Tiny drifting shells help scientists reconstruct what the ocean was like in the past. Now, a scan can measure not only their shape, but how densely built they are—without breaking them open. Tiny plankton shells have traditionally been used to reconstruct past and present marine environmental conditions.

Their shape and makeup carry clues about the ocean they lived in. Many planktonic foraminifera develop a secondary crust layer as they reproduce, covering the thinner primary layer they formed at the end of growth. That means one shell can preserve more than one stage of its life.

But here is the catch: measuring how densely a microfossil is built has not been discussed in enough detail. The key questions are whether the measurement is accurate and whether it can be repeated reliably. So the study sets out the precautions needed for these X-ray measurements, examines their stability, and develops new reference materials for measuring calcium carbonate density.

The same concept can be applied to a foraminiferal sample. Although the attenuation coefficient and gray value are different quantities, both relate to X-ray attenuation, and their relative relationship can be normalized into relative density. The shell’s scan value can then be compared with a standard sample and expressed as a relative density.

The same concept applies to calculating the bulk density of a foraminiferal sample. To make that comparison trustworthy, the study creates carbonate reference pieces with different amounts of empty space. Different pressing pressures produce different degrees of packing between the calcium carbonate particles.

The reference pieces are scanned under the same conditions as the foraminiferal shell, allowing the scan value to be connected to bulk density. This provides a dependable bridge from how tightly packed the calcium carbonate is to its CT reading: the points lie almost exactly along a straight relationship, supporting its use for calibrating density measurements.

The scan stayed stable over the two-month investigation period. Its variability showed no clear trend over that time, supporting the use of the setup for repeated measurements. That matters because the CT number's variability showed no clear trend over the two-month investigation period, providing evidence of stable measurements over time.

Using this approach, the mean bulk density of the shell was two point three grams per cubic centimeter, with a small spread around that value. The calculated weight also fell within the reported range for individual shells in modern water.

So the scan produced a mean bulk density of two point three plus or minus zero point zero two grams per cubic centimeter and a weight of twenty-one point eight plus or minus zero point two eight micrograms, consistent with earlier observations. But density is not a perfect damage detector.

In a shell with two layers, strong dissolution can remove the more vulnerable layer first and leave the tougher layer behind. That remaining shell could still have a high scan value, falsely suggesting that it escaped damage. Density must therefore be considered together with the shell’s visible internal structure.

The practical lesson is to read the scan number alongside the shell's visible internal structure, including when examining the thickness and volume of foraminiferal tests. The study shows that careful X-ray scanning can measure the density and shape of these shells accurately, offering a better way to read ocean history—provided density is interpreted alongside the shell’s structure.