Are foraminifera tests a good palaeothermometer?

Paleoclimate reconstructions commonly use oxygen isotope compositions from fossil foraminifera tests as proxies. However, the isotopic composition of these calcitic tests can be substantially altered during diagenesis. In the published paper the fluid-mediated isotopic exchange was examined in pristine tests of three modern benthic foraminifera species i.e., Ammonia sp., Haynesina germanica, and Amphistegina lessonii. Reacted tests remained texturally ‘pristine’ but their bulk oxygen isotope compositions revealed rapid and species-dependent isotopic exchange with the water; diagenetic alteration is correlated with test ultra-structure and associated organic matter.

The result implies that the tests that are considered texturally ‘pristine’ for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange; critical paleo-temperature record re-examination is warranted.