J Geophys Res 96(S01): 3343-3357
5. However, the ? 18 Osw did not show a distinct signature for each water mass because it is not possible to distinguish the water masses based only on their isotopic signatures. That isotopic signature masking can be attributed to the large distances between the study area and the sinking areas where those water masses were formed, which masked their initial isotopic characteristics. Despite the overlap in the standard deviations intervals, the current data have a widely scattered distribution if compared to the Pierre et al. (1991) and Ostlund et al. (1987) datasets, which suggests that in a regional scale the water masses do not have a homogeneous isotopic composition. Thus, the broader geographic distribution may reflect a natural larger dispersion of the isotopic data. Furthermore, the ? 18 Osw average values obtained by Pierre and Ostlund were higher than in the present work, especially at the surface. This seems to be a response from a seasonal variation pattern. The probable cause for this shift in the Salinity and ? 18 Osw relationship may be the intensification of the Brazil Current during the Summer season. An intensified BC would carry more Tropical Water to the subtropics and allow the occurrence of high ? 18 Osw values in the first 200 m of the water column.
The lines representing ? 18 Osw: Salinity in Fig. 5 get closer in layers with salinities near 34 and deviate in the high salinity layers, which is consistent with the seasonal differences in evaporation-precipitation budget and reinforced by the fact that the comparison datasets from Pierre and Ostlund are concentrated in austral Summer and this study in austral Winter. For example, a salinity of 37 (typical oceanic surface layer salinity) would have a difference of -0.43‰ between the Winter and Summer lines. However, for the layer of 35 psu, the difference becomes -0.16‰. This shows that the ratio is maintained in the deeper layers and has a larger variation in surface, which is most likely due to the evaporation and precipitation seasonal effects active in this layer.
Additionally, the reality that of the Coastal Liquids signature (built-up on the Cold weather) falls exactly in the summer line as the a sign this particular matchmaking are somehow influenced by the latest continental freshwater influx
The steeper slope of the Summer line indicates a negative E-P balance (Schmidt 1999), which should imply a ? 18 Osw depletion (Meredith et al. 1999). However, as shown above, the Summer line (especially the surface section) has higher ? 18 Osw levels than the Winter line, corroborating with the hypothesis that the increased volume of Tropical Water via Brazil Current may be responsible for the Summer ? 18 Osw enrichment and the seasonal shift in the Salinity:? 18 Osw surface relationship. Where excess freshwater intake is lowering salinity and stabilizing the upper ocean, mixing by mechanical turbulence is inhibited. Large enough freshwater inputs may lead to the formation of “barrier layers” in which a strong halocline inhibits the vertical exchange between the blend layer and the thermocline. Lukas LUKAS R and LINDSTROM E. 1991. The mixed layer of the western equatorial Pacific Ocean. and Lindstrom (1991) describe the barrier layer phenomena in the tropical Pacific, where the ITCZ is the source of fresh water. The tropical Atlantic also has barrier layers, where river discharge waters play a prominent role ( Hu HU C, MONTGOMERY E, SCHMITT R and MULLER-KARGER F. 2004. The dispersal of the Amazon and Orinoco River water in the tropical Atlantic and Caribbean Sea: Observation from space and S-PALACE floats. Deep-Sea Research II, p. 51. et al. 2004). The strong oceanographic component (advection) in the North Brazil Current (NBC), associated with the spatial and temporal variability of the ITCZ influence may contribute to the absence of isotope variation with the shoreline distance.
