Isotopic filtering reveals high sensitivity of planktic calcifiers to Paleocene-Eocene thermal maximum warming and acidification

Abstract

Ocean warming and acidification driven by anthropogenic carbon emissions pose an existential threat to marine calcifying communities. A similar perturbation to global carbon cycling and ocean chemistry occurred ∼56 Ma during the Paleocene-Eocene thermal maximum (PETM), but microfossil records of the marine biotic response are distorted by sediment mixing. Here, we use the carbon isotope excursion marking the PETM to distinguish planktic foraminifer shells calcified during the PETM from those calcified prior to the event and then isotopically filter anachronous specimens from the PETM microfossil assemblages. We find that nearly one-half of foraminifer shells in a deep-sea PETM record from the central Pacific (Ocean Drilling Program Site 865) are reworked contaminants. Contrary to previous interpretations, corrected assemblages reveal a transient but significant decrease in tropical planktic foraminifer diversity at this open-ocean site during the PETM. The decrease in local diversity was caused by extirpation of shallow- and deep-dwelling taxa as they underwent extratropical migrations in response to heat stress, with one prominent lineage showing signs of impaired calcification possibly due to ocean acidification. An absence of subbotinids in the corrected assemblages suggests that ocean deoxygenation may have rendered thermocline depths uninhabitable for some deeper-dwelling taxa. Latitudinal range shifts provided a rapid-response survival mechanism for tropical planktic foraminifers during the PETM, but the rapidity of ocean warming and acidification projected for the coming centuries will likely strain the adaptability of these resilient calcifiers.

Keywords: PETM; global warming; ocean acidification; planktic foraminifera; sediment mixing.

Fig. 1.

Site 865 location and CIE marking the PETM. (A) Paleogeographic map showing tropical location of open-ocean Site 865 (red star) during the late Paleocene (source: Ocean Drilling Stratigraphic Network, odsn.de). (B) Planktic foraminifer carbon isotope (δ¹³C) records for the 865C section constructed with pooled, multishell samples of M. velascoensis (orange squares), Acarinina spp. (green diamonds), and Subbotina spp. (blue triangles) (32). The red arrow denotes stratigraphic position (meters below sea floor = mbsf) of the benthic foraminifer extinction event associated with CIE onset (32). (C and D) Bulk-carbonate δ¹³C records of CIE in the 865C and 865B PETM sections, respectively. The break along vertical axis in panel D signifies a coring gap. The dark gray shading delimits the correlative lower CIE interval in the two PETM records. The light gray shading delimits the portion of 865B record sampled for planktic foraminifer assemblage counts. (E and F) Relative abundance curves showing acme of excursion marker taxa (M. allisonensis, A. sibaiyaensis, and A. africana) based on unfiltered census counts (>125 μm) for 865C and 865B PETM planktic foraminifer assemblages, respectively. Assemblage count data for the 865C PETM section taken from published literature (27). For comparative purposes, data shown are plotted relative (±cm) to stratigraphic level of CIE onset in bulk-carbonate δ¹³C records, which was prescribed a reference depth of 0 cm.

Fig. 2.

Frequency distributions of single-shell δ¹³C values for planktic foraminifers from the lower CIE of the Site 865C PETM record (n = 548). The bimodal distributions for shallow-dwelling genera Morozovella (n = 274) and Acarinina (n = 220) indicate that these taxa are mixtures of reworked, pre-CIE specimens with high δ¹³C values and in situ CIE specimens with relatively low δ¹³C values. The unimodal distribution of thermocline-dwelling genus Subbotina (n = 54) is centered on a pre-CIE value for this particular taxon, indicating that all subbotinids are reworked specimens.

Fig. 3.

Proportions of reworked specimens within the lower CIE of the 865C PETM section used to reconstruct population dynamics for individual planktic foraminifer taxa during the early stages of the CIE. (A–E) “Lazarus” taxa disappear over part or all of the early CIE only to subsequently reappear in the fossil record. (F and G) “Holdover” taxa occur continuously before, during, and after the PETM. (H and I) “Excursion” taxa occur almost exclusively during the early CIE. The red error bars delimit the 95% CI as determined by nonparametric bootstrapping (i.e., resampling) of proportions (1,000 iterations). The data are plotted relative (+cm) to stratigraphic level of CIE onset in Site 865C bulk-carbonate δ¹³C record.

Fig. 4.

Comparison of isotopically filtered and unfiltered planktic foraminifer assemblages from the 865B PETM section. Diversity metrics and relative abundances for the nine taxa sampled for single-shell stable isotope analyses (Materials and Methods). Comparisons include (A) diversity (Shannon-H) and (B) combined relative abundance of excursion taxa (M. allisonensis and A. sibaiyaensis). (C) Stacked-area plot showing changes in taxonomic composition and taxon relative abundances for isotopically filtered assemblages over the lower CIE in the 865B PETM section. The break along the vertical axis denotes coring gap in the 865B section (mbsf = meters below sea floor).