Nannoplankton Deformation from the Paleocene-Eocene Thermal Maximum: Evidence for Deep Photic Zone Undersaturation
Abstract:The PETM is characterized by a transient group of nannoplankton whose range is restricted to the interval defined by the carbon isotope excursion. Many of these so-called excursion taxa belong to the genus Discoaster, a group characterized by multi-rayed taxa. The origin of the Discoaster excursion taxa is debated, with ocean acidification a potential cause1. The excursion taxa are not cosmopolitan and the controls on their distribution is also poorly understood. We have observed excursion Discoaster taxa in the PETM from shelf sections in the US Atlantic Coastal Plain and obtained new information on their morphology and relationship to environmental change. The excursion taxa show progressive ray deformation starting near the base of the event with inter-ray irregularity, followed by ray-tip deformation and ray malformation, and, ultimately, ray loss. Tip deformation and ray malformation and loss characterize the dominant excursion taxa whose ranges extend through the peak of the event.
The deformational features shown by Discoaster are consistent with decreased calcification. Excursion Discoaster constitutes a minor component of the nannoplankton assemblage, with co-existing nannoplankton displaying no morphological change during the PETM. An explanation for this dichotomy is provided by the depth habitat of Discoaster. The genus is thought to have inhabited the deeper photic zone near the nutricline2, and we postulate that the excursion taxa are adaptations to undersaturated waters in this depth range. We have tested this explanation with EMIC simulations using GENIE in coastal and UVic in open ocean sites. In general, our results suggest that the excursion taxa are present at locations where calcite saturation shows a pronounced decrease during the first ~10kyr of the event. We conclude therefore that the excursion Discoaster represents an adaptation to diminished saturation within the lower part of the photic zone.
1 Raffi, I., Backman, J., Zachos, J.C., and Sluijs, A., 2009. Marine Micropaleontology, 70, 201-212, doi:10.1016/j.marmicro.2008.12.005.
2 Schueth, J. D., and Bralower, T. J., 2015. Paleoceanography, 30, doi:10.1002/2015PA002803.