Three-dimensional simulations of the impact of Southern Ocean nutrient depletion on atmospheric CO2 and ocean chemistry

Abstract

Surface nutrient concentrations in the Southern Ocean are an important indicator of the atmosphere-ocean chemical balance that played a key role in ice-age reduction of atmospheric pC0, and would play a role in any Fe fertilization scenario for increasing oceanic uptake of anthropogenic CO,. The response of the ocean and atmosphere to a scenario of extreme depletion of Southern Ocean surface nutrients by an increase in the organic matter flux to the deep ocean is examined with a three-dimensional model of ocean circulation coupled to a one-box model of the atmosphere. After 100 yr, the increase in the organic matter flux is 6-30 Gt C yr-l-about twice the global new production determined by the same model for the present ocean. The removal of nutrients from surface waters of the Southern Ocean reduces the nutrient content of the near-surface and intermediate depth waters of the entire ocean, resulting in a 0.5-l .9 Gt C yr-’ reduction of low-latitude new production. The deep circumpolar waters, enriched in nutrients by regeneration of organic matter, spread into the deep and bottom waters of the remainder of the ocean, giving an overall downward shift of nutrients from surface and intermediate to circumpolar and deep waters. The oceanic total C distribution is also shifted downward, resulting in uptake of atmospheric CO, of 46-85 ppm (98-181 Gt C) in the first 100 yr. The oxygen content shifts upward in the water column, approximately mirroring the downward shift of nutrients. Some of the oxygen shifted to the upper ocean escapes to the atmosphere. As a consequence, the global average oceanic content of oxygen, presently 168 pmol kg-l, is reduced by 6-20 pmol kg-l, with anoxia developing in the southwestern Indian Ocean. CO2 is unusual compared to other atmospheric gases because most of the combined atmosphere-ocean inventory (98.5%) is in the ocean. By contrast, oxygen, a more typical atmospheric gas, has only 0.6% of its atmosphere-ocean inventory in the ocean. CO2 is m 30 times as soluble as oxygen, due mainly to hydrogen bonding with water molecules; however, the primary reason for the large amount of CO2 in the ocean is that it reacts with water to form HCO,and Acknowledgments This work was supported by the U.S. Department of Energy under contract DEFG 02-90ER611052, by the National Science Foundation (OCE 9012333), and by GFDWNOAA through the generosity of K. Bryan and J. Mahlman. The perturbation approach for modeling total C was developed in collaboration with U. Siegenthaler. The collaboration of J.L.S. with F. Joos and U. Siegenthaler in earlier high-latitude, nutrient-depletion studies gave us considerable insight into how to go about our threedimensional experiment and how to interpret the results. B. Flannery was the first member of our group to do a nutrient-depletion experiment (summer 1989), following the basic modeling approach developed by R. Najjar. Collaboration of J.C.O. with R. Najjar, R. Slater, and K. Dixon aided in modeling and analysis. COS2-. Only + 1% of the dissolved inorganic C is in the non-ionic CO, form (e.g. Broecker and Peng 1982). Another important contributor to reduced atmospheric CO, levels results from the distribution of total inorganic CO2 (i.e. the sum of the concentrations of C02, and HC03and C032ions), which is IV 12% lower at the surface than at depth. Part of this surface-to-deep difference is due to what is referred to as the “solubility pump” (Volk and Hoffert 1985). CO, is more than twice as soluble in the cold, high-latitude waters which sink to form the deep waters of the ocean, than it is in warm, low-latitude waters (Weiss 1974). However, the primary source of the surface to deep difference is the “biological pump” (Volk and. Hoffert 19 8 5). Photosynthesis produces organic C from CO, in the euphotic zone of the ocean, depleting surface nutrients to near-zero levels over most of the surface ocean. Although most of the photosynthetically produced organic C is recycled in the euphotic zone, -30% either sinks to the deep ocean or is transported there by circulation before being regenerated to CO,.