Extreme warmth and heat-stressed plankton in the tropics during the Paleocene-Eocene Thermal Maximum

Abstract

Global ocean temperatures rapidly warmed by ~5°C during the Paleocene-Eocene Thermal Maximum (PETM; ~56 million years ago). Extratropical sea surface temperatures (SSTs) met or exceeded modern subtropical values. With these warm extratropical temperatures, climate models predict tropical SSTs >35°C-near upper physiological temperature limits for many organisms. However, few data are available to test these projected extreme tropical temperatures or their potential lethality. We identify the PETM in a shallow marine sedimentary section deposited in Nigeria. On the basis of planktonic foraminiferal Mg/Ca and oxygen isotope ratios and the molecular proxy [Formula: see text], latest Paleocene equatorial SSTs were ~33°C, and [Formula: see text] indicates that SSTs rose to >36°C during the PETM. This confirms model predictions on the magnitude of polar amplification and refutes the tropical thermostat theory. We attribute a massive drop in dinoflagellate abundance and diversity at peak warmth to thermal stress, showing that the base of tropical food webs is vulnerable to rapid warming.

Keywords: PETM; Plankton; SST; Tropics; dinoflagellate; heat; heat stress; paleocence - eocene; polar amplification; temperature proxy.

Fig. 1. Temperature and environment for the SQ (A) and IB10B core (B to E).

(A) SQ surface and bottom-water temperature reconstructions based on Mg/Ca (triangles), δ¹⁸O (circles) of selected foraminifer species, and ${\text{TEX}}_{86}^{\mathrm{H}}$ (squares). Error bars represent analytical errors. Conservative 1σ calibration errors: 1.6°C for δ¹⁸O; 1.7° and 2°C for planktonic and benthic Mg/Ca, respectively; and 2.5°C for ${\text{TEX}}_{86}^{\mathrm{H}}$. (B) IB10B stable carbon isotope record of TOC (δ¹³C_TOC) and palynological residue (δ¹³C_paly). The phases of the CIE are denoted by O, B, and R for onset, body, and recovery, respectively. PFZ, planktonic foraminifer zone. (C) ${\text{TEX}}_{86}^{\mathrm{H}}$-based SST (error bars based on replicate measurements). (D) Absolute concentrations of dinocysts per gram of dry sediment. Note the scale break between 4,000 and 10,000. (E) Indicators of anoxia: Organic carbon over total phosphorus ratio (Corg/Ptot) (blue squares). Presence of isorenieratene and its derivative. Concentrations of redox-sensitive trace elements Mo (light green circles) and Cd (dark green triangles) in parts per million (ppm). Note that Mo concentrations are close to the detection limit, and absolute values should be treated with caution.

Fig. 2. Model-data comparison for latest Paleocene (2240 ppm) and PETM (4480 ppm) CO₂ scenarios.

(A) Vertical temperature profile for the site locations. Solid and dashed lines represent latest Paleocene and PETM model estimates, respectively. Data from Fig. 1 (A and C) for comparison, including error bars for depth: Morozovella (0 to 30 m), Acarinina (0 to 50 m), ${\text{TEX}}_{86}^{\mathrm{H}}$ (0 m; surface), and benthic (100 to 150 m). (B) Modeled latest Paleocene temperature along an inshore-offshore transect representing the study site. (C) Modeled warming from latest Paleocene to PETM along the same transect. (D) ${\text{TEX}}_{86}^{\mathrm{H}}$-derived (orange), δ¹⁸O-derived (blue), and Mg/Ca-derived (red) SSTs and modeled Paleocene meridional SST gradient (solid line). Gray dots represent point-by-point model-data comparisons. (E) Interpolated absolute SST reconstructions. (F) Modeled latest Paleocene (2240 ppm) temperatures. (G) PETM absolute SST changes and ${\text{TEX}}_{86}^{\mathrm{H}}$-derived (orange), δ¹⁸O-derived (blue), and Mg/Ca-derived (red) SST changes between the latest Paleocene and the PETM. Gray dots represent point-by-point model-data comparisons. (H) Interpolated change in absolute SST. (I) Modeled SST change from the latest Paleocene to the PETM (4480 to 2240 ppm). (J) PETM SST changes, normalized to tropical (20°N to 20°S) SST changes (2.7°C) from the data and ${\text{TEX}}_{86}^{\mathrm{H}}$-derived (orange), δ¹⁸O-derived (blue), and Mg/Ca-derived (red) SST changes between the latest Paleocene and the PETM. Gray dots represent point-by-point model-data comparisons. (K) SST changes from the latest Paleocene to the PETM from the data, normalized to tropical SST changes in the data (2.7°C). (L) SST changes from the latest Paleocene to the PETM, normalized to tropical SST changes in CESM1 (2.4°C). Black circles indicate site locations.