ALBERT

Description: Highlights • Seafloor massive sulphides vary in mineralogy and oxidise at different rates. • Galvanic cells play a role in increasing dissolution rates. • SMS deposits that have been inactive for some time may have lost economic value. • The geochemistry of oxidation products has potential to be an exploration tool. • Potential for anthropogenic release of heavy metals during seafloor mining. • Any risk needs addressing by the ISA prior to the formation of mining regulations. The weathering process of seafloor massive sulphide (SMS) deposits can be considered analogous to weathering of terrestrial volcanogenic massive sulphides (VMS) deposits. However, in the context of SMS deposits, the process occurs in chemically buffered waters of near neutral pH, resulting in the formation of insoluble Fe oxy-hydroxide minerals including goethite and hematite as well as sulphates such as jarosite. As a result of this precipitation, it is commonly assumed that any SMS deposit is unlikely to exhibit a significant loss of metals (dissolution and release of heavy metals) into the water column. However, galvanic interactions have never been considered in this seafloor context, whilst they have already been shown to have the ability to increase dissolution significantly in terrestrial deposits. If heavy metal release is not temporally balanced by precipitation of oxide phases, there is the potential that these metal occurrences lose economic value. This is specifically significant if there is an industrial focus on exploiting deposits associated with hydrothermal vents that have been inactive for some time. Not only this, but the geochemistry of weathering products – ‘gossans’ – that are formed have the potential to be used as tools for exploration. Furthermore, it is unknown what impact galvanic coupling may have with regards to anthropogenic release of heavy metals during seafloor mining of deposits associated with either active or inactive vents (disturbance of sediment, plume generation and dewatering process). This environmental impact needs to be addressed prior to the formulation of regulations for deep-sea mining by the International Seabed Authority. The present review examines our current understanding of oxidation and dissolution of a mixed sulphide ore, bringing together lines of evidence from a range of literature sources. Based on this review, different seafloor sulphide ore deposits will dissolve by oxidation and release a variety of different metals (economic and/or toxic), all at different rates, with galvanic cells playing a role by increasing dissolution rates. While precipitation of oxide and oxy-hydroxide phases will occur, it is unknown in both a natural weathering and anthropogenic (mining) context whether heavy metal release (including economic and toxic metals) is temporally balanced by this precipitation and any subsequent adsorption. Based on our current understanding, certain sites will be more predisposed to oxidation as a result of their mineralogy (those containing significant pyrrhotite, marcasite, galena and secondary copper sulphides) and/or environment (higher temperatures and oxygen concentrations, lower pH's). Furthermore, certain sites pose more of an environmental risk (in terms of toxicity) than others, with arc-related SMS deposits associated with higher concentrations of As, Pb, Sb, Cd and Hg and ultra-mafic hosted SMS deposits associated with high concentrations of Co and Ni. The review highlights the need for subsequent studies that investigate the natural weathering process of seafloor sulphide ore deposits, including how this process impacts their economic value and consequent geochemical signature of oxides that are produced over time. Moreover, this review underlines the necessity for experiments to elucidate the oxidative dissolution of ore throughout any mining process.

Description: Heinrich events are intervals of rapid iceberg-sourced freshwater release to the high latitude North Atlantic Ocean that punctuate late Pleistocene glacials. Delivery of fresh water to the main North Atlantic sites of deep water formation during Heinrich events may result in major disruption to the Atlantic Meridional Overturning Circulation (AMOC), however, the simple concept of an AMOC shutdown in response to each freshwater input has recently been shown to be overly simplistic. Here we present a new multi-proxy dataset spanning the last 41,000 years that resolves four Heinrich events at a classic mid-depth North Atlantic drill site, employing four independent geochemical tracers of water mass properties: boron/calcium, carbon and oxygen isotopes in foraminiferal calcite and neodymium isotopes in multiple substrates. We also report rare earth element distributions to investigate the fidelity by which neodymium isotopes record changes in water mass distribution in the northeast North Atlantic. Our data reveal distinct geochemical signatures for each Heinrich event, suggesting that the sites of fresh water delivery and/or rates of input played at least as important a role as the stage of the glacial cycle in which the fresh water was released. At no time during the last 41 kyr was the mid-depth northeast North Atlantic dominantly ventilated by southern-sourced water. Instead, we document persistent ventilation by Glacial North Atlantic Intermediate Water (GNAIW), albeit with variable properties signifying changes in supply from multiple contributing northern sources.

Description: Due to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies.