The shape of the oceanic nitracline

[摘要] In most regions of the ocean, nitrate is depleted near the surface byphytoplankton consumption and increases with depth, exhibiting a strongvertical gradient in the pycnocline (here referred to as the nitracline). Thevertical supply of nutrients to the surface euphotic zone is influenced bythe vertical gradient (slope) of the nitracline and by the verticalseparation (depth) of the nitracline from the sunlit surface layer. Hence itis important to understand the shape (slope and curvature) and depth of theoceanic nitracline. By using density coordinates to analyze nitrate profilesfrom autonomous Autonomous Profiling EXplorer floats with In-Situ Ultraviolet Spectrophotometers (APEX-ISUS) and ship-based platforms (World Ocean Atlas – WOA09; Hawaii OceanTime-series – HOT; Bermuda Atlantic Time-series Study – BATS; andCalifornia Cooperative Oceanic Fisheries Investigations – CalCOFI), we areable to eliminate much of the spatial and temporal variability in theprofiles and derive robust relationships between nitrate and density. Thisallows us to characterize the depth, slope and curvature of the nitracline indifferent regions of the world's oceans. The analysis reveals distinguishingpatterns in the nitracline between subtropical gyres, upwelling regions andsubpolar gyres. We propose a one-dimensional, mechanistic model that relatesthe shape of the nitracline to the relative depths of the surface mixed layerand euphotic layer. Though heuristic, the model accounts for some of theseasonal patterns and regional differences in the nitrate–densityrelationships seen in the data.

[学科分类] 地球化学与岩石