The biogeochemical structuring role of horizontal stirring: Lagrangian perspectives on iron delivery downstream of the Kerguelen Plateau

[摘要] Field campaigns are instrumental in providing ground truth for understandingand modeling global ocean biogeochemical budgets. A survey however can onlyinspect a fraction of the global oceans, typically a region hundreds of kilometers wide fora temporal window of the order of (at most) several weeks. Thisspatiotemporal domain is also the one in which the mesoscale activity inducesthrough horizontal stirring a strong variability in the biogeochemicaltracers, with ephemeral, local contrasts which can easily mask the regionaland seasonal gradients. Therefore, whenever local in situ measures are usedto infer larger-scale budgets, one faces the challenge of identifying themesoscale structuring effect, if not simply to filter it out. In the case ofthe KEOPS2 investigation of biogeochemical responses to natural ironfertilization, this problem was tackled by designing an adaptive samplingstrategy based on regionally optimized multisatellite products analyzed inreal time by specifically designed Lagrangian diagnostics. This strategyidentified the different mesoscale and stirring structures present in theregion and tracked the dynamical frontiers among them. It also enabledback trajectories for the ship-sampled stations to be estimated, providingimportant insights into the timing and pathways of iron supply, which wereexplored further using a model based on first-order iron removal. This contextwas essential for the interpretation of the field results. The mesoscalecirculation-based strategy was also validated post-cruise by comparing theLagrangian maps derived from satellites with the patterns of more than onehundred drifters, including some adaptively released during KEOPS2 and asubsequent research voyage. The KEOPS2 strategy was adapted to the specificbiogeochemical characteristics of the region, but its principles are generaland will be useful for future in situ biogeochemical surveys.

[学科分类] 地球化学与岩石