Nannoplankton malformation during the Paleocene-Eocene Thermal Maximum and its paleoecological and paleoceanographic significance

Abstract

The Paleocene‐Eocene Thermal Maximum (PETM) is characterized by a transient group of nannoplankton, belonging to the genus Discoaster. Our investigation of expanded shelf sections provides unprecedented detail of the morphology and phylogeny of the transient Discoaster during the PETM and their relationship with environmental change. We observe a much larger range of morphological variation than previously documented suggesting that the taxa belonged to a plexus of highly gradational morphotypes rather than individual species. We propose that the plexus represents malformed ecophenotypes of a single species that migrated to a deep photic zone refuge during the height of PETM warming and eutrophication. Anomalously, high rates of organic matter remineralization characterized these depths during the event and led to lower saturation levels, which caused malformation. The proposed mechanism explains the co‐occurrence of malformed Discoaster with pristine species that grew in the upper photic zone; moreover, it illuminates why malformation is a rare phenomenon in the paleontological record.

Evidence for Shelf Acidification During the Onset of the Paleocene‐Eocene Thermal Maximum

“…araneus (Ar), and base D. salisburgensis var. anartios (An; all defined by Bralower & Self Trail, ), and base Rhomboaster spp. (R).…”

“…The distribution of stratigraphically significant taxa in BR, WL, and SDB is based on previous studies (Bralower & Self‐Trail, ; Gibbs, Bralower, et al, ; Self‐Trail et al, ) and on additional observations of samples from BR. Smear slides were prepared using standard techniques and viewed in a light microscope at magnification of 1,250X.…”

“…Smear slides were prepared using standard techniques and viewed in a light microscope at magnification of 1,250X. We compile the acmes of Calciosolenia aperta and Hornibrookina arca in the latest Paleocene and the first occurrences of the excursion nannoplankton in the early Eocene, including Rhomboaster spp., malformed Discoaster multiradiatus and D. salisburgensis (stage T2I of Bralower and Self‐Trail, ), Discoaster salisburgensis var. anartios , D. salisburgensis var.…”

Ecological Changes in the Nannoplankton Community Across A Shelf Transect During the Onset of the Paleocene‐Eocene Thermal Maximum

“…In MCBR, its abundance remains high throughout the section. (3) The diversity and abundance of the genus Discoaster increase above the CIE onset in all cores, especially in BR as a result of the origination of the excursion species (Bralower & Self‐Trail, ). (4) Chiasmolithus bidens decreases in abundance and becomes sporadic above the CIE onset in CD, BR, WL, and MCBR; it disappears entirely from SDB, not returning until the upper part of the PETM.…”

“…(4) Chiasmolithus bidens decreases in abundance and becomes sporadic above the CIE onset in CD, BR, WL, and MCBR; it disappears entirely from SDB, not returning until the upper part of the PETM. (5) Excursion taxa, which include the D. salisburgensis plexus of Bralower & Self‐Trail (), Rhomboaster spp., Bomolithus supremus , Coccolithus bownii , and Toweius serotinus , are present in all cores above the CIE onset except for MCBR, where the combined effects of environment and dissolution adversely affected the nannofossil assemblages (Self‐Trail et al, ). (6) Finally, species thought to be adapted to eutrophic conditions (including Zygodiscus , Zeugrhabdotus , Campylosphaera , Coronocyclus , Neochiastozygus , Neococcolithes , Pontosphaera ) increase in abundance in the later part of the onset.…”