Resupply of mesopelagic dissolved iron controlled by particulate iron composition

Abstract

Dissolved iron supply controls half of ocean primary productivity. Resupply by remineralization of sinking particles, and subsequent vertical mixing, largely sustains 2 this productivity. However, our understanding of the drivers of dissolved iron resupply, and their influence on its vertical distribution across the oceans, is still limited due to sparse observations. There is a lack of empirical evidence for what controls subsurface iron remineralization due to difficulties in studying mesopelagic biogeochemistry. Here, we present estimates of particulate transformations to dissolved iron, concurrent oxygen consumption and iron-binding ligand replenishment based on in situ mesopelagic experiments. Dissolved iron regeneration efficiencies (i.e., replenishment/oxygen consumption) were ten-to one hundred-fold higher in low-dust Subantarctic waters relative to higher-dust Mediterranean sites. Regeneration efficiencies are heavily influenced by particle composition. Their make-up dictates ligand release, controls scavenging, modulates ballasting, and may lead to differential remineralization of biogenic versus lithogenic iron. At high-dust sites these processes together increase the iron remineralization length-scale. Modelling reveals that in oceanic regions near deserts, enhanced lithogenic fluxes deepen the ferricline, which alter vertical patterns of dissolved iron replenishment, and set its redistribution at the global scale. Such wideranging regeneration efficiencies drive different vertical patterns in dissolved iron replenishment across oceanic provinces. Globally, the productivity of major phytoplankton groups, including diatoms and diazotrophs, is set by dissolved iron (DFe) supply 1. Twenty years of research has revealed diverse modes of DFe supply, from dust to hydrothermal vents, and their regional influences 2,3. Iron biogeochemistry is a rapidly evolving field, driving the development of global modelling initiatives 4. However, iron cycling in the oceans' interior, a fundamental component of iron biogeochemistry 5,6 , represents a major unknown, and critically is hindering model development 7 .

Changing Biogeochemistry of the Southern Ocean and Its Ecosystem Implications

“…For instance, whilst increased dust input from Australasia, Patagonia and South Africa would be expected to increase surface iron concentrations and enhance productivity, a large proportion of the dust may remain undissolved and act as additional ballast for sinking particulate organic carbon (POC). This would deepen the mean iron remineralisation depth, and possibly supply new particle surfaces for adsorptive scavenging (i.e., removal) of dissolved iron, reinforcing a possible reduction in the subsurface iron pool (Ellwood et al, 2014;Bressac et al, 2019). The vertical iron gradient and upward iron flux would then be reduced, even in Southern Ocean regions that experience reduced stratification associated with declining sea ice and/or intensification of westerly winds (Rintoul, 2018).…”

Regeneration of macronutrients and trace metals during phytoplankton decay: An experimental study

“…As the POC and PON gradually decreased ( Fig. 2c-e), dissolved Fe and other scavenged metals would be expected to increase (Barbeau et al 2001), accompanied by the release of Fe-binding ligands (Sato et al 2007;Bressac et al 2019), including siderophores (Gledhill et al 2004). Notably, Fe and Mn can both bind strongly to siderophores, leading to possible competitive binding and coupled biogeochemical pathways (Duckworth et al 2009).…”

“…Particle suspension experiments have offered further insight into trace metal sorption kinetics and partitioning (Balistrieri and Murray 1984;Nyffeler et al 1984;Garnier et al 1997), and radiotracer studies have measured release rates of some trace metals (e.g., Co, Zn, Cd) from phytoplankton in culture experiments (Lee and Fisher 1992;Fisher and Wente 1993). Recent dark incubation studies have examined the regeneration of Fe from mesopelagic particles collected in situ (Bressac et al 2019), and of Fe and Mn from benthic sediments (Cheize et al 2019). Substantial progress has also been made in modeling trace metal regeneration throughout the water column from particle flux studies (Twining et al 2014;Boyd et al 2017;Bressac et al 2019).…”

“…Recent dark incubation studies have examined the regeneration of Fe from mesopelagic particles collected in situ (Bressac et al 2019), and of Fe and Mn from benthic sediments (Cheize et al 2019). Substantial progress has also been made in modeling trace metal regeneration throughout the water column from particle flux studies (Twining et al 2014;Boyd et al 2017;Bressac et al 2019). Nevertheless, a gap still exists in understanding the timeline of trace metal regeneration over the course of phytoplankton decay, a process central to ocean water column distributions.…”

Phosphorites, glass ramps and carbonate factories: The evolution of an epicontinental sea and a late Palaeozoic upwelling system (Phosphoria Rock Complex)

“…Phosphorous could have been absorbed to iron‐(oxyhydr)oxides and clays (Filippelli, ; Wang et al., ; Drummond et al., ). Some of these nutrients would have been bioavailable and promoted primary productivity as they do in the modern oceans (Benitez‐Nelson, ; Bressac et al., ).…”