Mineralization of biogenic materials in the water masses of the South Atlantic Ocean. II: Stoichiometric ratios and mineralization rates

The variability of nitrate (N), phosphate (P), silicate (Si) and Apparent Oxygen Utilization (AOU) due to water mass mixing was objectively separated from the variability due to mineralization of biogenic materials in the western and eastern South Atlantic Ocean on basis of the constrained Optimum MultiParameter (OMP) analysis implemented in the companion manuscript. Using a consensus linear regression model, AOU/N/P/Si mineralization ratios and the corresponding oxygen utilisation rates (OURs) were obtained for the realm of each water mass defined after the OMP analysis. Combining these results with a stoichiometric model, the organic carbon to nitrogen (C/N) ratios and the biochemical composition (carbohydrates + lipids, proteins and phosphorus compounds) of the mineralized material, were derived. The vertical variability of the AOU/N, AOU/P and AOU/C mineralization ratios pointed to a significant fractionation during the mineralization of sinking organic matter. This fractionation was confirmed by preferential consumption of organic phosphorous compounds and proteins in shallower levels, which produced an increase of the C/N ratio of the mineralised materials of 0.5 ± 0.2 mol C mol N–1 every 1000 dbar. OURs in the twilight zone decreased quadratically with the C/N molar ratio of the mineralised material and exponentially with pressure (p, in 103 dbar) according to the following regression equation: Ln (OUR) = 6.2(±1.2) − 2.0(±0.7) * Ln (C/N) − 0.6(±0.2) * p (r2 = 0.87, p < 0.006, n = 8). This variability in the rates and stoichiometric ratios of the biogenic material mineralization compromises our capacity to predict the ocean biogeochemistry response to global change, including the CO2 uptake and storage and the corresponding feedback mechanisms.