainties in factors affecting initial test growth.…”

Evidence of Stable Foraminifera Biomineralization during the Last Two Climate Cycles in the Tropical Atlantic Ocean

“…Subject to the degree of segmentation, the X-ray microscopic analysis allows the determination and study of a number of biometric characteristics of the foraminifera shells, such as total shell volume ( Figure 2d), thus shell density (volume normalized weight) and calcite (test) volume, and thus test density [32] and calcite (test) surface (Figure 2a). The ratio of calcite volume/calcite surface provides a linear unidimensional quantity in length units and can thus serve as a measure of average test thickness.…”

“…The ratio of calcite volume/calcite surface provides a linear unidimensional quantity in length units and can thus serve as a measure of average test thickness. In this study, in addition to shell density, that is, the ratio of shell volume to shell mass, we use the "specific surface area", that is, the ratio of test volume/test surface, as a measure of average test thickness [32] and the test density, that is, the ratio of test volume to shell mass, as an indication of test porosity. Furthermore, by segmenting the area occupied by clay infillings, we were able to calculate by volume percent the degree of contamination in weight measurements.…”

Evidence of Stable Foraminifera Biomineralization During the Last Two Climate Cycles in the Tropical Atlantic Ocean

“…The segmentation resulted in the separation of the tomographs into shell area, area occupied by clay infillings (dirt) and internal shell (protoplasm) voids ( Figure 2). Subject to the degree of segmentation the X-ray microscopic analysis allows the determination and study of a number of biometric characteristics of the foraminifera shells, such as total shell volume ( Figure 2d) and thus shell density (volume normalized weight), calcite (test) volume and thus test density [30], calcite (test) surface (Figure 2a) etc. The ratio of calcite volume/calcite surface provides a linear unidimensional quantity in length units and can thus serve as a measure of average test thickness.…”

“…the ratio of shell volume to shell mass, we use the "specific surface area", i.e. the ratio of test volume/test surface, as a measure of average test thickness [30] and the test density, i.e. the ratio of test volume to shell mass as indication of test porosity.…”