Paleowave

Propagation of a Taxonomic Error

Taxonomic identification lies at the crux of foraminiferal paleoceanography. I have written before about the importance of properly identifying and reporting the species of foraminifera used for geochemical analysis in a study. This has implications not only for placing the extracted geochemical signals in a physical context with appropriate uncertainty bounds, but also for communicating the methodologies employed in the study for future replication and reproducibility purposes, a tenet of science.

Recently, Yi Ge Zhang and colleagues had an article in Science magazine about their work on a 12-million-year-old temperature reconstruction in the tropical Pacific Ocean. The study produces new, long and detailed tropical sea-surface temperature records using the TEX86 paleothermometer. Their results are remarkable as they show a sustained zonal tropical Pacific temperature gradient through the Pliocene, going against the paradigm of “permanent El Niño”. The Pliocene is a time period where the tropical Pacific gradient was thought to have collapsed, similar to what happens during a brief El Niño event. This inference comes from paleoceanographic records produced from the magnesium-to-calcium (Mg/Ca) ratios of planktic foraminifera. The authors make the case that this technique has limitations compared to TEX86 reconstructions on these time scales.

Discussing these previous studies that investigated Pliocene climate using planktic foraminifera, the authors write:

Published temperature records based on magnesium-to-calcium ratios (Mg/Ca) of the planktonic foraminifera Globoritalia sacculifer, from Ocean Drilling Program (ODP) site 806 (0°N, 159°E) (Fig. 1) (8), suggest that warm pool temperatures remained relatively constant as Earth cooled over the past 5 million years.

I was immediately struck by ‘Globoritalia’ - I had never heard of the genus, only Globorotalia. Furthermore, the only sacculifer species I was familiar with was Globigerinoides sacculifer. Being relatively inexperienced in non-Quaternary foraminifera, I assumed Globoritalia sacculifer was some kind of Pliocene planktic species. Skimming through foraminiferal identification books, I could not find any mention of the species. Google Scholar too yielded only five results for ‘Globoritalia sacculifer (including Zhang et al.) with all of them discussing the Pliocene mean state hypothesis. I was increasingly beginning to suspect that the species did not exist. The final nail came when I questioned my more experienced colleagues including foraminiferal expert Dick Poore, all of whom immediately recognized the error when I pointed it out: that it should be Globigerinoides sacculifer and not Globoritalia sacculifer (and Globorotalia not Globoritalia).

The origin of the taxonomic error seems to stem from another Science article, Permanent El Niño-Like Conditions During the Pliocene Warm Period by Michael Wara and colleagues, published in 2005. They write:

To track changes in the mean thermocline depth at the EEP site, we used ∆δ18O, the difference in δ18O between surface-dwelling Globoritalia sacculifer (without sac) and G. tumida (355 to 425 mm) (Fig. 2A), which occupies the base of the photic zone.

The error also seems to have propagated into a Journal of Climate article by Carl Wunsch (where Globigerinoides sacculifer is written correctly but Globorotalia tumida is written as Globoritalia tumida) and an article by Guodong Ji and others published in Geophysical Research Letters.

Though Globoritalia sacculifer was probably the result of an innocuous spelling oversight that somehow made its way past editing and though this error has no bearing on the scientific results contained therein, I find it very impressive that it has made its way into Science magazine, a highly prestigious journal, not once but twice over the last nine years.