EPSRC-SFI: An ocean microlab for autonomous dissolved inorganic carbon depth profile measurement

CO2 concentration in the atmosphere has increased significantly since pre-industrial times leading to global warming. There is now a major concern that ocean absorption of CO2 may be saturating, leading to more rapid global warming and much more serious consequences than predicted. Understanding the ocean CO2 system is of fundamental importance for climate change models that inform our predictions but ocean measurement of CO2, particularly in the form of dissolved inorganic carbon (DIC), is severely lacking due to technical challenges. We need regular measurements, down to a depth of 2 km, from thousands of locations world-wide. Accurate field measurements of DIC up to now have involved large and expensive surface instruments, e.g. infra-red absorption or mass spectrometry, and their miniaturisation is not feasible at the required accuracy. The aim of this project is to develop a new method of measuring DIC that is accurate, but which can also be miniaturised so that worldwide float deployment becomes a possibility.

At present, the Argo network consists of ~3000 untethered battery-operated floats located across the world's oceans. They operate autonomously, drifting at a park depth of 1.5 km and every 10 days they rise to the surface, measuring the temperature and salinity depth profiles on the way. This data is then transmitted to satellite and the cycle repeats. These two parameters can be measured instantaneously at each depth whereas DIC quantification requires time-consuming chemical analysis. In the laboratory, the standard calibration technique separates DIC from seawater as CO2 gas which then transfers across a membrane into a reagent (NaOH), resulting in a decrease in conductivity. With appropriate design and calibration, the measured change in conductivity can be converted to DIC concentration. The time required for gas exchange however prevents instantaneous measurement but with the Argo float cycle, there is a 10-day park window where this exchange could be allowed to occur, and with a large number of samples. Our objectives therefore are to miniaturise each of the functional units of the laboratory setup and integrate them into a single microfluidic lab on chip which can meet the severe size, power, cost and reliability limits imposed by the Argo float integration. This presents an immense challenge; microfluidics research up to now has focussed mainly on biomedical applications which have an entirely different set of criteria, essential ocean testing of ideas and refinements is very difficult and expensive, while technical challenges can appear insurmountable.

Conductivity measurement is relatively simple in concept and is readily miniaturised. However, the accuracy is much lower compared to optical techniques and this is exacerbated by the need to use extremely small sample volumes, (~100 nL). The depth resolution depends on the number of samples collected, stored, and subsequently analysed within float rise and park times respectively. The ultimate preference is ~100 samples, giving a depth resolution of 20m. This requires 100 fluid circuits and at least 100 valves to be fabricated in a 10 x 10 x 2 cm device. Such high-resolution channel patterning creates major difficulties with regards to bonding and sample leakage between channels, exacerbated by the extremely harsh environment, high pressure and the long-term deployment. This situation is further challenged by the need to seal a membrane within a multilayer structure. The best membrane materials (gas permeable and ion blocking) are very hydrophobic and resist bonding to other materials. Finally, there is no nano/micolitre valve technology that could operate in an environment where pressures vary up to 200 atmospheres. Most of the limited research to date has focussed on pneumatic valves. In this project we need to discover and develop new stimuli responsive valve materials and find a way to incorporate these into multiple microfluidic channels.

The proposed project combines ocean science & technology, microfluidics, nanomaterials/nanocomposites and plasma-liquid science & technology with the aim of developing a series of prototypes for a new microanalysis instrument that would allow for the first time, long-term autonomous ocean chemical analysis to quantify the depth variation of dissolved inorganic carbon (DIC) concentrations. This is a critical ocean parameter, with is seriously undersampled due to the current absence of any sub-surface continuous measurement capability. The project focus is directed at size and power miniaturisation so that ultimately, system integration into the Argo worldwide depth profile float network can become a reality. Successful completion of the project will bring the prospects of a step change in our long-term or extended ocean chemical analysis capability. In order that the potential impact of the proposed technological innovations can be realised in a reasonable timeframe, we aim to demonstrate staged prototype operation through a series of field, including ocean, trials. These represent our main and tangible pathway to impact.

The essential scientific and political support for implementing global policies to limit climate change depend on accurate ocean chemical analysis, where important parameters are chronically undersampled. The latest IPCC special report warns that a 1.5oC average increase by 2030 is likely and the associated CO2 ocean acidification is projected to amplify the adverse effects of warming. Climate-related risks to health, livelihoods, food security, water supply, human security, and economic growth are projected to increase with disadvantaged and vulnerable populations at disproportionately higher risk of adverse consequences. Human health will experience primarily negative consequences, e.g. heat-related morbidity and mortality, ozone-related mortality with increased risk of vector-borne diseases, and shifts in their geographic range, reductions in food availability and water stress.

A 2016 Ocean Warming report presents a stark outlook and highlights significant gaps in fundamental science and capability needs. Until very recently, the debate on climate change has focused on specific themes, only occasionally has the ocean been mentioned. Up to now, the ocean has shielded us from the worst impacts of climate change at significant cost to its chemistry as it absorbed large amounts of the extra carbon dioxide and warmed at an alarming rate. This report repeatedly highlighted fundamental and massive gaps in our understanding that already compromise even our basic ability to understand and predict with any confidence what changes, already underway, may mean to our wellbeing. The proposed project aims to tackle such gaps and its impact will be realised when a fully-deployable microanalysis system is available to ocean scientists and their data supplied to climate modellers. This will occur through a series of increasingly advanced component innovations, component integrations and proof of principle field trials of increasing challenge and realism.

There are many UK ocean technology companies but with limited exposure to microfluidics. There is also increasing commercial activity in microfluidic component, services and equipment supply but this is predominantly focussed on biomedical applications. A new and successfully trialled DIC profiling system is likely to have a rapid uptake and a predictable captive market. There is considerable scope for IP and hence the field is attractive for an expert new start company and also for partnership with established commercial interests in ocean technology and microfluidic engineering.