Ferromanganese phases and the chemistry of contemporary seawater

We report on a correlation between oceanic residence times of chemical elements and their enrichment in marine ferromanganese deposits. When a logarithmic plot of oceanic residence time vs. atomic number is placed above a similar plot of enrichment factor vs. atomic number, the plots exhibit a clear mirror image relationship. We suggest that this relationship is not accidental, but rather implies an important role for ferromanganese phases in the regulation of seawater chemistry, especially the residence times of the minor elements. The formation of ferromanganese phases, as nodules, crusts, particles and coatings on pelagic clays, is dependent upon oxic seawater conditions similar to those existing today. The absence of oxic conditions throughout much of geologic time insures that ferromanganese minerals would have been rare or nonexistent sedimentary deposits. If these phases are as important as contemporary residence time correlations suggest, the minor element chemistry of earlier oceans must have been radically different from that of the present. Moreover, in times of deep water stagnation over the Phanerozic, conditions would have been thermodynamically unsuitable for ferromanganese mineral formation and hence suitable for the accumulation of high concentrations of certain toxic metals. We suggest that metal poisoning resulting from deep water turnover may have contributed to some of the great marine extinctions occurring over Phanerozoic Time.[PUBLICATION ABSTRACT]