Marine Biogeochemistry

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.

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.

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.

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.

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.

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.

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.