Institution

)
TH E CARBONATES
Foraminiferal tests generally constitute more than 90
percent of the carbonate in the cores from the Greater
Antilles Outer Ridge, and consequently there is a strong
correlation between foraminiferal abundance determined in
visual core descriptions and the carbonate content (Fig. 4.7).
Microscopic examination of smear slides shows that the
non-forami ni feral carbonate consi sts mostly of nannopl ankton
remai nswi th very small amounts of carbonate detritus which
may be transported from shallower areas such as the Bahama
Banks. The paucity and very fine size (~5 ~m) of this
material make identification difficult, but coralline-algal
fragments and dolomite rhombs have tentatively been
identified.
Since the carbonate consists dominantly of foraminiferal
tests, the cyclicity of carbonate content in the cored sedi-
ment (Figs. 4.7, 4.8) may be caused by any of four factors:
1) dilution by terrigenous, carbonate-free sediment, 2)
current transportation of Foraminifera, 3) variations in
carbonate dissolution, or 4) variations in zooplankton
productivity of the overlying surface water.
Any effect of dilution by terrigenous sediment is
1 argely rul ed out by data on the observed rates of accumul a-
tion (see Chapter V); radiocarbon dates of the upper portions
of cores indicate that average sedimentation rates are
generally lower in the low-carbonate zones than in the
high-carbonate zones. It is also unlikely that abyssal
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