Elements Covers

Thematic Articles

Changing Trace Element Cycles in the 21st Century Ocean

Human activity is altering the ocean. This is happening through climate change, the release of pollutants, and direct exploitation of the marine environment. Recent advances in understanding the chemical cycling of trace elements within the global ocean comes at a critical time. Society is now increasingly viewing the ocean as a resource while also recognising that ocean systems are vulnerable to change.

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Trace-Metal Contaminants: Human Footprint on the Ocean

Anthropogenic activities have increased the fluxes of many trace metals into the oceans, changing their concentrations and distribution patterns. Despite their low dissolved concentrations, a number of these metals can still pose human and ecological risks. Some of these metals are well known (e.g. Pb, Hg), while others, such as the rare earth elements, represent emerging problems that impose new analytical challenges and environmental concerns. Defining the baselines of trace contaminants, identifying and quantifying the processes that control their transport, fate, and cycling are important issues to protect the ocean environment, safeguard human health, and support national and international marine decision-making.

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Discovering the Ocean’s Past Through Geochemistry

Trace elements and isotopes underlie many of the proxies used to reconstruct past ocean conditions. These proxies, recorded in diverse archives, are used to reconstruct seawater properties such as temperature, pH, and oxygen, or oceanic processes such as circulation, nutrient uptake, and biological productivity. Proxy calibration and validation requires a combination of ocean sediment core-top measurements, sediment trap studies, and laboratory- or field-based observations. New measurements of proxies in the modern ocean are rapidly illuminating the scope and limitations of each proxy while also helping to identify and evaluate new geochemical proxies that are based on trace elements and their isotopes.

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Oceanic Micronutrients: Trace Metals that are Essential for Marine Life

Trace metals are essential for life in the oceans but are present in extremely low concentrations. The availability of trace elements in surface waters frequently regulates the growth of microscopic marine plants called phytoplankton. As phytoplankton are responsible for taking up atmospheric carbon dioxide and exporting this to the deep ocean, trace elements are key components regulating the carbon cycle. New observations of the distribution of trace metals across all ocean basins from the GEOTRACES program have revealed a fascinating story of how the combination of trace metals interact with the ocean to regulate biological activity in new and surprising ways.

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Processes that Regulate Trace Element Distribution in the Ocean

Trace elements are powerful tracers – and in some instances drivers – of ocean interactions with the atmosphere, the hydrological cycle, the geology of the seafloor, and life on Earth. The concentration and the isotopic composition of trace elements are, therefore, diagnostic tools for the state of the ocean and its role as part of Earth’s dynamic system. Dissolved and particulate transport mechanisms determine how fast the ocean responds to change. The new wealth of data from the international GEOTRACES programme reveals new sources and sinks at all ocean boundaries, highlighting a much more dynamic equilibrium between the seafloor and the ocean than previously thought.

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New Tools, New Discoveries in Marine Geochemistry

Progress in the development of seawater sampling systems, analytical procedures and mass spectrometry have allowed measurements of trace elements and their isotopes in the ocean at spatial resolutions and at concentrations never achieved before. Marine geochemists are now exploiting these new developments to measure, for the first time, the stable isotopes of trace metals that are essential for marine life (e.g. Fe, Cu, Ni, Mo, Zn). The new data have already produced new insights into the interaction between ocean life and dissolved trace constituents and into the sources and sinks of those trace elements.

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GEOTRACES: The Marine Biogeochemical Cycle of Trace Elements and Their Isotopes

GEOTRACES is an international programme that was created to accelerate research on the global marine biogeochemical cycles of trace elements and their isotopes. Research encompasses four principle themes: 1) micronutrient elements (e.g. Fe, Co, Zn, Mn, Cu), which are essential for life; 2) tracers of processes that supply elements to the ocean; 3) anthropogenic contaminants (e.g. Hg, Pb); 4) geochemical proxies used in paleoceanography (e.g. trace-metal stable isotopes, naturally occurring radionuclides). The strategies adopted by GEOTRACES provide a template that can be adopted by other programmes in emerging fields of geochemistry.

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Deep-Sea Mining: International Regulatory Challenges and Responses

Deep-sea mining presents complex regulatory challenges due to its multi-faceted political, economic, technological, scientific, environmental, social, industrial and legal aspects. These must all be sensitively addressed to achieve a commercially viable and socially responsible industry. Furthermore, these aspects are either governed by, or must take into account, the burgeoning regulatory regime promulgated by the International Seabed Authority. This paper addresses the regulatory challenges associated with the three types of deep-ocean mineral deposits of greatest interest to the deep-sea mining industry: polymetallic nodules, ferromanganese crusts, and polymetallic sulfides. We review current resource interest, the international regulatory context, selected regulatory challenges, and the International Seabed Authority’s innovative responses to selected issues.

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Mining Deep-Ocean Mineral Deposits: What are the Ecological Risks?

A key question for the future management of the oceans is whether the mineral deposits that exist on the seafloor of the deep ocean can be extracted without significant adverse effects to the environment. The potential impacts of mining are wide-ranging and will vary depending on the type of metal-rich mineral deposit being mined. There is, currently, a significant lack of information about deep-ocean ecosystems and about potential mining technologies: thus, there could be many unforeseen impacts. Here, we discuss the potential ecological impacts of deep-ocean mining and identify the key knowledge gaps to be addressed. Baseline studies must be undertaken, as well as regular monitoring of a mine area, before, during, and after mineral extraction.

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Metal Extraction from Deep-Ocean Mineral Deposits

The future extraction of deep-ocean mineral deposits depends on being able to recover the metals in an economic and environmentally sensitive way. Metal production is one of the most energy intensive industrial sectors. The characteristics of some deep-ocean mineral deposits permit them to be readily dissolved and to release their contained metals into solution. Current innovations in hydrometallurgy, including metal leaching with ionic liquids and solvent extraction in non-dispersive phase contactors, demonstrate how metals could potentially be extracted from Fe–Mn deposits with increased energy efficiency and a reduced environmental footprint compared with traditional processing techniques. The importance of biological processes in the formation of deep-ocean Fe–Mn deposits is poorly understood. However, understanding how microorganisms select and deposit metal ions could further enhance targeted extraction of ‘critical’ metals.

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