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The periodic deposition of distal turbidites at a site on the Madeira Abyssal Plain causes the development of a nonsteady-state diagenetic system in which an oxidation front migrates downward into the sediment. Data presented here show that iron, manganese, and particulate organic carbon are oxidized at this front by oxidants (molecular oxygen and nitrate) diffusing from above. A numerical model of systems of this type predicts the formation of iron-rich layers under certain nonsteady-state conditions. The layers predicted by the model are closely comparable in thickness and general morphology to iron-rich layers found in certain ocean sediments, the origin of which has been until now unexplained.

Standard seawater (SSW) has been employed by oceanographers as a reference material in the determination of salinity for over a century. In all that time, this is the first study to determine the uncertainty of the SSW manufacturing process. SSW is calibrated in reference to carefully prepared solutions of potassium chloride (KCl). All uncertainties in the preparation and measurement of KCl solutions and of new SSW are calculated. The expanded uncertainty of the SSW conductivity ratio is found to be 1 ϫ 10 Ϫ5 , based on a coverage factor of 2, at the time of manufacture. There is no discernible "within batch" variability. No significant variability of quality within or between batches of KCl is found. Measurement of SSW "offsets" from the label conductivity ratio as long as 5 yr after the SSW batch manufacture are reported, and no significant change in label conductivity ratio for SSW batches P130 through P144 outside the expanded uncertainty of 1 ϫ 10 Ϫ5 is found. This last result is in contrast to some other studies, and herein are suggestions as to why this may be the case.

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