Plankton DNA in marine sediments should obviously decay with time and certainly not last for over a million years.
However, a recent paper published in the journal Geology reports on the discovery of traces of diatom DNA in samples dug up from the Bering Sea in sediments believed to be 1.4 million years old.
Younger sediments contained more DNA, but even the “1.4 million year old” had some DNA left.
While plankton are featured in some Darwinian stories, this time they're absent.
The researchers do not have a logical explanation for their findings:
“We do not know the mechanism behind the apparent relative slowdown of DNA degradation with age. Whether this decreased turnover is due to decreased lability of residual DNA, an overall decrease in enzyme activity, a decrease in spontaneous decay rates, or some combination of these and other factors remains presently unknown.”
Kirkpatrick, John B., Emily A. Walsh and Steven D’Hondt. 2016. Fossil DNA persistence and decay in marine sediment over hundred-thousand-year to million-year time scales.Geology 44 (8) (August).