sedimentation rates over much of the ocean, as low as 1 cm 1000 yr−1, coupled with
mixing (“bioturbation”) of the upper 2 to 10 cm by benthic animals. Thus, annual and
even centennial events and cycles are almost always obscured. There are useful
exceptions, especially deposits in anoxic basins with no benthic animals.
(^) Careful sorting of sediments from most locations will yield a substantial number of
microfossils. These are the siliceous shells of diatoms and radiolaria; the calcareous
shells of prymnesiophytes (coccoliths) and foraminifera; and aragonitic pteropod
shells. Microfossils present in a given sedimentary sequence vary in two ways. On
long time scales individual species evolve. On shorter time scales the relative
abundances of different fossil types vary markedly with depth in the sediments.
Evolutionary changes cannot be expected to tell us much about environmental
conditions in the past until the adaptive significances of the observed changes in
microfossil morphology are known, significances that are extremely difficult to
determine. Shorter-term abundance variations, however, are powerful indicators of
environmental history, particularly during the Pleistocene. We will use a single, old
example to illustrate the relation between short-term variations and zoogeography.
(^) Globorotalia menardii, a foraminiferan, is a circumglobal, temperate–tropical
species (Bé & Tolderlund 1971). It constitutes a substantial fraction of the planktonic
foraminifera in many warm-water areas. Phleger et al. (1953) showed that the spatial
distributions of many kinds of foraminifera from the tops of sediment samples are
coincident with the main hydrographic provinces of the oceans, just as we have seen
to be the case with living animals. Thus, sediment distributions for G. menardii (Fig.
10.17) from core tops, that is, from the surface of the sediment, resemble the pelagic
pattern closely. Not only are the overall ranges the same, but the zones of maximum
relative abundance around the circumference of the North Atlantic central gyre were
also observed by Bé and Tolderlund (1971) in the plankton. The similarity of the
patterns implies that the shells do not drift very far from the near-surface sites at
which they originate before sinking to the bottom. The appearance of G. menardii in
the sediment is, therefore, an indication of its presence in the overlying water.
Fig. 10.17 Distribution pattern of relative abundance (%) of Globorotalia menardii
plus G. tumida (which is similarly distributed) in sediments from the tops of cores
collected at the scattered stations represented by dots.
(^) (After Kipp 1976.)