Stable isotopes in surface waters of the Atlantic Ocean: Indicators of ocean-atmosphere water fluxes and oceanic mixing processes

The surface ocean hydrological cycle is explored based on similar to 300 new delta O-18 and delta D measurements from surface waters of the Atlantic Ocean and the Mediterranean Sea over the period 2010-2016. Our approach combines these surface observations with salinity (S) and stable isotope measurements of atmospheric water vapor. The distinct regional S-delta distributions are used to identify different surface water masses and their horizontal advection. Moreover, based on assumptions on the delta-S characteristics of seawater sources and the isotope composition of the evaporative (delta(e)) and meteoric water (delta(MW)) fluxes, the delta-S distribution is used to indicate the relative importance of evaporation (E) and meteoric water inputs (MW). Here delta(e) is estimated from the Craig and Gordon's equation using 120 days of measurements of the ambient air above the Atlantic Ocean collected during three cruises. To provide quantitative estimates of the E: MW ratio, we use the box model from Craig and Gordon (1965). This identifies the subtropical gyre as a region where E: MW similar to 2 and the tropical ocean as a region were MW: E similar to 2. Finally, we show that the delta O-18-delta D distribution is better represented by a linear fit than the delta-S relationship, even in basins governed by different hydrological processes. We interpret the delta O-18-delta D distribution considering the kinetic fractionation processes associated with evaporation. In the tropical region where MW exceeds E, the delta O-18-delta D distribution identifies the MW inputs from their kinetic signature, whereas in regions where E exceeds MW, the delta O-18-delta D distribution traces the humidity at the sea surface.