Neutron Computed Tomography: A novel high-resolution, non-destructive method for screening fossil coral for diagenetic alteration for geochronologic and paleoclimatic reconstructions

A novel high-resolution, non-destructive method for diagenetic screening of fossil corals for geochronologic and paleoclimatic studies using neutron computed tomography (NCT) is proposed. NCT circumvents limitations of traditional techniques, such as destructive sampling and 2-D imaging by providing detailed 3-D visualizations of coral structure and carbonate mineral phases. This method differentiates aragonite and calcite phases in fossil coral, crucial for identifying well-preserved sections suitable for dating and paleoclimatic reconstructions. A key advantage of NCT is its ability to map hydrogen content, providing a reliable indicator for identifying regions of well-preserved skeletal aragonite, since aragonite typically retains more water organic-matter than calcite. NCT scans conducted on a Holocene Porites coral (ca. 1.36–1.87 ka BP) from Muschu Island, Papua New Guinea, successfully distinguished between secondary low-magnesium calcite and aragonite skeletal material. This technique was also applied to an Isopora palifera fossil coral (ca. 39.4 to 44.8 ka BP) from Ashmore Reef, Northwest Shelf, Australia, which presented a more complex diagenetic history. Comparisons were made with results from hyperspectral imaging, X-Ray CT, scanning electron microscopy, and geochemical and petrological analyses, following calibration using a modern Porites coral from One Tree Reef, Southern Great Barrier Reef, Australia. Additionally, NCT was applied to an altered Acropora humilis coral (ca. 600 ± 280 ka BP) from Ribbon Reef 5, Great Barrier Reef, revealing small, hidden aragonite sections undetected by surficial hyperspectral imaging. This study demonstrates the advantages of combining NCT with traditional screening methods in identifying well-preserved aragonite for accurate geochronologic and paleoclimatic reconstructions. Recommendations for applying NCT in fossil coral screening are provided.