Biogeochemical cycling of iron and chromium in the North Atlantic Ocean: Insights from stable iron and chromium isotopes

Abstract

Many trace metals are essential for phytoplankton growth, but there is ongoing debate about the consequences of global warming on primary productivity because the processes that regulate the supply of these metals are poorly understood. This thesis utilises the stable isotope compositions of iron (Fe) and chromium (Cr) to provide new insights as to the provenance of metal inputs to the ocean, and the effects of biogeochemical cycling and redox processes on metal availability.
Chemical processes in the hydrothermal plume are demonstrated to regulate the evolution of the Fe isotope signature (δ56Fe) of hydrothermal Fe at hydrothermal vent fields (Beebe and Von Damm) in the Caribbean Sea. The δ56Fe of total dissolvable Fe evolves to higher values as Fe-sulfide particles fall out of the plume at Beebe. The δ56Fe value of dissolved Fe (dFe) in the near-field plumes was lower (as low as −4.08‰) than it was in the hydrothermal vent fluids (−0.28‰), due to oxidation of Fe(II) and precipitation of the Fe-(oxyhydr)oxides that form. The vent fluid Fe/H2S ratio and the Fe(II) oxidation rate are shown to be the principal controls on the δ56Fe signature of dFe that is delivered to the ocean interior.
For the first time, the Fe and Cr isotopic compositions of seawater in samples collected through hydrothermal plumes on the Mid-Atlantic Ridge (TAG and Rainbow vent fields) were measured. It is shown that the Fe and Cr isotope profiles through the hydrothermal plumes are the mirror image of each other, providing compelling evidence for coupled cycling of Fe and Cr. Oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides account for the low δ56Fe values of dFe (as low as −1.91‰ at TAG and −6.95‰ at Rainbow). At the same time, Cr(VI) appears to be reduced to Cr(III), as indicated by elevated Cr isotope (δ53Cr) values compared to background seawater (by up to +0.13‰ and +0.60‰ respectively). The δ56Fe of dFe evolves to heavier values (−0.44 to 0.23‰) in the distal part of the plume, likely controlled by exchange of Fe between the dissolved and particulate fraction. The δ56Fe value of the ‘stabilised’ Fe can nevertheless be used to distinguish input of hydrothermal Fe from other Fe sources (e.g. atmospheric dust). Scavenging of Cr(III) by Fe-(oxyhydr)oxide particles in the hydrothermal plume means that high-temperature hydrothermal systems are a sink for seawater Cr, potentially removing up to 20% of the riverine input.
Full water column depth profiles of dissolved Cr and δ53Cr were obtained for three stations in the sub-tropical North Atlantic. Subsurface waters were depleted in Cr, and enriched in heavy Cr isotopes, relative to deeper waters. High δ53Cr values (up to 1.4‰) in subsurface waters are not directly controlled by levels of oxygen or biological uptake, but are consistent with preferential removal of light Cr isotopes onto authigenic Fe particles. Regeneration of Cr in deeper waters leads to subtly increased levels of Cr at individual sites, but this trend is more obvious at the global scale. Removal and regeneration of relatively isotopically light Cr can account for the distributions of Cr and Cr isotopes in the global ocean and the systematic relationship between δ53Cr-ln[Cr] reported by other studies.

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Identifiers

ORCID for Wenhao Wang: orcid.org/0000-0003-4198-6681

ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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