4:30 PM - 4:45 PM
[MIS12-30] Micro X-ray CT technique for improving the performance of paleoceanographic proxies
Keywords:Microfocus X-ray Computed Tomography, foraminifera, Paleoproxy
Most paleoceanographic and paleoenvironmental analyses using deep-sea sediments are based on microfossil proxies, especially calcareous foraminiferal tests. However, geochemistry of foraminiferal tests can be easily modified by the surrounding physicochemical environments such as selective dissolution and diagenesis after deposition. This can also strongly affect the accuracy and reliability of paleoceanographic reconstruction. On the other hand, advances in molecular biology for foraminifers have revealed the existence of cryptic species within a single species (e.g. Darling, et al., 2000), and these morphological differences are generally difficult to distinguish under an optical microscope. Thus, the understanding of relationship between morphology, genetic types and geochemical signals recorded on the foraminiferal tests has been highlighted but remains unresolved in many cases.
Microfocus X-ray CT (µCT) has the potential to make a significant contribution to our understanding of above issues. µCT is a device that irradiates X-rays from an X-ray tube to an object from all directions and reconstructs the transmitted images using a computer to obtain a three-dimensional image. It can obtain precise morphological information of targeted object on the submicron order. Because it is a nondestructive analysis, it is possible to perform various analysis (like geochemistry) after the morphometric analysis. This is one of the great advantages for improving paleoproxy by using µCT.
Recently, it has been focused on the quantitative density and dissolution of calcareous tests using µCT. Because foraminiferal tests are composed of homogeneous calcite, the grayscale value distribution (≈X-ray absorption) of the test depends on the density of the calcite. Based on this feature, it has been proposed to reconstruct the spatio-temporal carbonate ion concentration in past seawater from the calcite CT number of foraminiferal tests (Iwasaki et al., 2022). This is far more quantitative than the conventional methods like bulk weight method and/or estimation using fragmentation ratio of foraminifers. More recently, a methodology for determining bulk density and individual weights directly from calcite CT number using the external phantoms made from calcite has been established, and it is now possible to correctly evaluate the biological impacts in the marine environmental changes (Kimoto et al., 2023). As described above, brand-new analytical techniques of µCT has the potential to contribute to the study of material cycles in the present ocean, as well as to improve the accuracy and reliability of paleoproxy.
Microfocus X-ray CT (µCT) has the potential to make a significant contribution to our understanding of above issues. µCT is a device that irradiates X-rays from an X-ray tube to an object from all directions and reconstructs the transmitted images using a computer to obtain a three-dimensional image. It can obtain precise morphological information of targeted object on the submicron order. Because it is a nondestructive analysis, it is possible to perform various analysis (like geochemistry) after the morphometric analysis. This is one of the great advantages for improving paleoproxy by using µCT.
Recently, it has been focused on the quantitative density and dissolution of calcareous tests using µCT. Because foraminiferal tests are composed of homogeneous calcite, the grayscale value distribution (≈X-ray absorption) of the test depends on the density of the calcite. Based on this feature, it has been proposed to reconstruct the spatio-temporal carbonate ion concentration in past seawater from the calcite CT number of foraminiferal tests (Iwasaki et al., 2022). This is far more quantitative than the conventional methods like bulk weight method and/or estimation using fragmentation ratio of foraminifers. More recently, a methodology for determining bulk density and individual weights directly from calcite CT number using the external phantoms made from calcite has been established, and it is now possible to correctly evaluate the biological impacts in the marine environmental changes (Kimoto et al., 2023). As described above, brand-new analytical techniques of µCT has the potential to contribute to the study of material cycles in the present ocean, as well as to improve the accuracy and reliability of paleoproxy.