The rise and fall of the Cretaceous Hot Greenhouse climate

A compilation of foraminiferal stable isotope measurements from southern high latitude (SHL) sites provides a novel perspective important for understanding Earth's paleotemperature and paleoceanographic changes across the rise and fall of the Cretaceous Hot Greenhouse climate and the subsequent Paleogene climatic optimum. Both new and previously published results are placed within an improved chronostratigraphic framework for southern South Atlantic and southern Indian Ocean sites. Sites studied were located between 58° and 65°S paleolatitude and were deposited at middle to upper bathyal paleodepths. Oxygen isotope records suggest similar trends in both bottom and surface water temperatures in the southern sectors of the South Atlantic and in the Indian Ocean basins. Warm conditions were present throughout the Albian, extreme warmth existed during the Cretaceous Thermal Maximum (early-mid-Turonian) through late Santonian, and long-term cooling began in the Campanian and culminated in Cretaceous temperature minima during the Maastrichtian. Gradients between surface and seafloor δ¹⁸O and δ¹³C values were unusually high throughout the 11.5 m.y. of extreme warmth during the Turonian-early Campanian, but these vertical gradients nearly disappeared by the early Maastrichtian.

In absolute terms, paleotemperature estimates that use standard assumptions for pre-glacial seawater suggest sub-Antarctic bottom waters were ≥21 °C and sub-Antarctic surface waters were ≥27 °C during the Turonian, values warmer than published climate models support. Alternatively, estimated temperatures can be reduced to the upper limits of model results through freshening of high latitude waters but only if there were enhanced precipitation of water with quite low δ¹⁸O values. Regardless, Turonian planktonic δ¹⁸O values are ∼1.5‰ lower than minimum values reported for the Paleocene-Eocene Thermal Maximum (PETM) from the same region, a difference which corresponds to Turonian surface temperatures ∼6 °C warmer than peak PETM temperatures if Turonian and Paleocene temperatures are estimated using the same assumptions. It is likely that warm oceans surrounding and penetrating interior Antarctica (given higher relative sea level) prevented growth of Antarctic ice sheets at all but the highest elevations from the late Aptian through late Campanian; however, Maastrichtian temperatures may have been cool enough to allow growth of small, ephemeral ice sheets. The standard explanation for the sustained warmth during Cretaceous Hot Greenhouse climate invokes higher atmospheric CO₂ levels from volcanic outgassing, but correlation among temperature estimates, proxy estimates of pCO₂, and intervals of high fluxes of both mafic and silicic >volcanism> are mostly poor. This comparison demonstrates that the relative timing between events and their putative consequences need to be better constrained to test and more fully understand relationships among volcanism, pCO₂, temperature ocean circulation, Earth's biota and the carbon cycle.