Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry

Abstract

Past greenhouse periods with elevated atmospheric CO were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Ca throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30-36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.

Midlatitude Temperature Variations in the Oligocene to Early Miocene

“…However, these simulations maintain a latitudinal SST gradient of ~8°C between the locations of Sites 925 and 1406 (Kiehl & Shields, ), still much lower than inferred from the Oligocene TEX 86 temperatures, although temperatures could be underestimated at these equatorial location due to uncertainties in the TEX 86 proxy. Our observations are consistent with a recent a data‐model comparison for the Eocene (Evans et al, ) and highlight the difficulty of simulating the absolute temperature and latitudinal temperature gradients of warm earth climates, including the Eocene and early Miocene as shown here.…”

The Magnitude of Surface Ocean Acidification and Carbon Release During Eocene Thermal Maximum 2 (ETM‐2) and the Paleocene‐Eocene Thermal Maximum (PETM)

“…Absolute temperature records for ETM‐2 are reconstructed using planktic Mg/Ca from Sites 1209 and 1210, and 1265 (Harper et al, ) and early Eocene Mg/Ca estimates of seawater of Mg/Ca sw = 2.24 mol/mol (Evans et al, ). Coarse resolution Mg/Ca data from Site 1210 generally agree with data from Site 1209, although the two records slightly diverge during the CIE recovery.…”

“…To compare our SST estimates for ETM‐2 to those of the PETM, we reconstruct absolute temperature for the PETM at ODP Site 1209 using foraminiferal Mg/Ca ( A. soldadoensis and M. velascoensis ; Penman et al, ), Mg/Ca sw of 2.24 mol/mol consistent with the value applied to ETM‐2, following the early Eocene estimates of Evans et al (), and δ 11 B‐based pH using our δ 11 B A. soldadoensis data and the δ 11 B M. velascoensis data of Penman et al (). SST reconstructions indicate warming from an initial pre‐PETM temperature of ~34 to ~40 °C during the main CIE at Site 1209 (Figure ) and are consistent with midlatitude TEX 86 ‐based SST reconstructions from higher midlatitude sites, Bass River and Wilson Lake, which yield pre‐PETM SSTs in the upper 20s (°C) and warming to the mid‐30s (°C) during peak warmth (e.g., Sluijs et al, ).…”

Equilibrium state and sensitivity of the simulated middle-to-late Eocene climate

“…Prior to the EOT, not only the exceptionally warm early Eocene (Greenwood and Wing, 1995;Bijl et al, 2009;Huber and Caballero, 2011), but also the later part of the Eocene were characterised by a low equator-to-pole temperature gradient (Bijl et al, 2009;Hollis et al, 2012;Douglas et al, 50 2014; Evans et al, 2018). In general, the Eocene greenhouse climate has proven challenging to simulate adequately with climate models (Huber and Sloan, 2001;Huber and Caballero, 2011).…”

“…CC BY 4.0 License. glis et al, 2015;Evans et al, 2018). Meanwhile, estimates of high-latitude temperatures are possibly biased towards warm season conditions (Sluijs et al, 2006;Bijl et al, 2009;Schouten et al, 2013).…”