During the Late Cretaceous, Earth was a greenhouse world characterized by extreme temperatures and high concentrations of atmospheric carbon dioxide. Records from the Falkland Plateau in the southern Atlantic Ocean suggest that during the period of peak warmth, which lasted from about 100 to 90 million years ago, sea surface temperatures at middle to high southern latitudes exceeded 30°C, conditions that were significantly hotter than today’s mean annual average of 0°C. But because these data have been difficult to reconcile with climate models and other proxy records, scientists have debated whether they truly reflect global climatic conditions.
Now O’Connor et al. further explore this question using TEX86, an organic paleothermometer regularly used to reconstruct past sea surface temperatures. The team applied this technique to Late Cretaceous sediment samples collected at Deep Sea Drilling Project drilling Sites 327 and 511 on the Falkland Plateau as well as Ocean Drilling Program Sites 1138 and 1135 on the Kerguelen Plateau in the southern Indian Ocean. All four locations lay between 50°S and 60°S from about 100 to 66 million years ago.
The data indicate that Late Cretaceous sea surface temperatures ranged from 27°C to 37°C at these locations. Because these results are comparable to other local proxy data as well as the global TEX86 records, the authors conclude their data accurately reflect long-term global trends and therefore corroborate previous interpretations of extreme warmth during the Late Cretaceous at Earth’s subpolar latitudes. The study results also show that after sea surface temperatures peaked during the Cenomanian-Turonian time interval, around 94 million years ago, Earth’s climate experienced a slow and steady cooling trend that lasted for at least 16 million years.
These findings also highlight the discrepancy between proxy records and climate model simulations, which struggle to reproduce such warm conditions at middle to high latitudes. Although modeling studies suggest that a combination of drivers was necessary to sustain these high temperatures, the relative influence of volcanic carbon dioxide emissions, water vapor, methane, and other potential agents is still hotly debated. These high temperatures have important implications for global heat transport and polar ice in a high–carbon dioxide world, an understanding of which is important for modeling both past and future climate change. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003546, 2019)
—Terri Cook, Freelance Writer