The C:N:P stoichiometry of exported organic matter regulates the magnitude and efficiency of the biological carbon pump, yet its global distribution remains poorly constrained. Conventional ocean biogeochemical models commonly assume fixed Redfield stoichiometry, potentially overlooking systematic variations in organic matter production, export, and remineralization. Here, we develop a three-dimensional inverse ocean biogeochemical model that couples the marine carbon, nitrogen, phosphorus, and oxygen cycles. The model is simultaneously constrained by global observations of dissolved inorganic phosphorus, dissolved inorganic nitrogen, dissolved organic nitrogen, dissolved inorganic carbon, alkalinity, dissolved organic carbon, and oxygen.

The optimized model generally reproduces the observed global distributions of major biogeochemical tracers and the organic matter stoichiometric patterns observed at time-series stations and across latitudinal gradients. The estimated globally averaged C:N:P ratio of exported particulate organic matter is approximately 129:20:1, substantially higher than the canonical Redfield ratio but broadly consistent with the observational estimate of 137:21:1. Export C:P and N:P ratios exhibit pronounced spatial and basin-scale variability, with lower values at high latitudes and higher values in nutrient-poor subtropical regions. The C:N ratio is comparatively stable but displays a similar spatial pattern. The model further estimates global particulate and dissolved organic carbon export fluxes of approximately 9.72 and 5.19 Pg C yr⁻¹, respectively.

These results demonstrate that inverse modeling constrained by multiple biogeochemical observations provides an effective framework for diagnosing the spatial heterogeneity of marine organic-matter stoichiometry. Future ocean warming and enhanced stratification may expand subtropical oligotrophic regions characterized by elevated C:P and C:N ratios. Consequently, incorporating flexible C:N:P stoichiometry into global biogeochemical models is therefore important for constraining the biological carbon pump and evaluating its response to environmental and climatic change.