Ferromanganese phases and the chemistry of contemporary seawater

We report on a correlation between oceanic residence times of chemical elem ents and their enrichment in marine ferromanganese deposits. When a logarit hmic plot of oceanic residence time vs. atomic number is placed above a sim ilar plot of enrichment factor vs. atomic number, the plots exhibit a clear mirror image relationship. We suggest that this relationship is not accide ntal, but rather implies an important role for ferromanganese phases in the regulation of seawater chemistry, especially the residence times of the mi nor elements. The formation of ferromanganese phases, as nodules, crusts, p articles and coatings on pelagic clays, is dependent upon oxic seawater con ditions similar to those existing today. The absence of oxic conditions thr oughout much of geologic time insures that ferromanganese minerals would ha ve been rare or nonexistent sedimentary deposits. If these phases are as im portant as contemporary residence time correlations suggest, the minor elem ent chemistry of earlier oceans must have been radically different from tha t of the present. Moreover, in times of deep water stagnation over the Phan erozic, conditions would have been thermodynamically unsuitable for ferroma nganese mineral formation and hence suitable for the accumulation of high c oncentrations of certain toxic metals. We suggest that metal poisoning resu lting from deep water turnover may have contributed to some of the great ma rine extinctions occurring over Phanerozoic Time.