An ITRAX geochemical study of ferromanganiferous sediments from the southwestern pacific Ocean
|Location||International Geological Congress,oslo 2008|
|Author||Cronan, David۱; Rothwell, Guy۲; Croudace, Ian۲|
|Holding Date||23 September 2008|
An exhaustive analysis of manganese nodule variability throughout the southwestern Pacific has confirmed that Cook Islands manganese nodules are potentially the most economically viable of such deposits in the region.They occur in the Penrhyn Basin where they reach abundances in excess of 30 kg per sq meter.In the south of the Basin they rest on dark brown manganiferous clays,while in the northern part the substrate becomes gradually more calcareous and siliceous.Nine box cores representing this variability,each up to half a 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 at a resolution as fine as 200 microns using photography,X-radiography and XRF analysis.As the sediments in the present study will be badly disturbed down to at least 50 cm during any future manganese nodule mining in the Penrhyn Basin,a detailed knowledge of their nature is a pre-requisite to environmentally sound recovery.
The cores were collected along a transect at about 158 W,between 12 S and the equator.South of about 4 S the sediments are uniform brown to dark brown ferromanganiferous clay down to at least 50 cm below the sea floor,averaging about 5-7.5% Al,17-21% Si,4-7.5% Fe and 1-2% Mn.Most of these elements occur at higher concentrations in the south compared to the north.North of 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 progressively replaces calcium carbonate as an important sediment builder under highest productivity waters at the equator.
Ferromanganese oxide associated trace elements are higher in the clays than in the other sediments with Ni averaging 200-600 ppm,Cu 350-650 ppm and Co 120-250 ppm.Other trace elements such as Cr,V,Zn,Pb and As are also higher in the clays than in the other sediments and show little variation downcore.
The sediments studied exhibit important differences from those in the other main proposed manganese nodule mining area in the South Pacific,the Peru Basin.There 5-15 cm of oxic brown mud overlies siliceous-calcareous ooze down to below 50 cm.Elevated concentrations of divalent Mn occur in the pore waters of the latter,potentially available for release into seawater during mining.In the Peru Basin,the greatest potential discharge of toxic trace elements into seawater will occur if oxic sediments are removed during mining and porewaters are released directly into seawater from underlying sub-oxic sediments.
Sediments in the Penrhyn Basin are overall more oxic than those in the Peru Basin and thus toxic element pollution during nodule mining there will probably be minimal.