Anthropogenic Carbon, CFC-12 and SF6 in the Tropical Atlantic.
Final Report Abstract
An analytical system able to simultaneously measure dichlorodifluoromethane (CFC-12) and sulphur hexafluoride (SF6) on seawater samples (directly on board or back in the laboratory) was further developed. We substantially extended the transient tracer database in the tropical North Atlantic and with the help of these tracer measurements, water mass ventilation processes and the input of anthropogenic carbon (Cant) to the ocean from the atmosphere were studied.Water sampling on four cruises to the tropical and the equatorial Atlantic between 2006 and 2009 served as basis for the subsequent work. With the so called transit time distribution (TTD) method the mean ages of the water masses and the anthropogenic carbon content has been estimated. The combined measurements of CFC-12 and SF6 allowed to assigning the Δ/Γ ratio to unity. The age calculations show that at the Equator at around 1000 m depth, an old water mass, the Antarctic Intermediate Water is present. Below that depth the younger upper North Atlantic Deep Water brings younger water into the area. The anthropogenic carbon concentrations of this water mass are 5 - 10 μmol/kg. Contrary, the anthropogenic carbon content of the AAIW and the deep waters below 2500 m stays below 5 μmol/kg. An analysis of mean age and Cant distribution on isopycnals highlights differences between the equatorial belt and the area north of it: a) The mean age on an isopycnal decreases towards the south and the Guinea Dome area exhibits a pronounced local increase in mean age. b) The Cant concentrations are higher in the equatorial region. Column inventories of anthropogenic carbon for the upper 1200 m range between 18 and 28 mol/m². North of 5°N the column inventories are on average 3 mol/m² higher than between 5°N and 5°S. This trend have not been described in the past. The total anthropogenic carbon inventory of an area of 106 km² has been determined to be 2.5 Pg. Partitioning this area into two regions of same size, one north of 4.5°N and the other one south of 4.5°N, gave an Cant inventory of 1 Pg in the northern area and 1.4 Pg in the southern area. Thus, the Cant inventory in the equatorial region is increased by 40% compared to the Guinea dome area. With the help of past tracer measurements in the tropical Atlantic, the increase of anthropogenic carbon could be assessed. On a 23°W section crossing the equator, measurements of CFC-11 from 1999 were available and with these we estimated the Cant concentrations for the year 1999 using the TTD method. The results were compared to the Cant concentrations based on measurements of CFC-12 and SF6 on the 23°W section from the present study ( in the years 2006-2009) and reveal an increase of anthropogenic carbon in the upper 700 m of the water column. In the mixed layer a Cant increase of up to 15 μmol/kg for a period of 8.5 years was found. No Cant increase is observed in the AAIW, whereas the upper LSW again shows a positive change.
Publications
- Tracers and anthropogenic carbon in the ocean. DISCO symposium, Honolulu, Hawaii, USA, 3 – 7 October 2010
Schneider, A.
- Water mass ages and anthropogenic carbon in the tropical Atlantic. GEO-Carbon Conference: Carbon in a changing world, hosted by FAO and the FP7 Coordination Action on Carbon Observing Systems, FAO, Rome, Italy, 24 – 26 October 2011
Schneider, A., T. Tanhua, A. Körtzinger, D.W.R. Wallace
- 2012. An evaluation of tracer fields and anthropogenic carbon in the equatorial and the tropical North Atlantic, Deep-Sea Research-I
Schneider, A., T. Tanhua, A. Körtzinger, D.W.R. Wallace
(See online at https://doi.org/10.1016/j.dsr.2012.05.007)
