8 Perspectives on the North Atlantic Carbon Cycle

Catherine Goyet

Woods Hole Oceanographic Institution

Department of Marine Chemistry and Geochemistry

Woods Hole, MA 02543

In order to better quantify the variation of the carbon cycle in the ocean, it is essential to make observations on at least decadal and seasonal time scales, and in several different ocean areas.

Studies on a decadal time scale are necessary for the quantification of the sequestration of carbon in deep waters, while studies on a seasonal time scale are as important for the quantification and prediction of air-sea CO2 fluxes, carbon transformations within the upper ocean and the transport of carbon by ocean currents.

Although the North Atlantic Ocean is one of the most studied ocean basins, we still do not know the amplitude of the seasonal variations of p CO2 in surface seawater over the whole basin; we know it only at the BATS location. If we are to predict the monthly means of pCO2 within ± 15 µatm on a 1 grid, then we need to make seasonal observations in several representative ocean areas with thermally and biologically driven seasonal cycles, and in ocean currents (areas of deep water formation and the Gulf Stream). Results of such intensive studies in these areas should be useful in determining the algorithms needed to best extrapolate our local observations over large spatial scales.

PCO2 in surface seawater is highly variable (up to approximately 1/3 of its mean value). It is very sensitive to sea surface temperature and chlorophyll concentration (Poisson et al., 1993; Robertson et al., 1993). No simple relationships yet exist to compute pCO2 from sea surface temperature and chlorophyll (or ocean color). However, pCO2 can be computed from total alkalinity (TA) and total CO2 (TCO2) in seawater using the thermodynamic model. Total alkalinity does not vary widely in the ocean and is known to be linearly linked with salinity (Brewer et al., 1986; Kroopnick et al., 1986). The seasonal variations of TCO2 in surface seawater depend upon the physical and biological processes involved in the studied area. These processes, namely ocean circulation and net production can be described by algorithms. The challenge is in developing better algorithms that could quantify monthly means of TCO2 and TA within a precision of ± 4 µmol/kg.

Recent measurements at the BATS station indicate that TA in surface seawater can be calculated using a simple relationship with salinity with an uncertainty of ± 4 µmol/kg. This assumes salinity is known to within 0.05. However, this relationship needs to be verified and extended over the whole North Atlantic Ocean.

Below the mixed layer depth, TCO2 in the North Atlantic Ocean can be computed to within ± 8 µmol/kg using nutrients, temperature, salinity and oxygen data. In the upper ocean there is currently too little TCO2 data (especially during the winter months) to parameterize the monthly variations of TCO2. There is a critical need to acquire monthly TCO2 data in the upper layer of the North Atlantic Ocean. The JGOFS program offers an unprecedented opportunity to do so. State of the art technology such as remote sensing and buoy-based pCO2 measurement should also be used to extend our knowledge beyond short time-and-space scale results from shipboard measurements.

Since we need both monthly coverage over large areas and a better understanding of the processes involved in the upper ocean of the North Atlantic Ocean, I would suggest conducting both:

1) an intensive study around one location.

This location could be anywhere in the North Atlantic Ocean; however, an area where nothing (or very little) is known about the carbon cycle would be a first choice. Buoys could be used in this area to monitor chemical properties, particularly when ships would not be in the area. We already have and will continue to have time-series measurements of the carbonate system at the BATS station. Comparison of these data with those from another site would be very helpful for extrapolating local processes over larger areas.

2) a monthly repeat survey cruise between Nova Scotia and Bermuda.

This cruise track would allow us to sample surface waters of various properties with strong seasonal signals. Overflights would further allow us to extrapolate shipboard measurements over larger areas. This cruise track would end in Bermuda where the seawater is well characterized and could serve as "calibration" for all the cruises.