Oxygen production and carbon sequestration in an upwelling coastal margin

Copyrighted by American Geophysical Union.We examined high-resolution cross-shelf distributions of particulate organic carbon\ud (POC) and dissolved O₂ during the upwelling season off the Oregon coast. Oxygen\ud concentrations were supersaturated in surface waters, and hypoxic in near-bottom\ud waters, with greatly expanded hypoxic conditions late in the season. Simplified time-dependent\ud mass balances on cross-shelf integrated concentrations of these two\ud parameters, found the following: (1) The average net rate of photosynthesis generated\ud 2.1 mmol O₂ m⁻³ d⁻¹ and (2) essentially none of the corresponding net carbon fixation of\ud 1.4 mmol m⁻³ d⁻¹ could be accounted for in the observed standing stocks of POC.\ud After examining other possible sinks for carbon, we conclude that most of the net\ud production is being exported to the adjacent deep ocean. A simplified POC budget\ud suggests that about a quarter of the export is via alongshore advection, and the remainder\ud is due to some other process. We propose a simplistic conceptual model of across-shelf\ud transport in which POC sinks to the bottom boundary layer where it comes into\ud contact with mineral ballast material but is kept in suspension by high turbulence. When\ud upwelling conditions ease, the BBL waters move seaward, carrying the suspended,\ud ballasted POC with it where it sinks rapidly into the deep ocean at the shelf break. This\ud suggests a mechanism whereby the duration and frequency of upwelling events and\ud relaxations can determine the extent to which new carbon produced by photosynthesis in\ud the coastal ocean is exported to depth rather than being respired on the shelf