The Geochemistry and Ecotoxicity of Offshore New Zealand Phosphorites
The New Zealand offshore seabed hosts diverse resources including phosphate rich rocks. Phosphate rock deposits on the Chatham Rise have been the focus of previous investigations into their composition and mining potential; however, the diversity of the geochemistry of phosphate deposits, including their wider distribution beyond the Chatham Rise, their trace metal budget, and potential for ecotoxicity, remain poorly characterised. This study addresses some of these gaps by presenting a geochemical investigation, including trace metals, for a range of phosphate nodules from across the Chatham Rise, Bollons Seamount and offshore southeastern South Island. Elutriate and reconnaissance bioaccumulation experiments provide insights into the potential for ecotoxic trace metal release and effects on biota should sediment disturbance through mining activities occur. The bulk chemistry of Bollons Seamount phosphorite nodules have been characterised for the first time, and show significant enrichment in first row transition metals; Co, Ni, Cu, Zn, in addition to Sr, Y, Mo, U, MnO, CaO and P2O5, and depletion in TiO2, Al2O3, MgO, K2O, FeO, SiO2, Sc, Cr, Ga, Rb, Cs, Hf, and Th relative to average upper continental crust. The cores of these nodules are dominated by apatite, quartz and anorthoclase phases, which are cross cut by Mn rich dendrites. The abundant presence of these minerals results in the significant differences in chemistry observed relative to Chatham Rise phosphorite nodules. The nodules also contain a secondary authigenic apatite phase, with a Mn crust rim. Significant rare earth element enrichment (REE) is most likely due to efficient scavenging by the Mn crust, resulting in seawater REE patterns characterised by negative Ce and Eu anomalies and heavy rare earth element enrichment. The bulk geochemistry of the Chatham Rise and offshore South Island phosphorite nodules is characterised by enrichment in CaO, P2O5, Sr, U, Y, Mo and depletion in TiO2, Al2O3, MnO, MgO, FeO, K2O, Sc, Cr, Cu, Ga, Rb, Cs, Ba, Hf, Ta, Pb and Th relative to average upper continental crust. The low concentrations of Cd in Chatham Rise, offshore South Island, and Bollons Seamount phosphorites make them potentially suitable sources for direct application fertilizers. The New Zealand marine phosphorite nodule deposits formed by repeated cycles of erosive bottom currents and phosphogenesis, resulting in the winnowing and concentration of the deposits. The iron pump model is proposed as a mechanism for the formation of apatite and associated mineral phases, giving the nodules their characteristic concentric zoning. The migration of the nodules through the oxic, suboxic, and anoxic zones of the sediment profile led to the formation of glaucony, apatite (suboxic zone), goethite (oxic zone), and pyrite with associated U enriched (anoxic zone) minerals. Rare earth elements (REE) in the Chatham Rise phosphorite nodules are associated with the glaucony rim minerals, and indicate that since the formation of the rims, very little diagenesis has occurred, preserving seawater REE patterns characterised by negative Ce and Eu anomalies and heavy REE enrichment. Site specific enrichments in trace elements Ba, V, Co, Ni, Cu, Zn, Y, Cd and Pb are attributed to either differences in incorporation of material into precursor carbonate e.g. volcanic materials, or higher fluxes of organic matter, delivering high concentrations of essential metals from biota, especially Cu and Zn. Direct pore water measurements from surficial sediment of the Chatham Rise show high concentrations of dissolved Fe and Mn, along with Cu, indicating suboxic conditions. High Cu concentrations measured in sediment pore water suggest that Cu release requires monitoring should seafloor surficial sediments on the Chatham Rise be disturbed. However, the elutriate experiments were not able to resolve if Cu release by sediment disturbance would exceed Australian and New Zealand Environment Conservation Council (2000) environmental guideline trigger values. The surrogate amphipod species Chaetocorophium c.f. lucasi shows promise as a biomonitor for disturbed marine sediments. Elements enriched in surficial sediments and phosphorite nodules, Hg, Pb, Fe, U and V, were not observed to bioaccumulate. Site specific differences in chemistry were observed, specifically in the different total relative bioaccumulation of Mo between amphipods exposed to sediments from two different sites. This suggests that future monitoring of chemical release during marine sediment disturbance requires the full geochemical characterisation of the substrate. Furthermore, fresh sediment and deep water should be used for future elutriate experiments, as storage of material by freeze-thawing and/or refrigeration causes mobilisation of some key trace metals such as U, V, Mo, Mn.