Observation of the dissolution process of Globigerina bulloides tests (planktic foraminifera) by X‐ray microcomputed tomography

Abstract

We performed a 9 day dissolution experiment with tests of the planktic foraminifer Globigerina bulloides at pH 6.7 ± 0.1 in water undersaturated with respect to calcite. The initial stage of the dissolution process, which is not recognizable from the surface structure of the tests, was quantitatively evaluated by X-ray microcomputed tomography (XMCT). XMCT revealed three distinct test structures: early-developed calcite formed during the juvenile stage of G. bulloides, an inner calcite layer, and an outer calcite layer. The test ultrastructure was observed by scanning electron microscopy, and CT number evaluated the density distribution in the test. The early-developed calcite and inner calcite layer had low CT numbers (500-1300; low density, porous) and were sensitive to dissolution, whereas the outer calcite layer had high CT numbers (<1300; high density) and resisted dissolution. Both the modes and the frequencies of the CT numbers decreased with progress of dissolution. Changes in the CT number histogram with progress of dissolution were quantified in terms of the percentage of calcite volume accounted for by low-density calcite (% Low-CT-number calcite). A clear linear relationship (R 2 = 0.87) between % Low-CT-number calcite and % Test weight loss was found. This relationship indicates that the amount of test dissolution can be estimated from the distribution of CT numbers. We propose that XMCT measurements will be useful for quantitatively estimating the amount of carbonate loss from foraminiferal tests by dissolution.

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

“…Here we employed XMCT scanning to evaluate the progression of test dissolution of multiple species of planktic foraminifera in the seafloor sediments in midlatitudes (~30°N) of the western North Pacific, around the northern limit of the wide distribution of carbonate sediment. We verified the applicability of this test dissolution index using the CT number histogram suggested by Iwasaki et al () with three additional species ( T. sacculifer , G. ruber , and N. dutertrei ) that are dominant species in this study area. The dissolution process of these three species of planktic foraminiferal tests was investigated with cross‐sectional isosurface images and CT number histograms of well‐preserved tests from sediment trap samples and partially dissolved tests from core‐top sediments in the western North Pacific.…”

“…The XMCT system (ScanXmate‐D160TSS105/11000, Comscantecno Co., Ltd., Kanagawa, Japan) at the Japan Agency for Marine‐Earth Science and Technology, Yokosuka, Japan, was used to investigate the physical characteristics of the foraminiferal tests (Iwasaki et al, ). For 3‐D observation of the foraminiferal tests, we used a high‐resolution setting (X‐ray focus spot diameter, 0.8 μm; X‐ray tube voltage, 80 kV; detector array size, 2,000 × 1,336; 1,500 projections/360°; 0.5 s/projection).…”

“…To estimate carbonate dissolution intensities of foraminiferal tests, we employed variations of CT number histograms obtained by XMCT scanning. Iwasaki et al () have found that CT number histograms of dissolved G. bulloides tests show bimodality of the dissolved and preserved calcite. Based on this result, they proposed the relative volume of low‐CT‐number calcite to the volume of calcite in the whole shell (%Low‐CT‐number calcite volume) as a more quantitative proxy for carbonate dissolution than the conventional test weight proxy.…”

“…They used variations of mean CT number (bulk density of test) and CT number histograms, which are measured by XMCT scanning, in order to evaluate the dissolution intensity of G. bulloides tests: the mean CT number of G. bulloides tests decreases and the CT number histogram shifts from a unimodal to a bi‐modal distribution with the progression of test dissolution. Thus, in order to quantify the shift of the CT number histogram with test dissolution, Iwasaki et al () suggested %Low‐CT‐number calcite volume, which is calculated based on the CT number histogram, as a new index of test dissolution intensity. Because the CT number histogram is independent of variations of test size and test wall thickness due to differences of initial test condition, this index is expected to be useful as a new proxy of carbonate dissolution at the deep seafloor; in the future, it might be developed as a quantitative proxy of deep seawater [CO 3 2− ].…”

Review: Potential catastrophic reduction of sea ice in the western Arctic Ocean: Its impact on biogeochemical cycles and marine ecosystems

“…The development of new quantification methodologies for accurate estimation of the physiological impacts of acidification on plankton are necessary to evaluate the impact of ocean acidification on organisms and marine ecosystems. One example is the Micro-focus X-ray Computing Tomography (MXCT) technique for measuring the X-ray attenuation of an object (calcifier), which is linearly related to its density (Iwasaki et al, 2015). The MXCT technique has the potential to be a robust method for quantifying the response of marine organisms to ocean acidification.…”