Boron isotope signals of benthic foraminifera during the Santonian

We investigated the δ¹¹Bisotope profiles in Cretaceous benthic foraminifera, focusing on the epibenthic species Notoplanulina rakauroana and Nuttallinella coronula. This study aims at understanding the implications of changes in δ¹¹B isotope signatures for pH, associated with paleoenvironmental factors during the beginning of the Late Cretaceous cooling in the Santonian in the southern high latitudes.

We examine 10 levels through the Santonian of International Ocean Discovery Program (IODP) Site U1513 in the Mentelle Basin, some kilometres offshore Western Australia. Following a biostratigraphic framework relying on planktonic foraminifera and calcareous nannofossils, foraminiferal samples of Notoplanulina and Nuttallinella, representing significant elements of bottom water fauna through this period of significant cooling in surface and bottom waters, have been processed.

The geochemical signals preserved in the tests of epibenthic foraminiferal species are supposedly less prone to representing vital effects as some stability in the paleohabitat in bottom waters is implicit (see Rae et al. 2011). The analysis of well-preserved foraminiferal tests by Inductively Coupled Plasma Mass Spectrometry (ICPMS) is imperative of a cleaning protocol that involves several steps of ultrasonic baths in clear water and alcohol (see Henehan et al. 2019).

The results of benthic foraminiferal δ¹¹B add information to our understanding of the relationbetween climate shifts and changes in the geochemical composition visible in foraminiferal tests. Results help to reconstruct prevailing pH of bottom waters during the documented reorganization of oceanic pathways in the southern high latitudes and offer a further perspective on the extent of paleoenvironmental change in the bottom waters.

References:
Rae, JWB, Foster, GL, Schmidt, DN, Elliott, T, 2011. Boron isotopes and B/Ca in benthic foraminifera: Proxies for the deep ocean carbonate system, Earth and Planetary Science Letters, 302, 3–4.
Henehan, MJ, Ridgwell, A, Thomas, E, et al., 2019. Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact. PNAS, 116, 45, 22500-22504.