A nutrient and carbon pump over mid-ocean ridges (RidgeMix)

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

Abstract

Phytoplankton are aquatic, single-celled plants that lie at the heart of the global cycling of carbon between the atmosphere and the oceans. Like other plants, phytoplankton require sunlight and nutrients to grow and flourish. However, in the ocean sunlight is confined to the upper few tens of metres, while nutrient concentrations are low at the sea surface and greatest at depths of a kilometre or more. The growth of phytoplankton is thus fundamentally dependent on processes that transfer nutrients from depth up to the sunlit surface.

Over the mid latitudes the problem of acquiring nutrients appears to be particularly stark. The winds at mid latitudes provide a widespread downward transport of water, which inhibits the transfer of nutrient-rich deeper waters up into the sunlit, surface waters. Thus, one might expect much of the mid latitude ocean to be a desert due to a lack of nutrients. However, phytoplankton growth in the mid latitude ocean is more than might initially be expected, and is globally very important as it drives about half of the oceans' biological removal of carbon out of the atmosphere. Oceanographers have calculated the amount of nutrient required to support this growth, based upon the concentrations of inert tracers in the upper ocean. However, adding together the known nutrient supplies falls significantly short of this total nutrient requirement. Hence, there is a conundrum as to how the biological growth over the mid latitude ocean is sustained. If we want to understand how carbon is cycled between the atmosphere and oceans, and how it affects our climate, we need to answer this problem.

In this proposal, we address the problem of how deep nutrients are transported into the surface waters in mid-latitudes. We propose to test a new view: tides passing over the mid-Atlantic ridge generate enhanced turbulence and mixing, which in turn provides a nutrient supply to the upper thermocline waters. These nutrients are then transported horizontally along density surfaces over the western side of the basin, probably being swept along the Gulf Stream and eventually passing into the winter mixed surface layer. When this surface layer shallows and warms in spring, the nutrients are then available to the phytoplankton. The work plan involves two main components. We will carry out a field programme collecting measurements of the turbulence and nutrient concentrations over and adjacent to the mid-Atlantic ridge. This fieldwork will involve collecting data from a novel long-term moored array of instruments on the ridge along with a focused 5 week research cruise. Our work involves sampling sufficiently quickly to be able to resolve tidal changes in currents and mixing over the ridge: this has never been done before, and we have brought together scientists with expertise in tidal measurements in shallower shelf seas with others who are expert in deep ocean mixing and transports in order to do this. The 2nd component of our work will use computer models of circulation in the Atlantic to explore the wider implications of the fieldwork observations, allowing us to decide whether or not mixing over the mid-Atlantic ridge really does provide enough nutrients to explain the phytoplankton production in the mid-latitude N Atlantic.

Planned Impact

Who:
While our work is not immediately relevant to policy making, we expect policy makers and civic leaders to have an interest in work that addresses the fundamentals of how the global carbon system works. We wish to use the big questions in our science, and the challenge of making observations in the open ocean, to engage with school children, students, teachers and the general public. Outreach to this last group will be the main focus of our pathways to impact.
Non-scientists, including school students and their career advisors as well as policy makers, are often familiar with the parts of marine science related to sea-level rise, fisheries, coral reefs and marine mammals. We want to continue our work in broadening peoples' views of oceanography to include the importance of microscopic phytoplankton as well as the role of physics in controlling our oceans and climate.

How:
We will disseminate our research to the research community in ocean sciences through publications and presentations at academic meetings such as the AGU/ASLO Ocean Sciences meeting and the UK Challenger Society biennial conference.
Engagement with policy makers, civic leaders and the wider community will be co-ordinated through the University of Liverpool Research Centre for Marine Sciences (www.liv.ac.uk/climate); Williams leads the Centre, while Sharples and Mahaffey sit on the Steering Group. It has a record of engagement, including with Archbishop Carey of Liverpool, Bishop James of Liverpool, Sir David King, Andrew Miller MP, local MPs and councilors.

Our main focus is to engage school children, students and the general public. We will invite a Merseyside schoolteacher to accompany the scientific team on the research cruise in May/June 2015. This opportunity will be advertised through MerseySTEM (http://www.merseystem.co.uk; Sharples and Mahaffey are both STEM Ambassadors). The teacher will contribute to many aspects of this impact plan including capture of media aboard ship and exploiting the final resources generated as part of a local lecture tour of Merseyside schools. Dr Andy Heath will produce a series of accessible videos that will contribute to a 'resource toolbox' for later use both by the teacher and other organisations. These videos, which will also be uploaded to YouTube for public access and embedding, will be a combination of interviews, documentary film and animations with commentary (where appropriate) supplied by the teacher. How fluid circulates in the ocean will be demonstrated via fluid experiments in bench-top tanks.

Milestones:
Success of this impact plan will be measured by the degree of interaction with the teacher during Skype sessions and the number of talks given to schools using the resources generated, YouTube analytics and measurement of hits on websites that embed our various video outputs and the degree of uptake of our products through MerseySTEM and the wider STEMNET network. We will also use the video outputs to disseminate our research highlights to civic leaders via future events organized by the University of Liverpool research theme of Living with Environmental Change. Similar media created by Heath were used during the University of Liverpool LWEC event 'Health Challenges in a Changing World' event (May, 2013) which included speakers such as Lord May (science and government), Sir Colin Blakemore (ageing) and David Pencheon (NHS sustainability and low carbon).

Summary of Resources:
We request £18.5k for 3 months support for Dr Andy Heath to implement the Pathways to Impact. Computing equipment and software for the editing of the videos has been provided by previous NERC support.
We request funds to purchase audio-visual equipment for use by the teacher on the cruise, and to cover travel costs and teacher cover for the schoolteacher who will participate in the research cruise. We also request £3,500 for building two rotating fluid experiment tanks. Total = £31,155

Publications

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Description Tides have long been known to play an important role in driving the ocean's overturning circulation, by mixing different oceanic layers through the turbulent breaking of submarine (internal) waves generated as tidal flows impinge on the rugged topography of mid-ocean ridges. To date, much of what we know about tidally-generated internal waves stems from a major experiment conducted by the U.S. oceanographic community around the Hawaiian Ridge. In that experiment, it was concluded that a large fraction of the tidally generated internal waves propagates over hundreds or thousands of kilometres before breaking and influencing the ocean's overturning circulation. Our findings in the RidgeMix project indicate that a very different paradigm applies to the Mid-Atlantic Ridge, whereby tidally generated internal waves break shortly after being generated, without significant propagation. This suggests that the relative significance of local and far-field internal wave breaking depends on the nature of the seafloor topography and its orientation with respect to the tidal flows.
Exploitation Route The above findings are likely to lead to a revision of the representation of tidally-generated mixing in climate-scale ocean models.
Sectors Environment