Program

With the release of the Global Ocean Data Analysis Project Version 2 dataset, the world’s open ocean surface DIC distribution is becoming clearer. Despite its importance to the ocean carbon sink and ocean acidification, the controlling mechanisms of DIC spatial patterns have seldom been studied on a global scale before. Of particular interest in this study is what controls the large latitudinal gradient in surface DIC (~300 µmol kg-1). Here we present a quantitative evaluation of three hypotheses: (a) temperature-driven CO2 gas exchange, (b) upwelling in the Southern Ocean, and (c) iron limitation at high latitudes which leads to HNLC conditions. The DIC concentrations in the surface global ocean were investigated for the reference year 2005 based upon GLODAPv2. Generally, sea surface DIC increases polewards, negatively correlated with the sea surface temperature, consistent with the first hypothesis. Water from depth is brought to the surface both in the Southern Ocean (meridional overturning circulation) and in subarctic regions (deep winter mixing), which leads to the second hypothesis. Given the correspondence between nDIC and nutrient patterns, this gives grounds for support of the third hypothesis.

Our ongoing study suggests that temperature-driven CO2 gas exchange can explain the majority of the surface DIC latitudinal gradient (~220 µmol kg-1 at 70°S). Southern Ocean upwelling would produce a gradient of ~270 µmol kg-1 in nDIC in the absence of solubility effect, however, it is only responsible for an observed excess of about 40-80 µmol kg-1 nDIC over and above air-sea gas exchange equilibrium. Seasonal biological drawdown of surface DIC decreases the latitudinal gradient by ~45 µmol kg-1 in the Southern Ocean but would decrease it by much more (~120 µmol kg-1) if not for iron limitation.