Contribution of resuspended sedimentary particles to dissolved iron and manganese in the ocean: An experimental study

Abstract

A number of trace metals play essential roles in marine ecosystem structure and biological productivity. Until recently, it has been argued that phytoplankton access primarily dissolved iron, while particulate iron was considered a refractory material with little use biologically and limited interaction with the dissolved pool. In order to assess the transfer mechanisms between sediment-sourced particulate trace metals and the dissolved pool, we conducted a 14-month incubation that reacted resuspended sediments with natural seawater, both originating from the Kerguelen area (KEOPS cruises; Southern Ocean), in the dark, and at concentrations replicating natural conditions. Three types of sediments were investigated (named BioSi, BioSi + Ca, and Basalt), mostly composed of (i) biogenic silica (bSiO2), (ii) bSiO2 and calcite, and (iii) basaltic fragments, respectively. The release of dissolved silicon (dSi), iron (dFe) and manganese (dMn) was monitored regularly throughout the incubation, as well as living bacteria density and Fe organic ligands. Depending on the origin and composition of the sediment, unique dFe and dMn fluxes were observed, including a strong decoupling between dFe and dMn. The basaltic sediment released up to 1.09 ± 0.04 nmol L−1 of dFe and 0.28 ± 0.09 nmol L−1 of dMn, while the biogenic sediments released a higher 3.91 ± 0.04 nmol L−1 and 8.03 ± 0.42 nmol L−1 of dFe and dMn, respectively. Several factors influencing the release and removal of dFe and dMn were discernable at the temporal sampling resolution of the incubation, including the structural composition of the sediment, bacterial abundance, and the formation of manganese oxides. The regular sampling over short Highlights ► Resuspended sedimentary particles contribute to a substantial flux of trace metals into the overlying oceanic water column. ► The release of dissolved Fe and Mn from resuspended sedimentary particles is decoupled through time

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

“…Opal rich sediments show the fastest dissolution rates, which reach a maximum of 3.7 × 10 −4 day observed between day 57 and day 71. Data from the Cheize et al's () experimental study then suggest a range of dissolution rates from 4.2 × 10 −5 to 3.7 × 10 −4 day depending on the nature (calcite, basalt, and opal) of the sedimentary particles, that all originate from the Kerguelen area. Those values are significantly lower than the indirect estimation of 6 × 10 −3 day inferred by Slemons et al () from equatorial Pacific in situ observations.…”

“…To set the dissolution rate ( ), we benefited from the results of dissolution experiments conducted by Cheize et al (), which used, for the first time, realistic suspended particle concentrations and were performed under trace metal clean conditions. In those dissolution experiments, particles of sediments from different origins were kept in seawater for a 14‐month incubation period at a constant temperature of 15°C.…”

“…Given the large uncertainties on the dissolution rate, three simulations with a PFe Inorg flux were run that differ in the prescribed value of the dissolution rate: the P slem simulation used a dissolution rate of 6 × 10 −3 day corresponding to the estimate by Slemons et al (). In P med and P min , dissolutions rates were chosen to bracket the values derived from Cheize et al () (see Table ) and were set to 4 × 10 −4 and 4 × 10 −5 day, respectively. The simulation with an intermediate dissolution rate (i.e., P med ) is subsequently used as a reference.…”

“…Indeed, to date, particulate iron has mostly been viewed for its role in the biogenic iron remineralization, the scavenging, and aggregation processes (Ohnemus & Lam, 2015). This traditional view has been challenged by modeling studies (e.g., Gorgues et al, 2009), and more recently, experiments (e.g., Cheize et al, 2019;Jeandel & Oelkers, 2015) have stressed the role of lithogenic particles dissolution as a slow release of DFe. Thus, it has been suggested that the lithogenic particles from ocean margins may act as a "continuous fertilizer" impacting open ocean phytoplankton communities.…”

“…Additional Data to theCheize et al (2019) Experiments Used to Compute the Dissolution Rates of the PFe Inorg . and Corresponding Mean and Maximum Dissolution Rate Observed During the 423 Days of the Dissolution Experiment Sediment types Opal (A3 in Cheize et al (2019)) Opal and calcite (C11 in Cheize et al (2019)) Basalt (C1 in Cheize et al (2019))…”