Abstract:
Planktonic foraminifera from plankton tows, sediment
traps and sediments from the central North Atlantic were
studied in order to understand how they acquire their oxygen
and carbon isotope compositions. Shallow dwelling planktonic
foraminifera (mostly spinose species), collected in plankton
tows in the photic zone, show light isotopic compositions
possibly in slight negative deviation from oxygen isotopic
equilibrium.
Radioactive tracer experiments using 14C and 45Ca were
conducted on shallow dwelling benthonic foraminifera and
hermatypic corals. They show that photosynthesis of
symbiotic algae within these organisms increases the amount
of metabolic C02 incorporated into the skeleton which
consequently becomes isotopically lighter. Because shallow
dwelling planktonic foraminifera contain symbiotic algae it
is suggested that their light isotopic compositions are also
caused by photosynthetically enhanced incorporation of
metabolic C02 in the skeleton.
Planktonic foraminifera collected in sediment traps
and sediments show heavier oxygen isotope compositions that
are in equilibrium for CaC03 deposited in the photic zone.
At the same time the weight/individual for these foraminifera
is almost doubled compared to those from plankton tows. I
suggest that these apparent equilibrium compositions are
achieved by a combination of light, perhaps non-equilibrium
skeletons deposited in the photic zone and isotopically
heavier calcite deposited below the photic zone. The latter
being isotopically heavy because temperatures are lower,
metabolic activity is reduced, and photosynthesis by the
symbiotic algae stops.
Dissolution of planktonic foraminifera on the ocean
floor removes first the light-weight thin shelled individuals
of a species population. Because these individuals are
isotopically lighter, the isotopic composition of the
surviving population is heavier.
The scheme described above is applied to explain the
effect of dissolution on the glacial-interglaical amplitude
of the Pleistocene isotopic record in the Atlantic and the
Pacific Oceans. The timing of dissolution cycles in the two
oceans is out of phase. Dissolution during the glacial in
the Atlantic and during the interglacial in the Pacific
makes the isotopic composition heavier. Preservation in the
Atlantic during interglacials and in the Pacific during the
glacials makes the isotopic composition lighter. The net
effect is amplification of glacial-interglacial amplitude in
the Atlantic and reduction of the amplitude in the Pacific.
