The nature of the exported iron plays a major role in its ocean cycle
For the first time, an international team, including our MIO colleague Thibaut Wagener, made in situ observations of bacterial iron regeneration in the mesopelagic zone - the "black" ocean where light does not penetrate - of the Southern Ocean and the Mediterranean Sea. By combining these observations with numerical simulations of the PISCES biogeochemical model, the researchers highlighted the contrasting roles of biogenic and lithogenic iron in the mesopelagic zone. They demonstrated that the biogenic or lithogenic nature of the particulate iron exported to the depths controlled the replenishment of this deep reservoir with dissolved iron on a global scale.
The mesopelagic zone (located between the base of the euphotic layer and 1000 m deep) represents an important reservoir of dissolved iron, a chemical element that plays a key role in controlling primary oceanic production on a global scale. As this area is difficult to explore, our knowledge of the mesopelagic iron cycle, a major component of its ocean cycle, is limited to the systematic decoupling observed between the remineralization depth of iron and that of other major elements such as carbon or nitrogen. A better understanding of iron regeneration in this deep ocean area is therefore an important scientific challenge to better describe the global distribution of dissolved iron and to improve its representation in biogeochemical models.
This lack of knowledge of mesopelagic iron regeneration is linked in particular to the difficulty of studying in situ the biogeochemistry of the mesopelagic zone. The recent development of the BREATH and TM-RESPIRE traps/incubators now makes it possible to measure simultaneously and in situ the bacterial remineralization rates of the flows of exported material to the depths and the release of dissolved iron associated with the degradation of this material.
Thanks to these innovative instruments, an international research team1 was able to carry out these measurements for the first time in contrasting areas of the ocean: the Southern Ocean and the Mediterranean Sea2. These two environments are characterized by very different surface productivity and atmospheric inputs of lithogenic material (from rocks): in the Austral, the exported flow of particulate iron is mainly carried by biogenic particles (from living organisms) while in the Mediterranean, the lithogenic component dominates.
The study shows that the efficiency of iron regeneration is one to two orders of magnitude higher when iron flows are essentially biogenic than when lithogenic material is present. The replenishment of the mesopelagic reservoir with dissolved iron is therefore strongly influenced by the composition of the exported particulate iron (i.e. biogenic or lithogenic). Although lithogenic particles of desert origin are considered a major source of dissolved iron for the surface ocean, it appears that several complex processes make these particles a trap for dissolved iron in the mesopelagic zone.
These in situ measurements allowed a first parameterization of the mesopelagic regeneration of iron which was integrated into the PISCES biogeochemical model. These new numerical simulations reveal that the combined effect of lithogenic particles and ocean dynamics is responsible for a major redistribution of dissolved iron in the first 1000 m of the water column, not only in regions near deserts but also on the scale of the global ocean. In a context of climate change and aridification, these effects would be accentuated, with significant implications for the efficiency of the biological carbon pump.
Bressac M., C. Guieu, M. J. Ellwood, A. Tagliabue, T. Wagener, E. C. Laurenceau-Cornec, H. Whitby, G. Sarthou, P. W. Boyd. Resupply of mesopelagic dissolved iron controlled by particulate iron composition. Nature Geoscience, 2019, doi:10.1038/s41561-019-0476-6.