Biological carbon fixation and iron isotopes in Pacific Ocean (MC-ICP-MS)

Biological carbon fixation

Biological carbon fixation is limited by the supply of iron (Fe) in vast regions of the global ocean. Dissolved Fe in seawater is primarily sourced from continental mineral dust, submarine hydrothermalism, and sediment dissolution along continental margins. However, the relative contributions of these three sources to the Fe budget of the open ocean remains contentious. By exploiting the Fe stable isotopic fingerprints of these sources, it is possible to trace distinct Fe pools through marine environments, and through time using sedimentary records.

The study presents a reconstruction of deep-sea Fe isotopic compositions from a Pacific Fe−Mn crust spanning past 76 Ma. It is found that there have been large and systematic changes in the Fe isotopic composition of seawater over the Cenozoic that reflect the influence of several distinct Fe sources to the central Pacific Ocean. Given that deeply sourced Fe from hydrothermalism and marginal sediment dissolution exhibit the largest Fe isotopic variations in modern oceanic settings, the record requires that these deep Fe sources have exerted a major control over the Fe inventory of the Pacific for the past 76 Ma. The persistence of deeply sourced Fe in the Pacific Ocean illustrates that multiple sources contribute to the total Fe budget of the ocean and highlights the importance of oceanic circulation in determining if deeply sourced Fe is ever ventilated at the surface.

Fe isotopic analyses were carried out on a Nu Plasma HR MC-ICP-MS at the University of Oxford. Corrections for instrumental mass bias was performed by sample-standard bracketing. The plot shows the reconstructed Fe isotopic history of central Pacific seawater over the past 76 Ma.

Data source: Horner et al., PNAS 112 (2015) 1292-1297