Carbon and Tracers in the Subpolar Gyre

Chlorofluorocarbons (CFCs) and Sulfur Hexafluoride (SF6) have well-understood atmospheric sources and have been used to calculate ventilation ages of water masses and to validate numerical models. The ventilation age is the estimate of when a water mass was last at the ocean surface in contact with the atmosphere. When the measured concentrations of tracers in water masses are compared with the history of their atmospheric mixing ratios, the year of water mass formation can be estimated. Further, from the known outcrop region and the assumed pathways to the sampling location, the transport speed can be calculated. The inert nature of CFCs and SF6 in seawater and presently available techniques for sensitive analysis, make these transient tracers powerful tools to study air-sea interaction processes such as deepwater formation and subsequent transport of ventilated waters. Transient tracers (CFC-12, CFC-11, CFC-13 and CCl4) have been measured on the extended Halifax Line since 2006, and the AR7W Line across the Labrador Sea since 1990, and SF6 from 2010 for both lines.

Lower concentrations of transient tracers (shown for SF6 in the first panel below) indicate older water. Correspondingly higher Total Inorganic Carbon (DIC) due to the remineralisation of organic matter can be seen at 500 m depth on the slope and extending offshore (second panel below). Emerald Basin on the Scotian Shelf (station 3) also shows this older, higher TIC and hence lower pH (third panel) water at depth due to replenishment by intermittent upwelling from the slope. Underneath the layer of old water, newly-ventilated Labrador Sea Water is present at around 1500 m depth, shown by higher concentrations of SF6 and lower TIC (and higher pH) signals.

Distributions of SF6
Distributions of SF6 over the Scotian Shelf, Slope and Rise from AZOMP profiles in May 2013.
Distributions of TIC
Distributions of TIC over the Scotian Shelf, Slope and Rise from AZOMP profiles in May 2013.
Distributions of pH
Distributions of pH over the Scotian Shelf, Slope and Rise from AZOMP profiles in May 2013.
Chemical oceanographic team in the laboratory on CCGS Hudson in 2013
Chemical oceanographic team in the laboratory on CCGS Hudson in 2013

For further information on related ocean monitoring, see Chemical Oceanographic Monitoring of the Labrador Sea and Chemical Oceanographic Monitoring of the Scotian Slope/Rise

For further information, contact Dr.Kumiko Azetsu-Scott