First recovery of the RAPID array’s novel biogeochemical component

A post by Pete Brown

As well as playing a fundamental role in the northwards transport of heat, the North Atlantic is also a key part of the global carbon cycle. Of all the extra carbon dioxide (CO2) that has been added to the atmosphere by human activities (fossil fuel burning, land-use change, concrete production etc) over the last 300 years or so, about a quarter has been and continues to be absorbed by the oceans, greatly reducing the speed at which atmospheric CO2 concentrations increase, and the rate at which their associated climate-related impacts develop.

Within this, the North Atlantic is the most effective global ocean location for CO2 absorption. Both the cooling of poleward-moving surface waters and intense biological activity act in concert to greatly reduce carbon dioxide concentrations in the shallowest water depths. To make up this deficit, CO2 is then absorbed from the atmosphere, with human-derived carbon being absorbed at the same time.

Strong biological activity (also ‘Net Primary Production’) in the North Atlantic leads to a large reduction in surface carbon levels, and a strong of uptake of CO2 from the atmosphere occurs to counter this.

Our current understanding of the magnitude of the air-sea CO2 flux in the North Atlantic (for instance, that it absorbs as much carbon each year as the total emissions of Japan and Germany combined) is derived mainly from sensors on board volunteer observing ships (such as ferries or container ships). While these do a good job at estimating the integrated annual signal, the sporadic nature of the observations they make (both in time and location) mean that much less is known about the processes that cause the flux to vary from week-to-week, month-to-month and year-to-year.


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.


In order to better predict how the North Atlantic carbon sink will respond in the future to a changing climate (and thus its ability to and continue mitigate further atmospheric CO2 increases), we need to improve our understanding of the processes and drivers of the air-sea flux of CO2. In particular, the transport and carbon and nutrients into the region, and the effects they have on export production and the accumulation of anthropogenic carbon. For this we need new observations at a much higher frequency. About 16 months ago, biogeochemical sensors and autonomous water samplers were deployed across the RAPID mooring array for the first time to help meet this need. And on this trip, we’re collecting our first data from them.

In sunnier times: temperature, salinity, pressure and oxygen sensors being attached to the mooring wire

Autonomous sampler and suite of sensors in the frame below being recovered from the Eastern Subtropical North Atlantic in March 2017.

 

As part of the Atlantic BiogeoChemical Fluxes program (www.rapid.ac.uk/abc), oxygen, pH and pCO₂ sensors were installed alongside autonomous samplers collecting water to be analysed for dissolved inorganic carbon, total alkalinity, inorganic nutrients (phosphate, nitrate and silicate) and organic nitrate. We’ve now collected samplers and sensors from two of the four locations where they were deployed, and installed replacements to make additional observations until Autumn 2018. Over the coming weeks and months, this fantastic new data set for the region will be analysed and investigated, numbers crunched and calculations made.

Combined pH, oxygen, temperature, salinity and pressure sensor recovered from 50m depth in March 2017.

 

Increasing 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) will allow us to massively increase our understanding of the variability of processes involved in ocean-atmosphere interaction in these locations. We’ll then combine this novel biogeochemical data with the estimates of water transport and the AMOC from the RAPID array, to investigate 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. Onwards and upwards!

Locations of the RAPID moorings, and those with novel biogeochemical samplers / sensors being recovered for the first time this year.