Persistence of deeply sourced iron in the Pacific Ocean

Abstract

Biological carbon fixation is limited by the supply of 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. We present a reconstruction of deep-sea Fe isotopic compositions from a Pacific Fe−Mn crust spanning the past 76 My. We find 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 My. 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.

Iron mineral structure, reactivity, and isotopic composition in a South Pacific Gyre ferromanganese nodule over 4 Ma

“…Although our measurement interval is too coarse to resolve the isotopic signature of Fe-rich ''matrix" versus Mnrich botryoidal structures, our results are consistent with similar isotope values for ''matrix" and ''botryoids" because there is more ''matrix" in the ''inner" part and more ''botryoids" in the ''outer" part of the nodule, yet the isotope values are the same within experimental error. Moreover, the Fe isotopes recorded in the SPG-2 nodule are within the range of Fe isotope compositions reported for slowly precipitated hydrogenetic ferromanganese crusts (1.2 to +1.6‰) (Zhu et al, 2000;Levasseur et al, 2004;Chu et al, 2006;Horner et al, 2015). the SPG-2 nodule are present in a wide variety of formsgoethite (a-FeOOH), goethite polymorphs feroxyhite (d 0 -FeOOH) and nano-lepidocrocite (c-FeOOH), and biogeniclike ferrihydrite.…”

“…For instance, the Fe isotope composition in a Pacific hydrogenetic ferromanganese crust over 76 Ma shows that deep Fe sources (e.g. hydrothermal Fe) may have strongly influenced the Fe supply to the oceans over the Cenozoic (Horner et al, 2015). These findings questioned the general assumption that aeolian dust particles are the predominant source controlling the biogeochemical cycling of Fe in modern oceans.…”

“…Despite a range of Fe isotope fractionation factors that were measured during laboratory experiments involving adsorption reactions and scavenging on Fe-phases (e.g. (Bullen et al, 2001;Skulan et al, 2002;Johnson et al, 2005;Wu et al, 2011)), Horner et al (2015) report that the isotopic fractionation caused by the precipitation of dissolved seawater Fe (most likely in the form of Fe (III (Conway and John, 2014)). Finally, implementation of our analytical approach to different type of nodules (e.g.…”

“…These sources of Fe have different d 56/54 Fe values that are further modified by chemical processes in the ocean (Beard et al, 2003;Chu et al, 2006;Dauphas and Rouxel, 2006;Severmann et al, 2010;Radic et al, 2011;John et al, 2012;Conway and John, 2014;Scholz et al, 2014;Horner et al, 2015). Hence, temporal Fe isotope variations in seawater, as recorded by ferromanganese hydrogenetic deposits, may help deciphering the variability of Fe sources to the oceans (Zhu et al, 2000;Chu et al, 2006;Horner et al, 2015). For instance, the Fe isotope composition in a Pacific hydrogenetic ferromanganese crust over 76 Ma shows that deep Fe sources (e.g.…”

Diagnosing oceanic nutrient deficiency

“…However, recent evidence, particularly from hydrothermal vent plumes [102][103][104], suggests that sub-surface transport of Fe may still occur over significant distances, presumably due to some processes stabilizing deep ocean Fe concentrations [105]. Such large-scale transport of persistent deeply sourced Fe [104,106] will act to reduce the overall magnitude of sub-surface Fe deficiency (figures 4e,f and 5) and likely contributes significantly to sustaining production following deep water upwelling in the SO [104].…”