Separated in time by usually years, repeat CTD sections provide spatially highly resolved snapshots of water properties along a given transect through the ocean. This data serve to, e.g., establish the volume of water belonging to a certain water mass and to estimate the amount of heat or gases and dissolved substances such as CO2 and nutrients stored therein. In Figure 7for example the volume of Antarctic Bottom Water (delineated by a lower black line) steadily decreases while this water mass slowly gets warmer, i.e. takes up heat.
Repeat sections employing high-precision CTDs (with absolute accuracies better than 0.001°C in temperature upheld for now approaching three decades) allow capturing the ever so slightly increase of ocean bottom temperatures as depicted in Figure 8 by one of the longest oceanic time series at the 61°S 00°E, in the northeastern-most extent of the Weddell Gyre, where at least some of the AABW is believed to escape to the North.
Along with (electronic) CTD measurements, water samples are collected throughout the water column to be retrospectively analyzed on board or back in the laboratory for concentrations of various trace gases and carbon constituents. Measurements of Total CO2 (also known as DIC) in the surface and bottom waters of the Weddell Sea, reveal a steady CO2 increase of anthropogenic origin, as surface water charged with anthropogenic CO2 is a major component in the formation of WSBW (Van Heuven et al. 2011). The findings reflect the deep-sea sequestration of anthropogenic CO2, thus contributing to diminishing the burden of excess CO2 in the atmosphere. The largest increase of CO2 is found in the surface layer (Van Heuven et al. 2014). Here, uptake of excess CO2, i.e. anthropogenic CO2, from the atmosphere has obviously occurred. Somewhat surprisingly, steady state tracer oxygen shows a decreasing trend in the WSBW. This confirms that the Weddell Gyre circulation is not in steady state, as also detected in potential temperature and salinity data collected at AWI (Fahrbach et al. 2011). A decrease of oxygen in the WSBW may be caused by a changing composition of the water mass, where the contribution of the one component surface water is decreasing compared to that of the other component (Modified) Warm Deep Water.