Latitudinal change of remineralization ratios in the oceans and its implication for nutrient cycles

A new three‐end‐member mixing model is introduced to obtain remineralization ratios of organic matter in the water column. Remineralization ratios (P/N/Corg/−O2) of organic matter in the deep water column change systematically from the northern Atlantic to the Southern Oceans, then to the equatorial Indian and the northern Pacific oceans, more or less along the global ocean circulation route of deep water. Average remineralization ratios of organic matter for the northern Atlantic Ocean are P/N/Corg/−O2 = 1/(16 ± 1)/(73 ± 8)/(137 ± 7), and for the Southern Oceans P/N/Corg/−O2 = 1/(15 ± 1)/(80 ± 3)/(133 ± 5). Those values are similar to the traditional Redfield ratios of P/N/Corg/−O2 = 1/16/106/138 for marine plankton, except for the low Corg/P ratio. Average remineralization ratios for the equatorial Indian Ocean are P/N/Corg/−O2 = 1/(10 ± 1)/(94 ± 5)/(130 ± 7), and for the northern Pacific Ocean P/N/Corg/−O2 = 1/(13 ± 1)/(124 ± 11)/(162 ± 11). The apparent low N/P ratio for both ocean basins suggests that organic nitrogen was converted partly into gaseous N2O and N2 by bacteria through nitrification/denitrification processes in a low‐oxygen or reducing microenvironment of organic matter throughout the oxygenated water column. The actual N/P ratio of remineralized organic matter is probably around 15 ± 1. The −O2/Corg ratio of remineralized organic matter also decreases systematically along the global ocean circulation route of deep water, indicating changes in relative proportions of biomolecules such as lipids, proteins, nucleic acids, and carbohydrates. No temporal trends of remineralization ratios are detected when comparing the results obtained by GEOSECS and WOCE data sets.