Biogeochemical cycling in the global ocean .2. New production, Redfield ratios, and remineralization in the organic pump

Analytic solutions for global ocean distributions of phosphate, nitrate, and dissolved oxygen are presented for the high-latitude exchange/interior diffusion-advection (HILDA) model. The organic pump, which together with ocean physics controls nutrient and oxygen cycling in the ocean, is characterized in the model by parameters representing element (Redfield) ratios of organic matter exported from the lighted surface layer of the ocean and by parameters representing depth scales of remineralization for each element. Estimates for these biological parameters for the organic pump of the modern ocean are obtained by seeking least mean squares best fits of model solutions to ocean phosphate, nitrate + nitrite, and dissolved oxygen data cast into the HILDA geometry. Values of free parameters characterizing ocean physics in HILDA are adopted from the earlier analysis of ocean temperature and carbon 14 data in part 1 of this series. The best fit to nutrient data yields a depth scale of remineralization of 1060 m for phosphate and 1110 m for nitrate. Estimates of N:P ratios in new production are 14.5 and 15.4 within and outside high latitudes. For the oxygen model a low- to middle-latitude N:P ratio of 16 and a remineralization scale for nitrate of 1060 were adopted after a discussion of denitrification. The best fit to oxygen data yields a depth scale of remineralization for the carbon + hydrogen fraction of 1370 m and a (C + H):P ratio in new production of 116. These results lead to estimates of -O-2:P remineralization ratios of 122, 142, 168, and 199 at 100-, 1000-, 2000- and 3000-m depths, respectively. Together with an overall O-2:C photosynthetic quotient of 1.4, the best estimate of the C:P ratio in new production is 105, the traditional Redfield value. Sensitivity calculations are carried out to estimate confidence ranges for the best fit parameter values and the dependence of these values on choices of external parameters such as high-latitude surface temperature and preformed nutrients. A critical reassessment of recent work on determining Redfield ratios in new production and remineralization ratios is presented. From this reassessment and from the more robust results of this paper, the following conclusions are reached: (1) Total ocean new production is probably at least 5 Gt C yr(-1). (2) The best estimate for ocean mean -O-2:C:N:P mole ratios in new production is about 148:105:15:1. (3) Significant vertical fractionation of organic matter occurs in the upper kilometer or so of the ocean; nutrients are released in the mean faster than carbon dioxide. Mean remineralization ratios for -O-2:P (C:P) increase from about 130 +/- 15 (90 +/- 15) at the base of the euphotic zone to about 170 +/- 10 (125 +/- 10) at 1000- to 1500-m depth and remain so deeper down. (4) The organic matter exported out of the surface layer of the high-latitude oceans is relatively poor in nitrogen but relatively rich in hydrogen.