Age determination of foraminifera shells

The causes for discordant radiocarbon results on multiple species of planktonic foraminifera from high-sedimentation-rate marine sediments are investigated. We have documented two causes for these anomalous results. One is the addition of secondary radiocarbon for which we have, to date, only one firm example. It involves an opal-rich sediment. The other is the incorporation of reworked material. Again, we have, to date, only one firm example. It involves a rapidly deposited ocean margin sediment. However, we have three other examples where reworking is the most likely explanation. On the basis of this study it is our conclusion that, where precise radiocarbon dating of high-deposition-rate marine sediment is required, a prerequisite is to demonstrate that concordant ages can be obtained on pairs of fragile and robust planktic shells. For sediment rich in opal, it is advisable to check for secondary calcite by comparing ages obtained on acid-leached samples with those on unleached samples.

Silicoflagellate abundance in ODP Holes 183-1138A and 183-1140A

The biostratigraphic distribution and abundance of lower Oligocene and Miocene to Pleistocene silicoflagellates are documented from Ocean Drilling Program Leg 183 Holes 1138A and 1140A, on the Kerguelen Plateau. The Distephanus speculum speculum forma pseudofibula plexus is found in the upper Miocene in Hole 1138A, but other important biostratigraphic markers are not available. Diversity and abundance of silicoflagellates vary considerably in Hole 1138A, with silicoflagellates more abundant in the Pliocene and Pleistocene and some intervals of the Miocene barren of silicoflagellates or containing only limited numbers of specimens. The silicoflagellates of Hole 1140A include a new skeletal morphology, described here as Distephanus speculum speculum forma cylindrus. Silicoflagellates were generally abundant throughout the lower and middle Miocene in Hole 1140A.