Fossilized remains of marine calcifiers constitute the physical basis for reconstructions of both deep
ocean and sea-surface temperatures going back millions of years, but paleoclimate records derived
from their isotope and trace-element chemistry can be biased by diagenesis. Experiments simulating
diagenesis in the presence of an 18O-rich seawater analogue were conducted with modern and 14
Myr old foraminifera (Ammonia sp.) tests to investigate their relative susceptibility to oxygen isotope
exchange. The fossilized tests were of exceptional preservation and similar to modern tests in terms
of structure and crystalline organization, but had experienced partial loss of embedded organic
structures, thus a priori offering fewer preferential pathways for porewaters to penetrate the tests.
NanoSIMS imaging revealed that oxygen isotope exchange was pervasive in fossil tests, with isotopic
exchange occurring at approximately half the rate of modern tests. The results unequivocally show
that fossil biocalcites are metastable and remain more susceptible to isotope exchange than abiotic
calcites millions of years after sedimentation and burial.