Iron Isotope Characteristics of Hot Springs at Chocolate Pots, Yellowstone National Park

Abstract

Chocolate Pots Hot Springs in Yellowstone National Park is a hydrothermal system that contains high aqueous ferrous iron [∼0.1 mM Fe(II)] at circumneutral pH conditions. This site provides an ideal field environment in which to test our understanding of Fe isotope fractionations derived from laboratory experiments. The Fe(III) oxides, mainly produced through Fe(II) oxidation by oxygen in the atmosphere, have high ⁵⁶Fe/⁵⁴Fe ratios compared with the aqueous Fe(II). However, the degree of fractionation is less than that expected in a closed system at isotopic equilibrium. We suggest two explanations for the observed Fe isotope compositions. One is that light Fe isotopes partition into a sorbed component and precipitate out on the Fe(III) oxide surfaces in the presence of silica. The other explanation is internal regeneration of isotopically heavy Fe(II) via dissimilatory Fe(III) reduction farther down the flow path as well as deeper within the mat materials. These findings provide evidence that silica plays an important role in governing Fe isotope fractionation factors between reduced and oxidized Fe. Under conditions of low ambient oxygen, such as may be found on early Earth or Mars, significantly larger Fe isotope variations are predicted, reflecting the more likely attainment of Fe isotope equilibrium associated with slower oxidation rates under low-O₂ conditions.

Iron Isotope Fractionation during Fe(II) Oxidation Mediated by the Oxygen-Producing Marine Cyanobacterium Synechococcus PCC 7002

“…12,58−60 Although iron atom and electron exchange has received the most attention as a process relevant to Fe(III)-reducing systems where Fe(II) is in contact with Fe(III) minerals, our data suggest this process could also be relevant in environments where Fe(II) is abundant during Fe(II) oxidation and Fe(III) mineral formation. This includes oxidizing environments with high enough fluxes of Fe(II) for Fe(II) to persist even in the face of rapid oxidation, 2,61 such as Fe(II)-rich springs or seeps and marine upwelling zones that tap ferruginous bottom waters, past or present. 33 Furthermore, our work documents atom and electron exchange in the presence of iron minerals whose formation pathways are biologically induced and when organic phases coat iron minerals.…”

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring, Yellowstone National Park

“…Previous studies of other thermal springs in YNP have identified Fe-cycling microorganisms, including those involved in DIR (Kashefi et al, 2002a;Kozubal et al, 2012). Although prior studies have suggested that DIR could play a role in Fe redox cycling in the Fe(III) oxide deposits at CP hot springs, (Pierson et al, 1999;Pierson & Parenteau, 2000;Wu et al, 2013), to date there have been no explicit studies of organisms capable of DIR in this particular hydrothermal environment. The goal of this study was to obtain a first look at the potential for DIR in CP materials through a combination of MPN enumerations and enrichment culturing, and to assess the potential for Fe isotope fractionation during microbial reduction of the siliceous Fe(III) oxides native to CP.…”

“…Recent studies of Fe isotope geochemistry are also suggestive of a reductive iron cycle at CP (Wu et al, 2011(Wu et al, , 2013. Iron occurs as four stable isotopes, where 54 Fe and 56 Fe are the most abundant at 5.84% and 91.76%, respectively .…”

Fractionation of Fe isotopes during Fe(II) oxidation by a marine photoferrotroph is controlled by the formation of organic Fe-complexes and colloidal Fe fractions

“…For instance, at Chocolate Pots Hot Springs in Yellowstone NP, the degree of fractionation between aqueous Fe(II) and Fe(III) minerals is less than expected given the extent of Fe(II) oxidation (Wu et al, 2013). In other words, the Fe ppt is lighter than it should be using published fractionation factors.…”