Constraints on the chemical composition of the ancient oceans

Addresses the problem of reconciling a Proterozoic “Canfield ocean” scenario (with globally significant euxinia), with the relatively rarity of pyrite-enriched reactive iron during this interval. Uses a simple box model to illustrate how the “electron tower principle” implies that globally significant sulphate reduction necessitates equivalently globally significant nitrate depletion (because denitrifiers will outcompete sulphate reducers). Proposes two distinct geochemical regimes for the low oxygen Proterozoic ocean (nitrate rich/ferruginous and nitrate depleted/euxinic), the latter of which may be partially reverted to in nitrate/oxygen depleted modern day upwelling zones. Suggests that a future empirical cross referencing exercise should find the iron speciation evidence for euxinia to be out of phase in time, with nitrogen isotope evidence for denitrification.





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