Definition, properties, and Atlantic Ocean distribution of the new tracer TrOCA

Natural and anthropogenic tracers in the ocean are widely used not only to better understand water masses circulation and mixing but also to understand and quantify the ocean uptake and storage capacity of greenhouse gases. However, since each tracer is different, it is best to use the complementarity of several tracers to unequivocally identify the various water masses. Here we illustrate the conservative properties and the spatial distribution of the new composite tracer TrOCA (Tracer combining Oxygen, inorganic Carbon, and total Alkalinity) using oxygen (O2), dissolved inorganic carbon (TCO2), and total alkalinity (TA), from the Atlantic Ocean. The significant accuracy improvement of TCO2 and TA measurements since the 1970s, combined to a large effort in measuring these parameters during large scale cruises throughout the Atlantic Ocean, makes this tracer TrOCA an additional tool in analyzing water mass distribution. This tracer is shown to be conservative in intermediate, deep, and bottom waters. For instance, we show that the independence of TrOCA from other tracers provides further information on the origin and mixing of the main Atlantic water masses. Furthermore, TrOCA combined with the composite tracer NO, in particular the ratio TrOCA/NO, can be used to unequivocally identify and separate the Antarctic Intermediate Water, the Antarctic Bottom Water, and the North Atlantic Deep Water.