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The problem of whether planktonic forams and associated microfossils dissolve during settling or on the sea floor was investigated in the eastern tropical Pacific Ocean. Net tow samples, from 3,000 meters depth and deeper, showed excellent preservation of most microfossils. Relatively rapid-settling forams (> 150 μ) were well preserved showing only minor etching, whereas effects of dissolution were noticeable in the finer fraction. Whole pteropod tests were common, in part showing considerable etching. Several tows collected abundant large centric diatom frustules, the valves being generally intact and the preservation excellent. Box cores recovered assemblages of foraminifera at the sediment-sea water interface at depths well below the regional calcite compensation depth. The assemblages contained a wide range of preservation states, that is, delicately spined individuals having experienced no dissolution as well as keel fragments of Globorotalia species being the final stages of solution of some of the most resistant species. Such mixed assemblages are expected if dissolution occurs mainly on the sea floor.

The results from deep net tow and box core samples indicate that larger forams and pteropods experience little solution during settling: solution occurs mainly at the sediment-sea water interface. Estimates of the average residence time of foram tests on the sea floor before disintegration yield a value on the order of one month for the area studied.

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