The age of iron and iron source attribution in the ocean

• Main Authors: Holzer, Mark, Frants, Marina, Pasquier, Benoît
• Other Authors: Naval Postgraduate School (U.S.), Engineering and Applied Sciences
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: AGU Publications 2016
• Subjects: Iron concentrations are partitioned according to source and time since injection into the ocean for a family of inverse-model estimates; Iron concentration anomalies due to source removal underestimate the true source contribution because scavenging is nonlinear; Most iron in the Southern Ocean euphotic zone is upwelled regenerated iron deposited from the atmosphere several hundred years earlier; Southern Ocean; Antarc*; Antarctica
• Online Access: https://hdl.handle.net/10945/57209

The article of record as published may be found at http://dx.doi.org/10.1002/2016GB005418 All iron data used in this study are available from GEOTRACES (http://www.bodc.ac.uk/geotraces/data) We use tracers to partition dissolved iron (dFe) into the contributions from each source within a numerical model of the iron cycle without perturbing the system. These contributions are further partitioned according to the time since injection into the ocean, which defines their iron-age spectrum and mean iron age. The utility of these diagnostics is illustrated for a family of inverse model estimates of the iron cycle, constrained by a data-assimilated circulation and available dFe measurements. The source contributions are compared with source anomalies defined as the differences between solutions with and without the source in question. We find that in the Southern Ocean euphotic zone, the hydrothermal and sediment contributions range from 15% to 30% of the total each, which the anomalies underestimate by a factorof∼2becauseofthenonlinearityofscavenging.Theironageisonlyresetbyscavengingandattains a mean of several hundred years in the Southern Ocean euphotic zone, revealing that aeolian iron there is supplied primarily from depth as regenerated dFe. Tagging iron according to source region and pathways shows that 70–80% of the aeolian dFe in the euphotic zone near Antarctica is supplied from north of 46∘S via paths that reach below 1 km depth. Hydrothermal iron has the oldest surface mean ages on the order of middepth ventilation times. A measure of uncertainty is provided by the systematic variations of our diagnostics across the family of iron cycle estimates, each member of which has a different aeolian sourcestrength. Australian Research Council grant DP120100674 Australian Research Council Government of Monaco Scientific Centre of Monaco Frères Louis et Max Principale Foundation Cuomo Foundation