A Data-constrained Estimate of the Global Ocean Iron Cycle: Budgets, Timescales, and Iron Limitation
Friday, 19 December 2014
The oceanic iron cycle is estimated by optimizing a simple steady-state model based on a data-assimilated global circulation, with a prescribed optimized phosphorus cycle and a prescribed aeolian source pattern. Key biogeochemical parameters are determined by minimizing a suitably weighted quadratic misfit between the model’s dissolved iron concentration and a global data set of sparse measurements. The global dissolved iron inventory is estimated to be (7.1±0.1)×1011 mol Fe, of which (6.9±0.1)×1011 mol Fe is bound to organic ligands and hence bioavailable, while the remainder is “free” iron. The aeolian iron input rate is estimated at (3.3±0.5)×109 mol Fe/year, corresponding to a bulk residence time for bioavailable iron of 215±40 years, comparable to the bulk biological cycling timescale estimated at 246±24 years. Iron limitation is quantified in terms of the difference [Fe∗] between the actual iron concentration and that needed to utilize the available phosphate. The optimized model captures the observed high-nutrient, low-chlorophyll regions of the ocean as iron-limited regions with [Fe∗]<0. We define an iron age, ΓFe, as the mean time since iron at a given point was last injected from the atmosphere and compute ΓFe using an equivalent linear formulation of the model. In the euphotic zone, ΓFe ranges from a few decades or less in regions of high aeolian input to ∼1800 years in the Southern Ocean. The patterns of ΓFe show that iron is supplied to the Southern Ocean euphotic zone primarily from depth rather than being advected within the thermocline following deposition from continental dust plumes. Because [Fe∗] is negative in the deep southern oceans, upwelling waters maintain Southern Ocean iron limitation.