Impact of Inorganic Particles of Sedimentary Origin on Global Dissolved Iron and Phytoplankton Distribution

Abstract

Iron is known to be the limiting nutrient for the phytoplankton growth over~40% of the global ocean and to impact the structure of marine ecosystems. Dissolved iron (DFe) is assumed to be the only form available to phytoplankton while particulate iron (PFe) has mostly been considered for its role in the biogenic iron remineralization and induced scavenging. Therefore, most studies focused on the nature of DFe external sources to the ocean (i.e., eolian dust, riverine fluxes, hydrothermal sources, and sediment) and their quantification, which still remain uncertain. Among these external sources, the sedimentary sources have been shown to be underestimated. Moreover, the iron supply from sediments has been documented to be often larger in the particle fraction. Here we test the impacts of an iron sediment source of inorganic particulate iron (PFe Inorg ) on global DFe and phytoplankton distribution. We use experimentally acquired knowledge to test a parameterization of a PFe Inorg pool in a global biogeochemical model and compare with published indirect estimation. Depending on the parameterization of its dissolution and sinking speed, the PFe Inorg can noticeably enrich water masses in DFe during its transport from the sediment to the open ocean, notably in regions not usually accessible to external DFe inputs. Indeed, the fact that DFe is prone to scavenging reduces the impact of equivalent Fe inputs from sediments in the dissolved form in those regions far from the sediment sources. PFe Inorg thereby has the potential to fuel the phytoplankton growth in offshore regions impacting the coastal-offshore chlorophyll gradient.

Dissolved Iron Patterns Impacted by Shallow Hydrothermal Sources Along a Transect Through the Tonga‐Kermadec Arc

“…However, even though this process acts throughout the water column, scavenging is likely enhanced in the particle-rich surface layer as the residence time of DFe relative to scavenging has been reported to be faster in this layer (10-100 days; Black et al, 2020) than at depth (70-270 years; Bergquist & Boyle, 2006;Bruland et al, 1979). In addition, biological activities are likely to produce high levels of biogenic particles and simulations from Beghoura et al (2019) showed that biogenic particles sink up to two orders of magnitude faster than small inorganic particles. Besides, some processes influencing DFe removal act exclusively in shallow environments that are characterized by higher temperature and pH levels than deeper environments.…”

DFe patterns impacted by shallow hydrothermal sources along a transect through the Tonga-Kermadec arc

“…However, even though this process acts throughout the water column, scavenging is likely enhanced in the particle-rich surface layer as the residence time of dFe relative to scavenging has been reported to be faster in this layer (10 to 100 days; Black et al, 2020) than at depth (70 to 270 years; Bergquist & Boyle, 2006;Bruland et al, 1979). In addition, biological activities are likely to release high proportion of biogenic particles and simulations from Beghoura et al (2019) reported a lower sinking rate of small inorganic pFe compared to biogenic pFe (up to 2 orders of magnitude). Besides, some processes influencing dFe removal act exclusively in shallow environments.…”

Unexpected Source and Transport of Iron from the Deep Peru Margin

“…Rates of nonreductive dissolution of lithogenic Fe would have to be faster than the enhanced scavenging rates from increased particle concentration in order to act as a net source of dFe but measured rates of nonreductive dissolution are wuite slow . Further, mesocosm and modeling , experiments have shown that the addition of mineral dust frequently decreases dFe concentrations because of enhanced scavenging by the high particle concentrations.…”

“…However, the slow dissolution of lithogenic Fe is precisely the characteristic that may explain why relatively insoluble Fe-bearing sedimentary particles may be particularly important as a source of dFe into the deep interior ocean: coastal sources of dFe are scavenged and removed before reaching the interior, but since overall particle concentrations are greatly reduced in the interior the slow lithogenic dissolution rate may be able to compete with the even slower scavenging rate in the interior and act as a net source . On the GP16 transect, pAl concentrations, indicative of lithogenic particles, are elevated above background at 2000 m to 89°W and possibly further (Figure ), more than 1200 km from the margin, potentially providing a slow release source of dFe into the interior.…”