PROCESO DE PLANIFICACIÓN:

My earliest experience of doing science was during my senior year in college at Miami University, Oxford, Ohio. I was invited by my environmental chemistry teacher (and first scientific mentor), Bill Green, to work on the nearby Acton Lake. I was completely taken by the idea of actually doing research in the environment and jumped at the chance, leaving any ideas of pursuing a career in inorganic chemistry far behind.

Lake versus Marine Chemistry. Like many lakes in the Midwest of the

United States, Acton Lake is diamictic, meaning that it mixes twice a year and that it is stratified during the summer. Also like many lakes in the Midwest and elsewhere, when summer stratified, the bottom waters go anoxic and accumu- late dissolved Fe(II). Some of my very first geochemical measurements were of oxygen and Fe(II) concentrations in Acton Lake bottom waters, leading to my very first publication (Canfield et al., 1984). This paper is no citation classic, but it started a 30 year on-and-off affair with stratified lakes, and it demonstrated an important difference between the anoxic waters of Acton Lake and anoxic basins of the ocean. Simply stated, in the modern realm, when lakes go anoxic, they tend to accumulate Fe(II), whereas when marine basins go anoxic, they tend to accumulate sulphide. The difference in the behaviour of these two systems, briefly mentioned in Section 8.4, forms the basis for understanding the evolution of different types of ocean chemistry through Earth history.

One obvious difference between Acton Lake and the oceans is sulphate concentration. In Acton Lake, sulphate concentration is low at about 300 µM (Green et al., 1985). Low sulphate concentrations are a general feature of lakes (with some notable exceptions), compared to the high average modern marine sulphate concentration of 28 mM. Therefore, we could imagine that sulphate would be less available in lakes, compared to oceans, allowing the oceans a better chance to go sulphidic when anoxic. A beautiful example of the interplay between sulphate, sulphide and Fe(II) in a permanently anoxic ferruginous lake has already been shown for Lake Matano, Indonesia in Figure 8.4. Recall that in Lake Matano, sulphate, beginning at a very low concentration in the upper waters, is depleted in the upper reaches of the chemocline, and that the small amounts of sulphide produced are overwhelmed by the massive supply of Fe(II) in the lake, which is thus Fe(II), not sulphide, dominated.

Relative Fluxes of Fe and S to the Oceans. Therefore, sulphate concentra-

tion has something to do with determining whether an anoxic system will tend towards sulphidic or ferruginous conditions. However, sulphate concentration is

just a proxy for the real driver, which is the relative fluxes of highly reactive Fe1

and sulphide (from sulphate reduction) to the anoxic system. If we have a system at circum-neutral pH2_{, and if the sulphide flux exceeds the highly reactive Fe flux}

by a factor of two (the stoichiometric ratio of Fe to S in pyrite, FeS2), then sulphide

should accumulate. If sulphide production does not exceed the highly reactive Fe flux by a factor of 2, then Fe(II) should accumulate. This simple idea is expressed in cartoon form in Figure 9.1.

Returning now to Acton Lake and Lake Matano, the low sulphate concen- trations in these lakes are a reflection of the low sulphate fluxes to them. The low sulphate fluxes are, furthermore, insufficient to drive enough sulphate reduction to remove the highly reactive Fe entering these lakes. The highly reactive Fe is dominantly composed of the iron (oxyhydr)oxides associated with detrital particles entering the lakes (see Section 3.5 for a more complete discussion of highly reactive Fe and footnote 1). The Fe(II) that accumulates is derived from the reductive dissolution of these highly reactive Fe phases. If there was no highly reactive iron flux to these

lakes, sulphidic conditions would dominate despite the low sulphate fluxes and sulphate concentrations.

I had some idea of this simple control principle when I proposed a transition from ferrug- inous to sulphidic marine condi- tions during the Proterozoic Era in 1998 (Canfield, 1998). I will discuss this idea in more detail in a later part of this section, but to evaluate it, I estimated the flux of highly reactive Fe to the global ocean (based on dithionite extractable iron in river particulates; see also Section 3.5) and compared this to

1 . Highly reactive Fe includes the Fe phases which are viewed to be reactive on early diage- netic time scales . In oxic waters, highly reactive Fe is comprised mostly of particulate iron oxide and oxyhydoxide phases . In anoxic waters , some of these oxides and oxyhydroxides will be converted to other diagenetic products, and the highly reactive Fe pool consists of unconverted Fe oxides and oxyhydoxides together with Fe sulphides and Fe carbonate phases as well as dissolved Fe(II) . See Sections 3 .5 and 3 .6 for further discussion . 2 . The solubility product of FeS is given as ksp = aFe2+aHS–/aH+ . As pH drops (aH+ increases),

the concentration of HS– _{increases to maintain FeS saturation at constant Fe}2+ _{. Further-}

more, the first dissociation constant for H2S is about 10–7 . Therefore, as pH drops below

7, the concentration of HS– _{begins to drop at any equal concentration of total H} 2S (H2S +

HS– _{+ S}2–_{) . This means that total H2S concentrations must increase further to maintain}

saturation with FeS as pH drops below 7 . Thus, below a pH of about 7, both ferruginous

Figure 9.1 Cartoon showing how the relative fluxes of reactive Fe and sulphide production can determine the nature of anoxic water chemistry.

modern rates of sulphate reduction. The comparison revealed a clear excess of sulphate reduction over highly reactive Fe delivery, and I argued that because of this, if the oceans went anoxic with the same flux of sulphate to the oceans as today, they would become sulphidic. In fact, I made the wrong calculation, or rather, I mixed two different ideas. The RIGHT calculation would have been to compare the highly reactive Fe flux to the ocean to the sulphate flux, and not to worry about modern rates of sulphate reduction.

In fact, not all of the sulphate entering the oceans is necessarily available to end up as pyrite. Indeed, the sulphate entering the oceans can be removed by two major pathways; either as pyrite sulphur or as evaporitic CaSO4. From

simple S isotope mass balance considerations, a very high percentage of the sulphate entering the Proterozoic oceans, particularly early in the Proterozoic, was removed as pyrite (Canfield, 2004), so we don’t need to worry so much about the CaSO4 removal pathways. Therefore, the river flux of sulphate into the oceans

provides the removal flux of pyrite from the oceans. How does this compare to the highly reactive Fe input flux?

Simon Poulton and I have discussed this at length. Our estimates rely heavily on previous Fe budget estimates generated by Rob and Simon (Poulton and Raiswell, 2002), along with best estimates for pre-anthropogenic sulphur delivery to the oceans (Poulton and Canfield, 2011). Our compilation is repro- duced in Table 9.1. We find that, in contrast to my earlier approach, there is a great excess in highly reactive iron input to the oceans compared to sulphur input, with a S/Fe input ratio of 0.5, compared to the pyrite ratio of 2/1. Therefore, if the WHOLE ocean went anoxic, ferruginous conditions would be predicted. However, there is a great excess of highly reactive Fe deposition on inner shelf sediments, so the ratio of S/Fe left for removal onto the outer shelf, slope and deep sea is

Table 9.1 Global FeHR and S budgets for the modern ocean.

FeHR S† S:FeHR Fluxes (× 1012 _{mol y}–1₎ Continental sources 6.5 ± 1.7 2.6 ± 0.6 Hydrothermal 0.3 ± 0.1 0.5 ± 0.4 Volcanic - 0.2 ± 0.1 Total 6.8 ± 1.7 3.3 ± 0.7 Sinks (× 1012 _{mol y}–1₎

Inner shore areas* _{5.5 ± 0.6 1.0 0.2}

Global ocean 1.3 ± 0.3 2.3 1.8

*_{Including the outer shelf, continental slope and the deep sea.} †_{Uncertainties on S sinks are not reasonably definable.}

about 1.8/1, very close to the ratio in pyrite formation. Therefore, the system is apparently balanced near a switching point. This means that if anoxia develops below the ocean mixed layer of about 100 meters, small changes in either the S or Fe fluxes could drive the system to either sulphidic or ferruginous states. As we shall see below, the discussion is more subtle than this, and mixed chemical states during many times in Earth history were most likely the rule.