The influence of differential production and dissolution on the stable isotope composition of planktonic foraminifera

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.