PRESERVATION INTEGRITY OF PLANKTONIC FORAMINIFERAL TESTS IN OCEANOGRAPHIC RECONSTRUCTION: EXAMPLES FROM THE LAST 6 MILLION YEARS

Fossil planktonic foraminifera are routinely used in oceanographic reconstructions of the upper water masses of the world's oceans. Perhaps the most widely used Neogene to recent species for tropical and sub-tropical regions is Globigerinoides sacculifer. This species has a complex history of nomenclature, but the original description is based on (fossil?) material from a ‘chalk' block collected on the coast of New Ireland, Papua New Guinea. Comparing fossil (Pliocene, Miocene), core top, and plankton tow G. sacculifer indicates significant differences in test calcification. Tow specimens, collected in vivo, are lightly calcified with translucent tests, calcite formed during ontogenesis. Core top and fossil specimens are heavily calcified, white opaque in appearance, and show evidence for thick gametogenic calcite veneers. These veneers add substantially to the mass of the tests. Fossil material, particularly from Miocene horizons, shows authigenic calcite, visible by scanning electron microscopy, though not apparent from light microscopy. We attempt to quantify by how much the original ontogenetic oxygen isotopic signature (d18O) of specimens is offset by these post-ontogenetic calcite phases, and provide a visual estimate of the preservation integrity of fossil and core top material as a guide to identifying pristine or near pristine material for use in oceanography.