Trace elements in New Zealand ferromanganese nodules: implications for deep sea environments

<p>Ferromanganese nodules are authigenic marine sediments that form over millions of years from the precipitation of Fe oxyhydroxides and Mn oxides from seawater (hydrogenetic-type growth) and sediment pore-water (diagenetic-type growth). Fe-Mn (oxyhydr)oxides grow in layers about nuclei, effectively scavenging minor metals such as Ni, Cu and Co from the waters they grow in. The uptake of different elements into the ferromanganese nodules reflects their environment and mechanism of growth, and these deposits are of interest both as a potential source of metals of economic interest, and as records of changing ocean conditions. This study investigates the composition of 77 ferromanganese nodules from the seafloor around New Zealand. Samples analysed come from locations several thousand kilometres apart under the same water mass (Lower Circumpolar Deep Water – LCDW), but with varying depth, current velocity, and sediment type. The outermost 1 mm rim of each nodule, representing near-modern growth, was sampled to compare with modern environmental parameters including substrate sediment composition and chemical and physical oceanography. Major, minor, and trace element analysis of nodule rims were undertaken, and the authigenic and detrital components examined via leaching experiments to evaluate their relative influence on growth mechanisms. Overall, New Zealand ferromanganese nodules are hydrogenetic in origin. However, there are systematic variations in composition that reflect variable diagenetic influence. Hydrogenetic endmember compositions are defined by samples from two localities in the Southern Ocean that have no evidence for diagenetic influence. Diagenetic influence on nodule composition is exemplified by samples from the two locations in the Tasman Sea, but also include nodules from the Campbell nodule field. Nodules from the Campbell nodule field come from two transects perpendicular to the Campbell Plateau, and the Deep Western Boundary Current (DWBC). Both sediment composition and nodule rim chemistry vary systematically across both transects. Areas closest to the slope have sediment profiles indicating high energy, erosive environments, continental-sourced sand components, and are dominated by nodules with hydrogenetic chemical characteristics similar to those of the Southern Ocean. Further from the slope, the sediment profiles indicate silt dominated sediments of a more oceanic crustal provenance, lower energy environment, and increased influence of oxic diagenetic processes on the major, minor and trace element profiles of the nodules. No hydrothermal contribution was identified in the chemistry of any of the nodules analysed. The physical and chemical properties of the sediment, along with current velocities, were found to be the key influences in diagenetic enrichment in the nodules. The influence of seawater chemistry was difficult to determine due to the lack of direct analyses in the area. Ferromanganese nodule chemistry is a function of the nodule environment, including water body, sediment composition and depth. The authigenic components of nodules can therefore be used to investigate the deep-sea environment. The redox conditions of sediments and the productivity of the overlying water will affect the trace metal constituents of the pore-waters of a sediment (Kuhn et al., 2017). Sediments with a larger fraction of labile organic matter may result in trace element enrichment of the pore-water. Sediments below the CCD will be higher in trace elements than sediments below the CCD (U1413, U1406B, U1402, U1398, U1398, and U1378) due to carbonate matter acting as a dilutant that can limit the supply of trace elements mobilised in the pore-water during diagenesis (Glasby, 2006). Terrigenous clasts such as quartz (Chester, 1990), will also reduce trace element enrichment in the pore-water due to their low reactivity, e.g. for the sediment U1406B, which has a high lithic component (Table 3.2). Sediments with a higher biogenic silica component (such as U1373, U1374, and U1378) (Table 3.2, Table 3.4) are predicted to produce nodules with higher trace element contents (ISA, 2010). In contrast to both the CCZ and Indian Ocean nodules, the Campbell nodule field samples formed above the CCD, and hence in sediments that include a significant carbonate component. This minimises the trace element pore-water enrichment and can account for the lower Cu+Ni+Co contents observed in the Campbell nodule field nodules compared with those that formed below the CCD (CCZ and Indian Ocean).</p>