Issues Archive

July/August 2020 Vol. 24
No. 4
 A strong effort has been  
  made recently for the  
  development of low power,  
  small in size and  
  accurate enough CO2  
  sensors, which can be  
  deployed in autonomous  
  platforms such as the  
  SeaExplorer glider 
 SeaExplorer underwater  
  glider trajectories  
  superimposed on maps of  
  sea surface temperature  
  (SST). Sea surface  
  temperature data source:  
  EU Copernicus Marine  
  Service Information 
 Data acquired by the  
  SeaExplorer during the  
  two deployments  
An innovative tool for assessing the impact of  
  climate change in the ocean

Measuring pCO2 at sea with the SeaExplorer underwater glider

Understanding the ocean’s role in the global carbon cycle and its response to a changing environment is of crucial interest. The ocean is indeed known to be the only true net sink for anthropogenic carbon dioxide (CO2) and without this oceanic uptake, atmospheric CO2 would be significantly higher today than what is currently observed. The impact of this ocean acidification can already be perceived and current projections suggest that those changes will persist in the future. The reliable measurement of oceanic pCO2 at large spatial and temporal scales is therefore becoming critical.

A strong effort has been made recently for the development of low power, small in size and accurate enough CO2 sensors, which can be deployed in autonomous platforms such as underwater gliders. ALSEAMAR has participated to this movement by integrating the Mini CO2 sensor from Pro-Oceanus, Canada, into the SeaExplorer glider rated to 1000 metres depth. The Mini CO2 sensor uses infrared detection to measure the partial pressure of CO2 gas dissolved in water.

In order to strengthen the understanding of CO2 dynamics in the ocean, a SeaExplorer underwater glider performed a campaign in the North Western Mediterranean Sea during the first half of 2020. For a comprehensive interpretation of the data, the glider was also equipped with a GPCTD-DO from Sea-Bird Scientific, USA. The first deployment was realised in January 2020 (winter) and the second in June 2020 (summer). This mission aimed at studying the seasonal distribution of CO2 at sea in both winter and summer, by performing an inshore-offshore transect between the south coast of France and the DYFAMED time-series station (SOERE MOOSE). Operating in this area offers the double advantage of being able to sample contrasted oceanic conditions and to consider intercomparison exercises between glider data and reference DYFAMED’s fixed mooring measurements.

Processing the CO2 data acquired during this mission required dealing with the response time of the membrane-based Mini CO2 sensor. Although the glider is a rather slow profiling device, the sensor response time led to a hysteresis in the obtained vertical pCO2 profiles. This was overcome by adapting the published algorithm, i.e., by considering a linear dependency of response time on water temperature and by correcting the raw signal applying an exponential fit (Fietzek et al., 2014).

Once corrected, the data acquired with the SeaExplorer glider revealed contrasted situations in the seasons, especially in the first hundred metres of the water-column. The most striking is that temperature and pCO2 were lower in winter and higher in summer, with a measured increase of about 50 µatm for pCO2. Such a co-variation is not surprising since changes in pCO2 in the mixed-layer are expected to be primarily driven by temperature changes. Another interesting feature is the subsurface pCO2 minimum (~370 µatm) that develops in summer, when the water-column is well stratified. This CO2-depleted layer is found between the 1028.2 and 1028.6 kg m-3 isopycnal levels and coincides well with an O2 maximum. This pattern is very likely to mirror the net biological effect, which includes primarily the CO2 utilisation by the phytoplankton (photosynthesis). Below the 1029 kg m-3 isopycnal level, waters are CO2-enriched (~440 µatm) whatever the season, as a result of respiration processes. At these depths, variations in the pCO2 vertical distribution follow for the most changes in the density fields.

The first results from this campaign are promising and highlight the great potential of measuring pCO2 with a glider for the scientific community. Further analysis and evaluation of the sensor performances will be conducted through a comparison with reference measurements made by autonomous Carioca sensors at the DYFAMED time-series station. The combination of a well-suited sensor with a highly enduring autonomous platform demonstrates the possibility to perform persistent observations at sea with greater spatial and temporal resolution than traditional methods and at a fraction of the cost, thanks to the reduced operational costs of the SeaExplorer glider. 

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