The goal of the SOCCOM float program is to produce a climate-quality data record for carbon cycling. That is a "time series of measurements of sufficient length, consistency and continuity to determine climate variability and change" (US NRC, Climate Data Records from Environmental Satellites, 2004). Such a record requires a sensor that is well characterized and calibrated to the property of interest before deployment, and whose calibration is assessed at deployment with high quality hydrographic measurements, or in which post-deployment calibration is possible. It must be possible to assess sensor stability and degradation with sufficient accuracy to allow the desired climate signal to remain detectable. This goal mandates that SOCCOM floats and their operation have four essential characteristics. These characteristics are as follows:
- Sensor suite. The float must carry sensors for two of the three chemical species: dissolved oxygen, nitrate, and pH. The overarching goal is to understand carbon cycling in the Southern Ocean and these sensors provide a direct link to the carbon system. Further, their sensor outputs can be directly related to SI units of moles per mass of seawater, which enables quantitative measurements. This characteristic is necessary for understanding long-term change. At least two sensors are required to provide some redundancy and a cross-check on data quality. Having all three sensors is preferred. Other sensors are desirable, but may not be essential in all cases. These include biooptical sensors for chlorophyll fluorescence, backscatter, light transmission or light intensity. As new sensors become available, it is possible that the definition of the core sensor suite may change, but new sensors must first be demonstrated to be capable of operation in large arrays, such as SOCCOM, for multiple years.
- Quantitative calibration of the core, biogeochemical sensor suite is essential to provide climate ready consistent data. Each float and sensor system must be accompanied by a quality predeployment calibration and an in situ calibration (typically a hydrocast and high-quality chemical analysis of the samples, or other acceptable procedure). This will be necessary for each sensor until it is demonstrated unequivocally that accurate calibration to within the following specifications is achieved with the "factory calibration". There must be means available to assess the changing performance of the sensor over time, such as comparisons to deep (>1000 m) properties, or measurements of air oxygen.
Sensor Oxygen Nitrate pH Chlorophyll Initial accuracy 2 µmol/kg 1 µmol/kg 0.01
N.b. – these specs are not as stringent as GO-SHIP measurements, but probably what can be achieved.
- A SOCCOM float is intended to be utilized for multi-year studies and should be operated in a mode that will allow the float to achieve a lifetime of at least 3 years. It should also be operated so that water at 1000 m or deeper is sampled on at least a monthly basis to enable assessment of sensor performance (drift). The focus on the Southern Ocean implies deployment south of 30°S.
- SOCCOM float data must be available in digital form in real time without restriction using a data policy similar to that of the Argo system. Real time data is necessary to allow intercomparison of the operation of each float with the entire array. This is essential for the rapid identification of sensor problems and the quality of data that it is produced. If there are adjustments to float raw data based on sensor calibrations during the deployment, then these adjusted data should also be available in near real time shortly after the float is deployed. SOCCOM floats in the Argo system should be identified by the word SOCCOM in the Project field, in addition to any other appropriate identifiers.
Deep-Sea DuraFET: A pressure tolerant pH sensor designed for global sensor networks
Johnson, K. S., H. W. Jannasch, L. J. Coletti, V. A. Elrod, T. R. Martz, Y. Takeshita, R. J. Carlson, J. G. Connery (accepted). Analytical Chemistry.
Testing the Honeywell Durafet® for seawater pH applications
Martz, T. R., J. G. Connery and K. S. Johnson (2010). Limnology and Oceanography: Methods, 8, 172-184.
An evaluation of pH and NO3 sensor data from SOCCOM floats and their utilization to develop ocean inorganic carbon products
Wanninkhof, R., K. Johnson, N. Williams, J. Sarmiento, S. Riser, E. Briggs, S. Bushinsky, B. Carter, A. Dickson, R. Feely, A. Gray, L. Juranek, R. Key, L. Talley, J. Russel, and A. Verdy. SOCCOM Carbon System Working Group white paper.
Air Oxygen Calibration of Oxygen Optodes on a Profiling Float Array
Johnson, K. S., Plant, J. N., Riser, S. C., & Gilbert, D. (2015).Journal of Atmospheric and Oceanic Technology, 32(11), 2160–2172.
Evaluation of a lifetime-based optode to measure oxygen in aquatic systems
Tengberg, A., J. Hovdenes, H.J. Andersson, O. Brocandel, R. Diaz, D. Hebert, T. Arnerich, C. Huber, A. Körtzinger, A. Khripounoff, F. Rey,C . Rönning, J. Schimanski, S. Sommer, A. Stangelmayer (2006). Limnology and Oceanography: Methods, 4, 2.
Long-term nitrate measurements in the ocean using the In Situ Ultraviolet Spectrophotometer: sensor integration into the Apex profiling float
Johnson, K. S., L. J. Coletti, H. W. Jannasch, C. M. Sakamoto, D. Swift, S.
C. Riser (2013). J. Atmos. Oceanic Technol., 30, 1854-1866.
Observations of pigment and particle distributions in the western North Atlantic from an autonomous float and ocean color satellite
Boss, E., D. Swift, L. Taylor, P. Brickley, R. Zaneveld, S. Riser, M.J. Perry, P.G. Strutton (2008). Limnology and Oceanography, 53, 5part2.
Primer regarding measurements of chlorophyll fluorescence and the backscattering coefficient with WETLabs FLBB on profiling floats
Boss, E., and N. Haëntjens, (2016). SOCCOM Tech. Rep. 2016-1.