On the Chemical Purity of Marine Microfossils

As a paleontologist, I was pleased to see the 35-million-year-old shell of the planktonic foraminiferan Cribrohantkenina inflata grace the cover of Physics Today (December 2001; story on page 16 ). Calcium carbonate–secreting microfossils are a powerful proxy for estimating sea-surface temperature change through time using stable oxygen isotopes. Another challenge that biogeochemists and geoscientists face is the alteration of in vivo isotopic values by mineral and chemical changes after an organism’s death. Implicit in any isotopic analysis using calcite shells, or fossil mammal teeth, or even dinosaur bones, is the assumption that the original stable isotopes survived the passage of millions of years.

A hemisphere away from the late Cretaceous marine localities of Paul Pearson’s work, ¹ dinosaur bones lie buried in late Cretaceous clays above the Arctic Circle on the North Slope of Alaska. ² They are so well preserved that an oil geologist in the 1960s originally identified them as the fossil remains of Ice Age mammals. A collection of those bones is cataloged in the University of California Museum of Paleontology. Under high magnification, the Alaska fossils reveal a beautifully preserved system of bone cells that are identical to the micron-scale architecture of a modern vertebrate bone. Similar to the Pearson group’s foraminifera, the dinosaur bones are entombed in clay sediments and do not appear altered by secondary remineralization or recrystallization. Microbeam particle-induced x-ray emission analysis revealed, however, that the bones showed significant postmortem enrichment by metals, primarily iron and manganese, more than 10⁴ times greater than in the bones of modern crocodilians and birds (a dinosaur’s closest living relatives). ³ Clay surrounding the Alaska dinosaurs was also markedly enriched in these metals from the burial environment.

Understandably, such factors are site specific and most likely vary by sediment layer and location. Their range and potential influence on a paleobiological isotopic signature remains largely unknown. Until that situation changes, the potential alteration of stable isotope values in fossil shells, bones, and teeth prevents the rejection of the null hypothesis that all fossil material is altered.

Beauty is in the eye of the beholder, and exceptional fossil preservation, even at the micron level, does not guarantee pristine chemistry.