A Biogeochemical Southern Ocean State Estimate (B-SOSE) is introduced that includes carbon and oxygen fields as well as nutrient cycles. The state estimate is constrained with observations while maintaining closed budgets and obeying dynamical and thermodynamic balances. Observations from profiling floats, shipboard data, underway measurements, and satellites are used for assimilation. The years 2008-2012 are chosen due to the relative abundance of oxygen observations from Argo floats during this time. The skill of the state estimate at fitting the data is assessed. The agreement is best for fields that are constrained with the most observations, such as surface pCO(2) in Drake Passage (44% of the variance captured) and oxygen profiles (over 60% of the variance captured at 200 and 1000 m). The validity of adjoint method optimization for coupled physical-biogeochemical state estimation is demonstrated with a series of gradient check experiments. The method is shown to be mature and ready to synthesize in situ biogeochemical observations as they become more available. Documenting the B-SOSE configuration and diagnosing the strengths and weaknesses of the solution informs usage of this product as both a climate baseline and as a way to test hypotheses. Transport of Intermediate Waters across 32 degrees S supplies significant amounts of nitrate to the Atlantic Ocean (5.57 +/- 2.94 Tmol yr(-1)) and Indian Ocean (5.09 +/- 3.06 Tmol yr(-1)), but much less nitrate reaches the Pacific Ocean (1.78 +/- 1.91 Tmol yr(-1)). Estimates of air-sea carbon dioxide fluxes south of 50 degrees S suggest a mean uptake of 0.18 Pg C/yr for the time period analyzed.