Biogeochemical sensors added to the RAPID array (part 1)

For the first time, biogeochemical sensors and samplers are being deployed at selected locations across the RAPID array to help us answer questions regarding the North Atlantic’s part in slowing the onset of global climate change.

As well as being key for the northward transport of heat, the North Atlantic also plays a very important role in the global carbon cycle. Cooling waters and intense biological activity lead to a strong reduction in carbon dioxide concentrations at the surface. CO₂ is then absorbed from the atmosphere to make up the deficit, with human-derived carbon (from fossil fuel burning, land-use change, concrete production etc) being absorbed at the same time. This ocean ‘sink’ vastly slows down the increase of CO₂ levels in the atmosphere caused by human activities. It’s thus important to understand the processes and drivers of how this happens now so we can better predict how it will behave in the future.

Biological activity in June 2014, observed here through the proxy of average chlorophyll concentration. Note the logarithmic scale and thus the intensity of the activity in the North Atlantic. This is thought to be sustained by the northward transport of nutrients across 24.5°N (black line).

The location of the accumulation of anthropogenic carbon within the global oceans. The North Atlantic holds 20% of the water volume, but 25% of the anthropogenic carbon inventory. Each year, it is thought that approximately half of the new accumulation is transported into the region across 24.5°N by ocean circulation.

Over the last two decades or so, instruments measuring seawater CO₂ levels on board volunteer observing ships (such as ferries or container ships) have allowed us to learn a lot about the magnitude of these air-sea exchanges (for instance, the size of the annual North Atlantic carbon sink is roughly equivalent to the annual emissions of the EU, Russia and India combined). However, less is known about the processes that cause it to vary from week-to-week, month-to-month and year-to-year.

A volunteer observing ship, the M/V Santa Maria, travelling between the UK & the Caribbean

It is thought that a large part of this variability is driven by the ocean, with its transport of carbon affecting the surface ocean-atmosphere concentration gradient and storage of anthropogenic carbon, and its transport of nutrients fuelling biology activity. But transport estimates are currently restricted to only every 5 to 6 years when transatlantic research cruises undertake full surveys of deep-ocean physics and chemistry across 24.5°N (which is what the trip after this RAPID cruise will be conducting).

This is where the new biogeochemical sensors and sampling technologies being deployed across the RAPID mooring array are seeking to fill the gaps. As part of the Atlantic BiogeoChemical Fluxes program (www.rapid.ac.uk/abc), oxygen, pH and pCO₂ sensors are being installed alongside autonomous samplers collecting water to be analysed for dissolved inorganic carbon, total alkalinity, inorganic nutrients (phosphate, nitrate and silicate) and organic nitrogen.

The biogeochemical sensor & sampler suite ready for deployment at the Mid-Atlantic Ridge

Firstly, these will greatly improve the temporal resolution of observations across the subtropical gyre from once every 5-6 years to once every 4 to 24 hours (for the sensors) or 11 days (for the samplers) - this will allow us to massively increase our understanding of the variability of processes involved in ocean-atmosphere interaction in these locations. Secondly, in combination with the estimates of water transport and the AMOC from the RAPID array, the new measurements will be used to calculate the transport of carbon and nutrients by the ocean at equally high frequency (approximately every 10 days). From here we’ll be able to look much more closely at the role of the North Atlantic in mitigating future atmospheric CO₂ increases.

In the next part, we’ll look at bit more at the new technologies being deployed to add a biogeochemical dimension to the RAPID array.

Written by Pete