Micro-Tomographic Investigation of a North-Western Pacific Polymetallic Nodule

Micro-computed tomography (μCT) and X-ray Fluorescence (XRF) were used to investigate a Polymetallic Nodule (PN) from the North-Western Pacific abyssal plain to gather more information concerning the environmental changes that could be reflected by the PN’s internal structure. Despite its small size, for example, an ovoid measured 48 × 38 mm, the μCT revealed the presence of four concentric layers with varying thicknesses and opacities to X-rays, all developed around a fragment of a tooth, most likely belonging to a Lamniformes shark. The same micro-tomograph, functioning as an XRF spectrometer, allowed for the determination of the mass fractions of Mn and Fe in the first two external layers. To estimate the PN age, a model that considers PN growth rate proportional to the ratio of Mn to the square of Fe mass fractions was used, and, by extrapolating it to the entire PN, its age was estimated at 1.56 ± 0.22 Ma, i.e., Early Pleistocene. Therefore, the correlated use of μCT and FRX, two noninvasive methods, allowed to highlight a shark tooth fragment as being the PN nucleus as well as determine its absolute age.