Iron in sea ice: Review and new insights

Abstract

The discovery that melting sea ice can fertilize iron (Fe)-depleted polar waters has recently fostered trace metal research efforts in sea ice. The aim of this review is to summarize and synthesize the current understanding of Fe biogeochemistry in sea ice. To do so, we compiled available data on particulate, dissolved, and total dissolvable Fe (PFe, DFe and TDFe, respectively) from sea-ice studies from both polar regions and from sub-Arctic and northern Hemisphere temperate areas. Data analysis focused on a circum-Antarctic Fe dataset derived from 61 ice cores collected during 10 field expeditions carried out between 1997 and 2012 in the Southern Ocean. Our key findings are that 1) concentrations of all forms of Fe (PFe, DFe, TDFe) are at least a magnitude larger in fast ice and pack ice than in typical Antarctic surface waters; 2) DFe, PFe and TDFe behave differently when plotted against sea-ice salinity, suggesting that their distributions in sea ice are driven by distinct, spatially and temporally decoupled processes; 3) DFe is actively extracted from seawater into growing sea ice; 4) fast ice generally has more Fe-bearing particles, a finding supported by the significant negative correlation observed between both PFe and TDFe concentrations in sea ice and water depth; 5) the Fe pool in sea ice is coupled to biota, as indicated by the positive correlations of PFe and TDFe with chlorophyll a and particulate organic carbon; and 6) the vast majority of DFe appears to be adsorbed onto something in sea ice. This review also addresses the role of sea ice as a reservoir of Fe and its role in seeding seasonally ice-covered waters. We discuss the pivotal role of organic ligands in controlling DFe concentrations in sea ice and highlight the uncertainties that remain regarding the mechanisms of Fe incorporation in sea ice.

Sea Ice Meltwater and Circumpolar Deep Water Drive Contrasting Productivity in Three Antarctic Polynyas

“…While low‐latitude environments of the Southern Ocean may receive atmospheric inputs of iron, coastal Antarctica generally does not (Heywood et al, ), except for places such as McMurdo Sound which is in close proximity to the dry valleys and Mount Erebus volcano (de Jong et al, ). The main sources of iron to coastal Antarctica are (1) melting ice shelves, glaciers, and icebergs, particularly if marine ice is present (Herraiz‐Borreguero et al, ; Lin et al, ); (2) upwelled iron‐rich mCDW interacting with Fe‐rich sediments (de Jong et al, ; Measures et al, ; Sherrell et al, ); and (3) sea ice (Lannuzel, Vancoppenolle, et al, ). We examine each of these sources in this and the following two sections.…”

Delineating environmental control of phytoplankton biomass and phenology in the Southern Ocean

“…We note however that seamounts and submerged plateaus do not enhance local or downstream PB, as already observed north of Kerguelen plateau [Graham et al, 2015] (Figures 1a and 4c), suggesting that not all shallow areas are active and/or bioavailable iron sources. Close to the Antarctic shelf and coastal polynyas, the potential role of the seasonal melting of sea ice [Arrigo et al, 2015;Lannuzel et al, 2016] as an iron source is revealed with enhanced PB (~0.4 mg m À3 ).…”

Physical and Biological Drivers of Biogeochemical Tracers Within the Seasonal Sea Ice Zone of the Southern Ocean From Profiling Floats

“…This increased stratification causes the depth of the surface mixed layer to shoal which in turn promotes conditions that trigger a phytoplankton bloom following Sverdrup's hypothesis where greater density of phytoplankton is within the euphotic zone (Smith & Nelson, 1986;Sverdrup, 1953). Other factors affecting the timing or magnitude of an ice edge phytoplankton bloom include release of iron and nutrients stored in ice upon ice melt (Lannuzel et al, 2016) as well as wind forcing (Fitch & Moore, 2007) and thermal convection (Ferrari et al, 2015).…”