Beyond Iron in the Ocean: Trace metal micronutrients and the carbon cycle (BIO-Trace)

Microscopic plants in the ocean, called phytoplankton, are responsible for about half of the solar-powered photosynthesis on Earth. As they grow and reproduce, phytoplankton take up dissolved carbon dioxide (CO2) from the surface ocean, where it is in balance with atmospheric CO2 gas, and convert it into solid organic carbon. When they die, this organic matter sinks into the deep ocean, and is converted back to dissolved CO2 via grazing by other plankton and bacteria. This process, called the biological pump, removes CO2 from the surface ocean-atmospheric reservoir and transfers it to the deep ocean, where it may be trapped for several hundred, or even thousand, years. The biological pump is pivotal to Earth's climate. Without it, pre-Industrial Revolution levels of atmospheric CO2 would have been more than 50% higher than observed, while today the oceans have already absorbed about 30% of human CO2 emissions. The key aim of this project is to understand the processes that determine the efficiency of the biological pump.

Like all living organisms, phytoplankton need a wide range of nutrients to grow, nutrients that they obtain from their external environment, i.e., from seawater. About 95% of all organic matter is made up of six 'macronutrients' (carbon, hydrogen, nitrogen, phosphorus, oxygen, and sulphur), which are used to make carbohydrates, proteins, fats, and nucleic acids (e.g., DNA). In addition, phytoplankton also require a range of trace metal 'micronutrients', which are used in an array of enzymes necessary to carry out the essential processes of life (including, for example, photosynthesis). The nutrient that is in shortest supply is called the 'limiting nutrient', because it limits phytoplankton growth and reproduction (or productivity). Limiting nutrients are one key control on the activity of the biological pump. For example, the micronutrient iron has long been known to be an important limiting nutrient in high latitude (polar) surface oceans. Other trace metals, like zinc, are likely also important, but have been less well studied.

The distribution of nutrients in the ocean is controlled by a complex interplay of their inputs (such as dust), biological uptake and sinking (the biological pump), and their redistribution laterally and vertically in the ocean via the movement of packages of water, called water masses. Of particular importance to this interplay are geographic regions where an exchange between the surface and deep ocean occurs, typically in the high latitudes. One of these regions is the Southern Ocean. Here, deep water masses, with a high nutrient content, return to the surface, while other water masses sink back from the surface towards intermediate depths and flow equator-wards. Critically, nutrients supplied to the low latitudes in these intermediate depth water masses are a key control on global ocean productivity, and hence atmospheric CO2. In my project, I will identify links between micronutrient supply to the surface ocean, the complexity of the chemical forms of trace metals present in seawater, and the community of phytoplankton present in the high latitude surface ocean, and evaluate how these factors combine with the ocean circulation to set the global distribution of nutrients in the ocean. The result will be a coherent, in depth understanding of how micronutrient limitation of the biological pump in the high latitude oceans impacts whole ocean carbon cycling.

The key objective of this proposal is to better understand the interactions between biology, ocean circulation, and carbon cycling, with direct ramifications for climate. The main impact of my research beyond the academic will therefore be societal, with the key beneficiaries likely to be policy makers and the general public.

1) Press/ Media/ Lay scientific organisations/ Wider public
Climate science is an emotive topic, with broad appeal. The global atmospheric CO2 concentration has increased dramatically as a result of human activities since 1750. Understanding the interactions between CO2, biology, and physical ocean circulation are vital to establish the degree to which the ocean will continue to 'soak up' manmade CO2 in the future. The research in this proposal will contribute directly to this understanding, and hence be of broad interest to press/media and the general public.

2) Policy Makers
This research will have value for policy makers seeking to act on the 2016 Paris Agreement. For example, iron fertilization of the Southern Ocean is one proposed means to draw down atmospheric CO2 concentrations by boosting biological productivity in this region. However, the consequences of such geo-engineering are likely to be far-reaching - this project will contribute valuable mechanistic understanding to if and why this is likely to be the case. On longer timescales, the incorporation of this mechanistic understanding in to climate models will improve the forecasts of future climate change that policy makers rely on.

3) Environmental consultants
Other potential beneficiaries of the trace metal stable isotope work proposed here are geochemists working in the environmental consulting sector. In particular, metal stable isotopes may provide tracers of sources of anthropogenic pollution. Better constraints are required on the isotopic variability of anthropogenic materials, however, including, for example, aerosols. This will be addressed as part of this proposal. In addition, the relative roles of biological uptake, adsorption on particle surfaces and organic complexation on these novel metal isotope systems remain poorly understood. Lessons from the marine realm from this project will be of value to those working in terrestrial aqueous systems.

4) Fisheries Management
Understanding nutrient distributions and phytoplankton community structure in the ocean is key to evaluating fish stocks and their likely evolution with global change. For example, nutrient influx from melting Arctic ice stimulates blooms of phytoplankton, which in turn boost the productivity of higher trophic levels. However, the extent and composition of this nutrient influx, as well as its potential ecological impact, is poorly understood. This is a topic that will be elucidated in this proposal.