Refined CT-Based Proxy for Foraminiferal Diagenesis Shows Evidence for Shallow Dissolution in North Atlantic Sediment Cores

Abstract. Geochemical reconstructions (e.g., δ¹⁸O, Mg/Ca) derived from the shells of the polar and subpolar planktonic foraminifera Neogloboquadrina pachyderma and Neogloboquadrina incompta are foundational to our understanding of high-latitude climate history. However, these species are highly vulnerable to diagenetic alteration—specifically partial dissolution and secondary calcification—which can significantly bias paleoceanographic signals. We introduce nCTDX%, a refined X-ray micro-computed tomography (μCT) proxy designed to quantify foraminiferal diagenesis with increased objectivity and time efficiency. By implementing mathematical thresholding and normalization to a density standard, nCTDX% allows for the direct comparison of shell density across different scans and study sites. Our results from North Atlantic, Nordic Sea, and North Pacific sediment cores confirm that both species dissolve from the inside out, preferentially removing internal structures previously shown to be rich in Mg and leaving behind resistant, low-Mg gametogenic crusts. While core-top nCTDX% correlates with bottom-water carbonate saturation (Δ[CO₃^2-]), we demonstrate that undersaturated pore waters can initiate significant dissolution within the top 2 cm of the sediment column, even at locations where overlying bottom waters are supersaturated. Furthermore, we show that nCTDX% is a sensitive indicator of secondary inorganic overgrowth, which appears in SEM images as low-density rhombohedral crystals on shell exteriors. The potential for pore water dissolution and inorganic calcification to overprint the bottom water Δ[CO₃^2-] signal contained in nCTDX% and other μCT and dissolution-based proxies suggests extreme caution must be taken when they are applied as proxies for bottom water carbonate ion saturation. Instead, given that even partial dissolution of internal shell walls can bias reconstructed temperatures by 1–2 °C, our findings suggest that μCT and dissolution-based proxies are better utilized as indicators of in situ diagenetic state. We therefore advocate for the routine application of nCTDX% to evaluate the preservation state of downcore samples and ensure the fidelity of geochemical paleoclimate records.