To discuss tests of these speciation hypotheses, some different background is
required: paleontological stratigraphy and molecular genetics. For the latter, please
refer back to Box 2.4.
Paleontological Stratigraphy of the Ocean Basins
in Relation to Planktonic Biogeography
Basic Notions
(^) There is a fundamental interdependence of studies of oceanic stratigraphy and studies
of the biogeography of plankton. Stratigraphy is the description and interpretation of
the content of layers of sediment that have accumulated on the seafloor. These
sediments may be lithified and uplifted, thus occurring on land; or they may remain
unconsolidated and submerged. The latter will be our main concern. Several devices
for collecting sediment sequences from the ocean bottom are in use, including free-
fall and piston corers and drilling devices. All of them amount to driving a pipe into
the sediment, then pulling it out to retrieve a vertical cylinder of sediment, a core. In
general, the farther down along a core that a sample is taken, the longer the interval
since its deposition. There are exceptions caused by slumping of sediments down
submarine slopes or winnowing by currents with subsequent redeposition. These
usually can be detected at a small price in evidential circularity. Thus, any process in
the water column that affects the character of the materials deposited will have its
history fuzzily recorded in the sediments. Fuzziness is caused by the extremely slow