Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996-1998

During the 1996–1998 Antarctic Environment and Southern Ocean Process Study (AESOPS), a component of US JGOFS, we obtained seasonal or longer-term data sets on the rates of production, vertical transport, remineralization and burial of particulate organic carbon (POC) and biogenic silica (BSiO₂) in the Southern Ocean at 170°W between 55°S and 68°S. The AESOPS data records enable us to construct vertical C and Si budgets for the water column and upper sediments, with all estimates derived from direct measurement of the relevant fluxes.

We constructed annual C and Si budgets for each of four ecologically distinct zonal bands within the system. For both POC and BSiO₂ the greatest annual delivery to the sea floor (∼200 and 1400 mmol m⁻² yr⁻¹, respectively) and burial (∼6 and 160 mmol m⁻² yr⁻¹, respectively) were observed in the southern Antarctic Circumpolar Current (ACC) between 61.5°S and 65.5°S. That pattern is consistent with our observation that a diatom bloom propagated southward through the southern ACC during the spring and summer of 1997–1998, following the receding ice edge, and that this bloom was the main source of both POC and BSiO₂ in the system on an annual basis. In the other zones the annual fluxes of POC and BSiO₂ to the sea floor ranged from 19% to 67% of those in the zone traversed by the summer diatom bloom. The higher benthic fluxes of both POC and BSiO₂ in the southern ACC imply that blooms similar to the one we observed in 1997–1998 occur commonly in the southern ACC, and that their high-productivity signature is transmitted to the sea floor.

The data show preferential preservation of BSiO₂ over POC throughout the water column and upper seabed. In the four zonal bands we consider, BSiO₂ and POC are produced in mole ratios of 0.1–0.4, exported from the upper 100 m in ratios of 0.2–0.6, arrive at 1000 m in ratios of 1.5–4.5, reach the sea floor in ratios of 2.2–7.6, and are buried in ratios of 11.6–28. Despite the preferential preservation of BSiO₂, accumulation of opal-rich sediments beneath the ACC does not result from unusually efficient preservation of siliceous material. The estimated BSiO₂ preservation efficiency (burial-production) ranges from 1.2% to 5.5%, indistinguishable from the global average of 3%. Instead, opal-rich sediment accumulation in this region reflects very high annual rates of BSiO₂ production in surface waters, along with very low accumulation rates of other sedimentary components (e.g., CaCO₃ and detrital material).

The observed high ratios of BSiO₂ production to POC production in surface waters are consistent with the known tendency for the Si/C ratio of diatoms to increase when [Fe] is low. If greater Fe availability during the last glacial maximum permitted diatoms in the Southern Ocean to grow with lower, more normal Si/C ratios, export of diatom-produced POC could have occurred at 2–3 times its present rate. A corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65°S under present conditions. Thus, even large increases in POC production and export during glacial periods would not be reflected in the opal accumulation record.