Back from the deep!

Article by Pete Brown (NOC), Sunke Trace-Kleeberg (U.Southampton) & Sara Fowell (NOC)

As part of EPOC, first steps have been undertaken in understanding how new biogeochemical (BGC) sensors can contribute to and improve our understanding of the large-scale transport of carbon and nutrients by the AMOC.

The recent DY174 expedition on the UK ship RRS Discovery marked an exciting new chapter in this work – 20 biogeochemical sensors were recovered from the RAPID Eastern Boundary Array following their deployment in February 2022 on DY146. A combination of off-the-shelf instruments and novel Lab-on-Chip sensors (LOCs) (co-funded by EPOC) were used to measure oxygen, pH, Total Alkalinity, nitrate, phosphate, and pCO2 multiple times per day at depths ranging between 50m and 3000m. This deployment marked the longest and deepest long-term deployment of the LOC technologies.

The novel sensor data will be used in EPOC to generate new regional estimates of regional BGC transports, and how they co-vary with the AMOC. This unprecedented increase in data resolution provided by the sensors represents a step-change in our abilities to measure the ocean for carbon and nutrient variables (an improvement from biennial discrete water samples, to 4 times per day frequency). Within EPOC we will evaluate the impact, importance and potential of such new BGC sensing technologies on transport mooring arrays, and their utility in uncovering new understanding of how the physical and biogeochemical spheres interleave.

Cruise update from Sara Fowell (NOC), who recovered the sensors:

Despite evidence of damage by fishing operations to one of the frames, all the instruments were successfully recovered – albeit in various states of repair. The data were downloaded immediately, giving us a sneak preview of the incredible high-resolution data collected during the deployment. It’s going to be really exciting to see how much more we can learn from such high-resolution datasets as opposed to discrete sampling, particularly since combining biogeochemical parameters will give us an overview of multiple processes occurring in this region.

We also deployed LOC pH and Total Alkalinity sensors on the CTD casts, measuring every 7 to 16 minutes respectively, allowing us to compare autonomously collected biogeochemical data collected in-situ, with traditional discrete samples analysed later in the laboratory.  

In addition to the in-situ measurements, discrete oxygen, dissolved inorganic carbon and nutrient samples were collected from the CTD casts carried out prior to mooring recovery. A total of 150 oxygen samples were analysed on board, while the carbon and nutrient samples returned to the National Oceanography Centre for analysis in the lab.

The EBH3 50m frame hosting an array of commercial and LOC biogeochemical sensors was successfully recovered, although heavily biofouled and damaged by fishing gear.
The look of relief when the last of your instruments are recovered!
A frame with 2 LOCs (grey), 2 titanium battery housings (silver), and 2 Sea-Bird MicroCATS are safely pulled on board. Far right: pH LOC sensor mounted on the CTD frame alongside Niskin bottles.

RAPID is a long-term AMOC observing programme situated in the subtropical North Atlantic comprised of a mooring array, measurements undertaken at the western boundary and outputs from a submarine cable across Florida Straits. It is a UK-US collaboration between the National Oceanography Centre, the University of Miami, and NOAA-AOML. It is funded by UK (NERC), and USA (NSF). Data are publicly available at https://amoc.rapid.ac.uk 

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EPOC is funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.

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