An ITRAX Geochemical Study of Ferromanganiferous Sediments from the Penrhyn Basin, South Pacific Ocean

Abstract

Manganese nodules reach abundances in excess of 30 kg per square meter in the Penrhyn Basin, central equatorial South Pacific. In the south of the Basin, they rest on dark brown ferromanganiferous clays, whereas in its northern part, the substrate becomes more calcareous and siliceous. Nine box cores representing this variability, each up to about one half meter in length, have been subjected to ITRAX XRF measurement, calibrated by laboratory WD-XRF analysis. ITRAX is an automated core scanning instrument that records optical, radiographic and elemental variations in sediment cores at a resolution as fine as 200 microns using photography, x-radiography and XRF analysis. Additional piston and gravity cores have been studied lithostratigraphically. As the sediments in the Penrhyn Basin will be badly disturbed during any future manganese nodule mining there, a detailed knowledge of their nature is a prerequisite to environmentally found nodule recovery.

The cores studied were mainly collected along a transect at about 158.5° W between 12° S and the equator. South of about 4° S the sediments are uniform brown to dark brown, and ferromanganiferous pelagic clays averaging about 5–7.5% Al, 17–21% Si, 4–7.5% Fe, and 1–2% Mn. North of about 4° S the sediments become progressively more calcareous, initially in the upper parts of the cores as biological productivity increases towards the equator, but by 2.20° S they are calcareous throughout. Finally, north of 2.20° S, the cores have increased Si and decreased Ca in their upper parts, as biogenic silica co-exists with calcium carbonate as an important sediment builder under highest productivity waters at the equator. Most trace elements are higher in the clays than in the biogenic sediments and show little variation downcore.

The sediments studied from the Penrhyn Basin exhibit important differences from those in other projected Pacific manganese nodule mining areas, such as the Clarion-Clipperton Zone and the Peru Basin. In the former, siliceous sediment is the main substrate in areas of high nodule abundance, whereas pelagic clay fills this role in the Penrhyn Basin. In the latter, 5–15 cm of oxic brown mud overlies sub-oxic siliceous/calcareous ooze down to below 50 cm, the expected minimum depth of sediment disturbance during manganese nodule mining. Elevated concentrations of divalent Mn occur in the pore waters of the latter. Sediments in the Penrhyn Basin do not exhibit this and are overall more oxic than those in the Peru Basin.

Keywords:

• ITRAX
• manganese nodules
• pelagic sediment
• penrhyn basin